Force 136 gallery in the Imperial War Museum London. Note the captured Japanese katana and Good Luck Flag.

Force 136 was the general cover name for a branch of the British World War II organization, the Special Operations Executive (SOE). The organisation was established to encourage and supply resistance movements in enemy-occupied territory, and occasionally mount clandestine sabotage operations. Force 136 operated in the regions of the South-East Asian Theatre of World War II which were occupied by Japan from 1941 to 1945.

Although the top command of Force 136 were British officers and civilians, most of those it trained and employed as agents were indigenous to the regions in which they operated. British, Americans or other Europeans could not operate clandestinely in cities or populated areas in Asia, but once the resistance movements engaged in open rebellion, Allied armed forces personnel who knew the local languages and peoples became invaluable for liaison with conventional forces. In Burma in particular, SOE could draw on many former forestry managers and so on, who had become fluent in Burmese or other local languages before the war, and who had been commissioned into the Army when the Japanese invaded Burma.

History[link]

SOE was formed in 1940, by the merger of existing Departments of the War Office and the Ministry of Economic Warfare. Its purpose was to incite, organise and supply indigenous resistance forces in enemy-occupied territory. Initially, the enemy was Nazi Germany and Italy, but from late 1940, it became clear that conflict with Japan was also inevitable.

Two missions were sent to set up (and assume political control of) the SOE in the Far East. The first was led by a former businessman, Valentine Killery of Imperial Chemical Industries, who set up his HQ in Singapore. A scratch resistance organisation was set up in Malaya, but Singapore was captured on 15 February 1942, soon after Japan entered the war.

A second mission was set up in India by another former businessman, Colin Mackenzie of J. and P. Coats, a clothing manufacturer. Mackenzie's India Mission originally operated from Meerut in North West India. Its location was governed by the fear that the Germans might overrun the Middle East and Caucasus, in which case resistance movements would be established in Afghanistan, Persia and Iraq. When this threat was removed late in 1942 after the battles of Stalingrad and El Alamein, the focus was switched to South East Asia.

The India Mission's first cover name was GS I(k), which made it appear to be a record-keeping branch of GHQ India. The name, Force 136 was adopted in March 1944. From December 1944, the organisation's headquarters moved to Kandy in Ceylon, and cooperated closely with South East Asia Command which was also located there.

Force 136 was wound up in 1946, along with the rest of SOE.

Operations[link]

Malaya[link]

The Oriental Mission of SOE attempted to set up "stay-behind" and resistance organisations from August 1941, but their plans were opposed by the British colonial governor, Sir Shenton Thomas. They were able to begin serious efforts only in January 1942, after the Japanese Invasion of Malaya had already begun.

An irregular warfare school, STS 101, was set up by the explorer and mountaineer Freddie Spencer Chapman. Chapman himself led the first reconnaissances and attacks behind Japanese lines during the Battle of Slim River. Although the school's graduates mounted a few operations against the Japanese lines of communication, they were cut off from the other Allied forces by the fall of Singapore. An attempt was made by the Oriental Mission to set up an HQ in Sumatra but this island too was overrun by the Japanese.

Malayan Communist Party[link]

Before the Japanese attacked Malaya, a potential resistance organisation already existed in the form of the Malayan Communist Party. This party's members were mainly from the Chinese community and implacably anti-Japanese. Just before the fall of Singapore, the party's Secretary General, Lai Teck, was told by the British authorities that his party should disperse into the forests, a decision already made by the party's members.

In isolation, the Communists formed the Malayan Peoples' Anti-Japanese Army (MPAJA). Their first arms and equipment were either donated by STS 101 before they were overrun, or recovered from the battlefields or abandoned British Army depots. The MPAJA formed rigidly disciplined camps and units in the forest, supplied with food by networks of contacts among displaced Chinese labourers and "squatters" on marginal land. Chapman had remained in Malaya after Singapore fell, but had no radio or means of contacting Allied forces elsewhere. Nevertheless, the MPAJA still regarded Chapman as the official British authority, and Chin Peng was appointed as liaison officer with Chapman.^[1]

Singaporean World War II hero Lim Bo Seng had returned to Malaya from Calcutta in 1942, and recruited some agents who had made their way to India by 1943. Force 136 attempted to regain contact with Chapman in Operation Gustavus, by infiltrating parties which included Lim Bo Seng and former STS 101 members John Davis and Richard Broome by sea into the area near Pangkor Island. Their radio was unable to contact Force 136 HQ in Ceylon and the MPAJA contacts on Pangkor Island were betrayed to the Japanese.

The radio brought in by Gustavus was finally made to work in February 1945. Chapman was able to visit Force 136 HQ in Kandy and report. By this time, Force 136 had substantial resources, and in the few months before the end of the war, they were able to send 2,000 weapons to the MPAJA and no less than 300 liaison personnel. About half of these were British who had worked or lived in Malaya before the war, the others were Chinese who had made their own way to India or who had been taken there by Force 136 for training. With these resources, the MPAJA was built up to become a substantial guerilla army with about 7,000 fighters.^[2] However, Japan surrendered before it had a chance to stage a major uprising.

In isolation in jungle camps for several years, the MCP and MPAJA had purged themselves of many members suspected of treachery or espionage, which contributed to their post-war hard-line attitude and led in turn to the insurgency known as the Malayan Emergency.

Kuomintang[link]

The Kuomintang also had a widespread following in the Malaysian Chinese community in the days before the War, but were unable to mount any significant clandestine resistance to the Japanese. Partly, this was because they were based among the population in the towns, unlike the MCP which drew much of its support from mine or plantation workers in remote encampments or "squatters" on the edge of the forest. Most of the KMT's supporters and their dependents were therefore hostages to any Japanese mass reprisal.

When Lim Bo Seng and other agents from Force 136 attempted to make contact with Kuomintang networks in Ipoh as part of Operation Gustavus, they found that the KMT's underground actions there were tainted by corruption or private feuding.^[3]

Malayan resistance[link]

This unreferenced section requires citations to ensure verifiability.

The force also collaborated with many Chinese Malayan villages. As a multi religious and multi-racial country, the population of Malaya was also strongly divided along communal and religious lines, with some portion of the populace loyal to the Allied forces, while others loyal to Nazi Germany and Fascist Japan. Thus, agents risked the constant threat of being betrayed.

Even though the Malays (who are Muslims) and Indians were not badly treated by Japanese forces in the beginning of the occupation, later they too felt the hardship of life under the occupation and this was magnified by the brutal treatment of anyone who was suspected of being anti-Japanese (although hardly any atrocities were inflicted on them). Thus the SOE found a suitable backing among a few Malays and sent their officers to train local resistance forces famously known as Harimau Malaya Force 136 (Tigers of Malaya of Force 136). However, certain individuals in Malaya were strong supporters of the Japanese, and were actively involved in the notorious Kempeitai "mopping up" operations and other atrocities.

It was due to these ill-treatment that prompted the local populace's involvement in Force 136. The main base for this group was near Gerik, a district in the state of Perak. The force's main task was to form an intelligence-gathering network and, should prospects be favourable, to establish a resistance movement in northern Malaya. The force also arranged the reception of other parties of Force 136 who landed by parachute, providing them with guides and local contacts in the areas of their planned operations.

A novel^[which?] loosely based on the exploit of the resistance force was produced in late 1980s and there were several known figures in the book including Lt. Colonel Peter Dobree, a well-known commander of the force.

China[link]

From 1938, Britain had been supporting the Republic of China against the Japanese, by allowing supplies to reach the Chinese via the Burma Road running through Burma. SOE had various plans regarding China in the early days of the war. Forces were to be sent into China through Burma and a Bush Warfare School under Michael Calvert was established in Burma to train Chinese and Allied personnel in irregular warfare. These plans came to an end with the Japanese conquest of Burma in 1942.

Strictly speaking, SOE was not tasked to operate inside China after 1943, when it was left to the Americans. However, one group, the British Army Aid Group under an officer named Lindsay "Blue" Ride did operate near Hong Kong, in territory controlled by the Communist Party of China.

In Operation Remorse, a businessman named Walter Fletcher carried out covert economic operations such as trying to obtain smuggled rubber, currency speculation and so on, in Japanese-occupied China. As a result of these activities, SOE actually returned a financial profit of GBP 77 million in the Far East. Many of these funds and the networks used to acquire them were subsequently used in various relief and repatriation operations, but critics pointed pointed out that this created a pool of money that SOE could use beyond the oversight of any normal authority or accountability.

Thailand[link]

On 21 December 1940, a formal military alliance between Thailand under Field Marshal Plaek Pibulsonggram and Japan was concluded. At noon on 25 January 1942, Thailand declared war on the United States and Great Britain. Some Thais supported the alliance, arguing that it was in the national interest, or that it was better sense to ally oneself with a victorious power. Others formed the Free Thai Movement to resist. The Free Thai Movement was supported by Force 136 and the OSS, and provided valuable intelligence from within Thailand. Eventually, when the war turned against the Japanese, Phibun was forced to resign, and a Free Thai-controlled government was formed. A coup was being planned to disrupt the Japanese occupying forces in 1945, but was forestalled by the ending of the war.

Burma[link]

Burma was the theatre in which the major Allied effort was made in South East Asia from late 1942 onwards, and Force 136 was heavily involved. Initially, it had to compete with regular formations such as the Chindits and other irregular organizations for suitable personnel, aircraft and other resources. It eventually played a significant part in the liberation of the country by slowly building up a national organization which was used to great effect in 1945.

Two separate sections of SOE dealt with Burma. One concentrated on the minority communities who mainly inhabited the frontier regions; the other established links with the nationalist movements among the majority Bamar peoples in the central parts of the country and the major cities. It has been argued that this division of political effort, although necessary on military grounds, contributed to the inter-community conflicts which have continued in Burma (Myanmar) to the present day.

There were Indians and Afghans who were part of Force 136 and were heavily involved in Burmese operation, like C. L. Sharma, an Indian Professor of Linguistics at British Army Headquarters in India who later became an active member of Force 136 and spent almost 6 years mainly in various missions of the Force in Burma.^{[citation needed]}

Karens, Chins, Arakanese and Kachins[link]

The majority community of Burma were the Bamar. Among the minority peoples of Burma, including Chins, Karens and Kachins, there was a mixture of anti-Bamar, anti-Japanese and pro-British sentiments. In 1942, the pro-Japanese Burma Independence Army raised with Japanese assistance, attempted to disarm Karens in the Irrawaddy River delta region. This created a large-scale civil conflict which turned the Karens firmly against the Japanese.

The Karens were the largest of the minority communities. Although many lived in the Irrawaddy delta, their homeland can be considered to be the "Karenni", a mountainous and heavily forested tract along the border with Thailand. They had supplied many recruits to the Burma Rifles (part of the British forces in Burma during the early part of the war), and in the chaos of the British retreat into India, many of them had been given a rifle and ammunition and three months' pay, and were instructed to return to their home villages to await further orders. The presence of such trained soldiers contributed to the effectiveness of the Karen resistance.

A few British army officers had also been left behind in the Karreni, in a hasty attempt to organise a "stay-behind" organisation. In 1943, the Japanese made a ruthless punitive expedition into the Karenni, where they knew a British Officer was operating. To spare the population, a British liaison officer, Hugh Seagrim, voluntarily surrendered himself to the Japanese and was executed along with several of his Karen fighters.

However, Force 136 continued to supply the Karens, and from late 1944 they mounted Operation Character, which organised large-scale resistance in the Karenni. In April 1945, Force 136 stage-managed a major uprising in the region in support of the Allied offensive into Burma, which prevented the Japanese Fifteenth Army forestalling the Allied advance on Rangoon. After the capture of Rangoon, Karen resistance fighters continued to harass Japanese units and stragglers east of the Sittang River. It was estimated that at their moment of maximum effort, the Karens mustered 8,000 active guerrillas (some sources claim 12,000), plus many more sympathisers and auxiliaries.

SOE had some early missions to Kachin State, the territory inhabited by the Kachins of northern Burma, but for much of the war, this area was the responsibility of the American-controlled China-Burma-India Theater, and the Kachin guerrillas were armed and coordinated by the American liaison organisation, OSS Detachment 101.

The various ethnic groups (Chins, Lushai, Arakanese) who inhabited the border areas between Burma and India were not the responsibility of Force 136 but of V Force, an irregular force which was under direct control of the Army. From 1942 to 1944, hill peoples in the frontier regions fought on both sides; some under V Force and other Allied irregular forces HQ, others under local or Japanese-sponsored organisations such as the Chin Defence Force and Arakan Defence Force.

Burmese political links[link]

The Burma section of Force 136 was commanded by John Ritchie Gardiner, who had managed a forestry company before the war and also served on the Municipal Council of Rangoon. He had known personally some Burmese politicians such as Ba Maw who had later formed a government which, although nominally independent, collaborated through necessity with the Japanese occupiers.

In 1942, when the Japanese invaded Burma, the majority Bamar (Burman) people had been sympathetic to them, or at least hostile to the British colonial government and the Indian community which had immigrated or had been imported as workers for newly created industries. Bamar volunteers flocked to the Burma Independence Army which fought several actions against British forces. During the years of occupation, this attitude changed. The Burma Independence Army was reorganised as the Burma National Army (BNA), under Japanese control. In 1944, Aung San, the Burmese nationalist who had founded the BIA with Japanese assistance and had been appointed Minister of Defence in Ba Maw's government and commander of the Burma National Army, contacted Burmese communist and socialist leaders, some of whom were already leading insurgencies against the Japanese. Together they formed the Anti-Fascist Organisation (AFO) under the overall leadership of Thakin Soe. Force 136 was able to establish contact with this organisation through links with Burmese communist groups.

During the final Allied offensive into Burma in 1945, there were then a series of uprisings in Burma against the Japanese, which Force 136 supported although it had little control or even influence over the rebellious BNA and its supporters. The first rebellion involved a locally recruited force known as the Arakan Defence Army turning on the Japanese in Arakan. The second involved an uprising by BNA units near Toungoo in Central Burma, beginning on 8 March 1945. The final uprising occurred when the entire BNA changed sides on 27 March.

The forces of the AFO, including the BNA, were renamed the Patriotic Burmese Forces. They played a part in the final campaign to recapture Rangoon, and eliminate Japanese resistance in Central Burma. The BNA's armed strength at the time of their defection was around 11,000. The Patriotic Burmese Forces also included large numbers of communists and other irregulars with loyalty to particular groups, and those Karens who had served in the BNA and Karen resistance groups in the Irrawaddy Delta.

In arranging the acceptance of Aung San and his forces as Allied combatants, Force 136 was in direct conflict with the more staid Civil Affairs Service Officers at South East Asia Command's headquarters, who feared the postwar implications of handing out large numbers of weapons to irregular and potentially anti-British forces, and of promoting the political careers of Aung San or the communist leaders. The AFO at the time of the uprising represented itself as the provisional government of Burma. It was eventually persuaded to drop this claim after negotiations with South East Asia Command, in return for recognition as a political movement (the AFPFL).

Indian National Army[link]

Another force operating under Japanese command in Burma was the Indian National Army, a force composed of former prisoners of war captured by the Japanese at Singapore and some Tamils living in Malaya. However, Force 136 was prevented from working with anyone in the Indian National Army, regardless of their intentions. The policy towards the INA was formed and administered by India Command, a British rather than Allied headquarters.

Field Operations[link]

Force 136 was also active in more conventional military-style operations behind Japanese lines in Burma. Such an operation could comprise a group of up to 40 infantry with officers and a Radio Operator, infiltrating Japanese lines on intelligence and discretionary search and destroy missions. Such missions, which could last several weeks (supplied by C47 transport aircraft) kept close wireless contact with operational bases in India, using high-grade ciphers (changed daily) and hermetically sealed wireless/morse sets.

Every day (Japanese permitting) at pre–arranged times, the Radio Operator (with escorts) climbed to a high vantage point, usually necessitating a gruelling climb to the top of some slippery, high, jungle-clad ridge, and sent the latest intelligence information and the group's supply requests etc., and received further orders in return. The Radio Operator was central to a mission’s success and his capture or death would spell disaster for the mission. To avoid capture and use under duress by the Japanese, every SOE operative was issued a cyanide pill.

One such Radio Operator was James Gow (originally from the Royal Corps of Signals), who recounted his first mission in his book "From Rhunahaorine to Rangoon." In the summer of 1944, the Japanese push toward India had been stopped at the Battle of Kohima. In the aftermath of the battle, Japanese forces split up and retreated deep into the jungle. As part of the initiative to find out if they were reforming for a further push, he was sent from Dimapur with a 40-strong group of Gurkhas, to locate groups of Japanese forces, identify their strengths and their organised status.

Discretionary attacks on isolated Japanese groups were permitted (no prisoners to be taken), as was destruction of supply dumps. One particular Gurkha officer under whom James Gow operated was Major William Lindon-Travers, later to become Bill Travers, the well-known actor of Born Free fame.

Other[link]

SOE's French Indo-China Section (1943–1945)[link]

Force 136 played only a minor part in attempts to organise local resistance in French Indochina, led mainly by Roger Blaizot, commander of the French Far East Expeditionary Corps (FEFEO) and General Eugène Mordant, chief of the military resistance. From 1944 to 1945 long-range B-24 Liberator bomber aircraft attached to Force 136 dropped 40 "Jedburgh" commandos from the French intelligence service BCRA, and agents from the Corps Léger d'Intervention also known as "Gaur", commanded by Lieutenant-Colonel Paul Huard, into Indochina. However Indochina was not originally part of the South-East Asian theatre, and therefore not SOE's responsibility. Notable French Force 136 members dropped in Laos in 1945 include: Jean Deuve (22 January), Jean Le Morillon (28 February), Jean Sassi (4 June)^[4], Bob Maloubier (August)^[4]

There were also American reservations over restoring the French colonial regime after the war, which led the Americans eventually to support the anti-French Viet Minh.^[5] Together with the complexities of the relationships between the Vichy-leaning officials in Indochina, and the rival Giraudist and de Gaullist resistance movements, this made liaison very difficult. SOE had few links with the indigenous Viet Minh movement.

This section requires expansion.

Dutch East Indies & Australia[link]

Except for the island of Sumatra, the Dutch East Indies were also outside South East Asia Command's area of responsibility until after the Japanese surrender. In 1943, an invasion of Sumatra, codenamed Operation Culverin, was tentatively planned. SOE mounted some reconnaissances of northern Sumatra (in the present-day province of Aceh). In the event, the plan was cancelled, and nothing came of SOE's small-scale efforts in Sumatra.

Another combined Allied intelligence organisation, Special Operations Australia (SOA), which had the British codename Force 137, operated out of Australia against Japanese targets in Singapore, the other islands of the Dutch East Indies, and Borneo. It included Z Special Unit, which carried out a successful attack on shipping in Singapore Harbour, known as Operation Jaywick.

Communications[link]

Until mid-1944, Force 136's operations were hampered by the great distances involved; for example, from Ceylon to Malaya and back required a flight of 2,800 miles (4,500 km). Such distances also made it difficult to use small clandestine craft to deliver supplies or personnel by sea (although such craft were used to supply the MPAJA in Perak late in the war). The Royal Navy made few submarines available to Force 136. Eventually, converted B-24 Liberator aircraft were made available to parachute agents and stores.

In Burma, where the distances involved were not so great, C-47 transport aircraft could be used. Westland Lysander liaison aircraft could also be used over shorter distances.

Notes[link]

Sources[link]

• Aldrich, Richard James (2000). Intelligence and the war against Japan: Britain, America and the politics of secret service. Cambridge University Press. ISBN 978-0-521-64186-9. 
• Allen, Louis. Burma: the longest War. J.M. Dent and sons. ISBN 0-460-02474-4. 
• Bayly, Christopher; Tim Harper. Forgotten Armies. Penguin. ISBN 0-14-029331-0. 
• Chapman, Freddie Spencer. The Jungle is neutral. Lyon Press. ISBN 1-59228-107-9. 
• Christie, Arthur. Mission Scapula: Special Operations Executive in the far east. ISBN 0-9547010-0-3. 
• Cruikshank, Charles (1983). SOE in the Far East. Oxford University Press. ISBN 0-19-215873. 
• Dear, Ian. Sabotage and Subversion: SOE and OSS at War. Cassell. ISBN 0-304-35202-0. 
• Foot, M. R. D. (1984). SOE. BBC Publications. ISBN 0-563-20193-2. 
• Hedley, John. Jungle Fighter. Tom Donovan Publishing Ltd. ISBN 1-871085-34-9. 
• Latimer, Jon (2004). Burma: The Forgotten War. John Murray. ISBN 978-0-7195-6576-2. 
• Smiley, David (1994). Irregular Regular. Norwich: Michael Russell. ISBN 0-85955-202-0. 
• Tan Chong Tee, Force 136, Story of a WWII resistance fighter, Asiapac Publications, Singapore, 1995, ISBN 981-3029-90-0

External links[link]

• Far East entry
• Burma Star entry
• Mission Scapula entry

Forces are also described as a push or pull on an object. They can be due to phenomena such as gravity, magnetism, or anything that might cause a mass to accelerate.

In physics, a force is any influence that causes an object to undergo a certain change, either concerning its movement, direction, or geometrical construction. In other words, a force is that which can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. Force can also be described by intuitive concepts such as a push or pull. A force has both magnitude and direction, making it a vector quantity. Newton's second law, F = ma, was originally formulated in slightly different, but equivalent terms: the original version states that the net force acting upon an object is equal to the rate at which its momentum changes.^[1]

Related concepts to force include: thrust, which increases the velocity of an object; drag, which decreases the velocity of an object; and torque which produces changes in rotational speed of an object. Forces which do not act uniformly on all parts of a body will also cause mechanical stresses,^[2] a technical term for influences which cause deformation of matter. While mechanical stress can remain embedded in a solid object, gradually deforming it, mechanical stress in a fluid determines changes in its pressure and volume.^[3][4]

Development of the concept[link]

Philosophers in antiquity used the concept of force in the study of stationary and moving objects and simple machines, but thinkers such as Aristotle and Archimedes retained fundamental errors in understanding force. In part this was due to an incomplete understanding of the sometimes non-obvious force of friction, and a consequently inadequate view of the nature of natural motion.^[5] A fundamental error was the belief that a force is required to maintain motion, even at a constant velocity. Most of the previous misunderstandings about motion and force were eventually corrected by Sir Isaac Newton; with his mathematical insight, he formulated laws of motion that were not improved-on for nearly three hundred years.^[4] By the early 20th century, Einstein developed a theory of relativity that correctly predicted the action of forces on objects with increasing momenta near the speed of light, and also provided insight into the forces produced by gravitation and inertia.

With modern insights into quantum mechanics and technology that can accelerate particles close to the speed of light, particle physics has devised a Standard Model to describe forces between particles smaller than atoms. The Standard Model predicts that exchanged particles called gauge bosons are the fundamental means by which forces are emitted and absorbed. Only four main interactions are known: in order of decreasing strength, they are: strong, electromagnetic, weak, and gravitational.^[3] High-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction.^[6]

Pre-Newtonian concepts[link]

Aristotle famously described a force as anything which causes an object to undergo "unnatural motion"

Since antiquity the concept of force has been recognized as integral to the functioning of each of the simple machines. The mechanical advantage given by a simple machine allowed for less force to be used in exchange for that force acting over a greater distance for the same amount of work. Analysis of the characteristics of forces ultimately culminated in the work of Archimedes who was especially famous for formulating a treatment of buoyant forces inherent in fluids.^[5]

Aristotle provided a philosophical discussion of the concept of a force as an integral part of Aristotelian cosmology. In Aristotle's view, the natural world held four elements that existed in "natural states". Aristotle believed that it was the natural state of objects with mass on Earth, such as the elements water and earth, to be motionless on the ground and that they tended towards that state if left alone. He distinguished between the innate tendency of objects to find their "natural place" (e.g., for heavy bodies to fall), which led to "natural motion", and unnatural or forced motion, which required continued application of a force.^[7] This theory, based on the everyday experience of how objects move, such as the constant application of a force needed to keep a cart moving, had conceptual trouble accounting for the behavior of projectiles, such as the flight of arrows. The place where forces were applied to projectiles was only at the start of the flight, and while the projectile sailed through the air, no discernible force acts on it. Aristotle was aware of this problem and proposed that the air displaced through the projectile's path provided the needed force to continue the projectile moving. This explanation demands that air is needed for projectiles and that, for example, in a vacuum, no projectile would move after the initial push. Additional problems with the explanation include the fact that air resists the motion of the projectiles.^[8]

Aristotelian physics began facing criticism in Medieval science, first by John Philoponus in the 6th century.

The shortcomings of Aristotelian physics would not be fully corrected until the 17th century work of Galileo Galilei, who was influenced by the late Medieval idea that objects in forced motion carried an innate force of impetus. Galileo constructed an experiment in which stones and cannonballs were both rolled down an incline to disprove the Aristotelian theory of motion early in the 17th century. He showed that the bodies were accelerated by gravity to an extent which was independent of their mass and argued that objects retain their velocity unless acted on by a force, for example friction.^[9]

Newtonian mechanics[link]

Sir Isaac Newton sought to describe the motion of all objects using the concepts of inertia and force, and in doing so he found that they obey certain conservation laws. In 1687 Newton went on to publish his thesis Philosophiae Naturalis Principia Mathematica.^[4][10] In this work Newton set out three laws of motion that to this day are the way forces are described in physics.^[10]

[edit] Newton's first law

Newton's First Law of Motion states that objects continue to move in a state of constant velocity unless acted upon by an external net force or resultant force.^[10] This law is an extension of Galileo's insight that constant velocity was associated with a lack of net force (see a more detailed description of this below). Newton proposed that every object with mass has an innate inertia that functions as the fundamental equilibrium "natural state" in place of the Aristotelian idea of the "natural state of rest". That is, the first law contradicts the intuitive Aristotelian belief that a net force is required to keep an object moving with constant velocity. By making rest physically indistinguishable from non-zero constant velocity, Newton's First Law directly connects inertia with the concept of relative velocities. Specifically, in systems where objects are moving with different velocities, it is impossible to determine which object is "in motion" and which object is "at rest". In other words, to phrase matters more technically, the laws of physics are the same in every inertial frame of reference, that is, in all frames related by a Galilean transformation.

For instance, while traveling in a moving vehicle at a constant velocity, the laws of physics do not change from being at rest. A person can throw a ball straight up in the air and catch it as it falls down without worrying about applying a force in the direction the vehicle is moving. This is true even though another person who is observing the moving vehicle pass by also observes the ball follow a curving parabolic path in the same direction as the motion of the vehicle. It is the inertia of the ball associated with its constant velocity in the direction of the vehicle's motion that ensures the ball continues to move forward even as it is thrown up and falls back down. From the perspective of the person in the car, the vehicle and everything inside of it is at rest: It is the outside world that is moving with a constant speed in the opposite direction. Since there is no experiment that can distinguish whether it is the vehicle that is at rest or the outside world that is at rest, the two situations are considered to be physically indistinguishable. Inertia therefore applies equally well to constant velocity motion as it does to rest.

The concept of inertia can be further generalized to explain the tendency of objects to continue in many different forms of constant motion, even those that are not strictly constant velocity. The rotational inertia of planet Earth is what fixes the constancy of the length of a day and the length of a year. Albert Einstein extended the principle of inertia further when he explained that reference frames subject to constant acceleration, such as those free-falling toward a gravitating object, were physically equivalent to inertial reference frames. This is why, for example, astronauts experience weightlessness when in free-fall orbit around the Earth, and why Newton's Laws of Motion are more easily discernible in such environments. If an astronaut places an object with mass in mid-air next to himself, it will remain stationary with respect to the astronaut due to its inertia. This is the same thing that would occur if the astronaut and the object were in intergalactic space with no net force of gravity acting on their shared reference frame. This principle of equivalence was one of the foundational underpinnings for the development of the general theory of relativity.^[11]

Though Sir Isaac Newton's most famous equation is
Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle{\vec{F}=m\vec{a}} , he actually wrote down a different form for his second law of motion that did not use differential calculus.

[edit] Newton's second law

A modern statement of Newton's Second Law is a vector differential equation:^[12]

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t},

where Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{p}

is the momentum of the system, and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{F}
is the net (vector sum) force. In equilibrium, there is zero net force by definition, but (balanced) forces may be present nevertheless. In contrast, the second law states an unbalanced force acting on an object will result in the object's momentum changing over time.[10]

By the definition of momentum,

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t} = \frac{\mathrm{d}\left(m\vec{v}\right)}{\mathrm{d}t},

where m is the mass and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{v}

is the velocity. 

In a system of constant mass, the use of the constant factor rule in differentiation allows the mass to move outside the derivative operator, and the equation becomes

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = m\frac{\mathrm{d}\vec{v}}{\mathrm{d}t}.

By substituting the definition of acceleration, the algebraic version of Newton's Second Law is derived:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} =m\vec{a}.

It is sometimes called the "second most famous formula in physics".^[13] Newton never explicitly stated the formula in the reduced form above.

Newton's Second Law asserts the direct proportionality of acceleration to force and the inverse proportionality of acceleration to mass. Accelerations can be defined through kinematic measurements. However, while kinematics are well-described through reference frame analysis in advanced physics, there are still deep questions that remain as to what is the proper definition of mass. General relativity offers an equivalence between space-time and mass, but lacking a coherent theory of quantum gravity, it is unclear as to how or whether this connection is relevant on microscales. With some justification, Newton's second law can be taken as a quantitative definition of mass by writing the law as an equality; the relative units of force and mass then are fixed.

The use of Newton's Second Law as a definition of force has been disparaged in some of the more rigorous textbooks,^[3][14] because it is essentially a mathematical truism. Notable physicists, philosophers and mathematicians who have sought a more explicit definition of the concept of force include Ernst Mach, Clifford Truesdell and Walter Noll.^[15]

Newton's Second Law can be used to measure the strength of forces. For instance, knowledge of the masses of planets along with the accelerations of their orbits allows scientists to calculate the gravitational forces on planets.

[edit] Newton's third law

Newton's Third Law is a result of applying symmetry to situations where forces can be attributed to the presence of different objects. The third law means that all forces are interactions between different bodies,^[16][17] and thus that there is no such thing as a unidirectional force or a force that acts on only one body. Whenever a first body exerts a force F on a second body, the second body exerts a force −F on the first body. F and −F are equal in magnitude and opposite in direction. This law is sometimes referred to as the action-reaction law, with F called the "action" and −F the "reaction". The action and the reaction are simultaneous:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}_{1,2}=-\vec{F}_{2,1}.

If object 1 and object 2 are considered to be in the same system, then the net force on the system due to the interactions between objects 1 and 2 is zero since

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}_{1,2}+\vec{F}_{\mathrm{2,1}}=0

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}_{net}=0.

This means that in a closed system of particles, there are no internal forces that are unbalanced. That is, the action-reaction force shared between any two objects in a closed system will not cause the center of mass of the system to accelerate. The constituent objects only accelerate with respect to each other, the system itself remains unaccelerated. Alternatively, if an external force acts on the system, then the center of mass will experience an acceleration proportional to the magnitude of the external force divided by the mass of the system.^[3]

Combining Newton's Second and Third Laws, it is possible to show that the linear momentum of a system is conserved. Using

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}_{1,2} = \frac{\mathrm{d}\vec{p}_{1,2}}{\mathrm{d}t} = -\vec{F}_{2,1} = -\frac{\mathrm{d}\vec{p}_{2,1}}{\mathrm{d}t}

and integrating with respect to time, the equation:

Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta{\vec{p}_{1,2}} = - \Delta{\vec{p}_{2,1}}

is obtained. For a system which includes objects 1 and 2,

Failed to parse (Missing texvc executable; please see math/README to configure.): \sum{\Delta{\vec{p}}}=\Delta{\vec{p}_{1,2}} + \Delta{\vec{p}_{2,1}} = 0

which is the conservation of linear momentum.^[18] Using the similar arguments, it is possible to generalize this to a system of an arbitrary number of particles. This shows that exchanging momentum between constituent objects will not affect the net momentum of a system. In general, as long as all forces are due to the interaction of objects with mass, it is possible to define a system such that net momentum is never lost nor gained.^[3]

Descriptions[link]

Free-body diagrams of an object on a flat surface and an inclined plane. Forces are resolved and added together to determine their magnitudes and the net force.

Since forces are perceived as pushes or pulls, this can provide an intuitive understanding for describing forces.^[4] As with other physical concepts (e.g. temperature), the intuitive understanding of forces is quantified using precise operational definitions that are consistent with direct observations and compared to a standard measurement scale. Through experimentation, it is determined that laboratory measurements of forces are fully consistent with the conceptual definition of force offered by Newtonian mechanics.

Forces act in a particular direction and have sizes dependent upon how strong the push or pull is. Because of these characteristics, forces are classified as "vector quantities". This means that forces follow a different set of mathematical rules than physical quantities that do not have direction (denoted scalar quantities). For example, when determining what happens when two forces act on the same object, it is necessary to know both the magnitude and the direction of both forces to calculate the result. If both of these pieces of information are not known for each force, the situation is ambiguous. For example, if you know that two people are pulling on the same rope with known magnitudes of force but you do not know which direction either person is pulling, it is impossible to determine what the acceleration of the rope will be. The two people could be pulling against each other as in tug of war or the two people could be pulling in the same direction. In this simple one-dimensional example, without knowing the direction of the forces it is impossible to decide whether the net force is the result of adding the two force magnitudes or subtracting one from the other. Associating forces with vectors avoids such problems.

Historically, forces were first quantitatively investigated in conditions of static equilibrium where several forces canceled each other out. Such experiments demonstrate the crucial properties that forces are additive vector quantities: they have magnitude and direction.^[4] When two forces act on a point particle, the resulting force, the resultant (also called the net force), can be determined by following the parallelogram rule of vector addition: the addition of two vectors represented by sides of a parallelogram, gives an equivalent resultant vector which is equal in magnitude and direction to the transversal of the parallelogram.^[3] The magnitude of the resultant varies from the difference of the magnitudes of the two forces to their sum, depending on the angle between their lines of action. However, if the forces are acting on an extended body, their respective lines of application must also be specified in order to account for their effects on the motion of the body.

Free-body diagrams can be used as a convenient way to keep track of forces acting on a system. Ideally, these diagrams are drawn with the angles and relative magnitudes of the force vectors preserved so that graphical vector addition can be done to determine the net force.^[19]

As well as being added, forces can also be resolved into independent components at right angles to each other. A horizontal force pointing northeast can therefore be split into two forces, one pointing north, and one pointing east. Summing these component forces using vector addition yields the original force. Resolving force vectors into components of a set of basis vectors is often a more mathematically clean way to describe forces than using magnitudes and directions.^[20] This is because, for orthogonal components, the components of the vector sum are uniquely determined by the scalar addition of the components of the individual vectors. Orthogonal components are independent of each other because forces acting at ninety degrees to each other have no effect on the magnitude or direction of the other. Choosing a set of orthogonal basis vectors is often done by considering what set of basis vectors will make the mathematics most convenient. Choosing a basis vector that is in the same direction as one of the forces is desirable, since that force would then have only one non-zero component. Orthogonal force vectors can be three-dimensional with the third component being at right-angles to the other two.^[3]

Equilibrium[link]

Equilibrium occurs when the resultant force acting on a point particle is zero (that is, the vector sum of all forces is zero). When dealing with an extended body, it is also necessary that the net torque in it is 0.

There are two kinds of equilibrium: static equilibrium and dynamic equilibrium.

Static equilibrium[link]

Static equilibrium was understood well before the invention of classical mechanics. Objects which are at rest have zero net force acting on them.^[21]

The simplest case of static equilibrium occurs when two forces are equal in magnitude but opposite in direction. For example, an object on a level surface is pulled (attracted) downward toward the center of the Earth by the force of gravity. At the same time, surface forces resist the downward force with equal upward force (called the normal force). The situation is one of zero net force and no acceleration.^[4]

Pushing against an object on a frictional surface can result in a situation where the object does not move because the applied force is opposed by static friction, generated between the object and the table surface. For a situation with no movement, the static friction force exactly balances the applied force resulting in no acceleration. The static friction increases or decreases in response to the applied force up to an upper limit determined by the characteristics of the contact between the surface and the object.^[4]

A static equilibrium between two forces is the most usual way of measuring forces, using simple devices such as weighing scales and spring balances. For example, an object suspended on a vertical spring scale experiences the force of gravity acting on the object balanced by a force applied by the "spring reaction force" which equals the object's weight. Using such tools, some quantitative force laws were discovered: that the force of gravity is proportional to volume for objects of constant density (widely exploited for millennia to define standard weights); Archimedes' principle for buoyancy; Archimedes' analysis of the lever; Boyle's law for gas pressure; and Hooke's law for springs. These were all formulated and experimentally verified before Isaac Newton expounded his Three Laws of Motion.^[3][4]

[edit] Dynamic equilibrium

Galileo Galilei was the first to point out the inherent contradictions contained in Aristotle's description of forces.

Dynamic equilibrium was first described by Galileo who noticed that certain assumptions of Aristotelian physics were contradicted by observations and logic. Galileo realized that simple velocity addition demands that the concept of an "absolute rest frame" did not exist. Galileo concluded that motion in a constant velocity was completely equivalent to rest. This was contrary to Aristotle's notion of a "natural state" of rest that objects with mass naturally approached. Simple experiments showed that Galileo's understanding of the equivalence of constant velocity and rest were correct. For example, if a mariner dropped a cannonball from the crow's nest of a ship moving at a constant velocity, Aristotelian physics would have the cannonball fall straight down while the ship moved beneath it. Thus, in an Aristotelian universe, the falling cannonball would land behind the foot of the mast of a moving ship. However, when this experiment is actually conducted, the cannonball always falls at the foot of the mast, as if the cannonball knows to travel with the ship despite being separated from it. Since there is no forward horizontal force being applied on the cannonball as it falls, the only conclusion left is that the cannonball continues to move with the same velocity as the boat as it falls. Thus, no force is required to keep the cannonball moving at the constant forward velocity.^[9]

Moreover, any object traveling at a constant velocity must be subject to zero net force (resultant force). This is the definition of dynamic equilibrium: when all the forces on an object balance but it still moves at a constant velocity.

A simple case of dynamic equilibrium occurs in constant velocity motion across a surface with kinetic friction. In such a situation, a force is applied in the direction of motion while the kinetic friction force exactly opposes the applied force. This results in zero net force, but since the object started with a non-zero velocity, it continues to move with a non-zero velocity. Aristotle misinterpreted this motion as being caused by the applied force. However, when kinetic friction is taken into consideration it is clear that there is no net force causing constant velocity motion.^[3]

Special relativity[link]

In the special theory of relativity, mass and energy are equivalent (as can be seen by calculating the work required to accelerate an object). When an object's velocity increases, so does its energy and hence its mass equivalent (inertia). It thus requires more force to accelerate it the same amount than it did at a lower velocity. Newton's Second Law

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = \mathrm{d}\vec{p}/\mathrm{d}t

remains valid because it is a mathematical definition.^[22] But in order to be conserved, relativistic momentum must be redefined as:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{p} = \frac{m_0\vec{v}}{\sqrt{1 - v^2/c^2}}

where

Failed to parse (Missing texvc executable; please see math/README to configure.): v

is the velocity and

Failed to parse (Missing texvc executable; please see math/README to configure.): c

is the speed of light

Failed to parse (Missing texvc executable; please see math/README to configure.): m_0

is the rest mass.

The relativistic expression relating force and acceleration for a particle with constant non-zero rest mass Failed to parse (Missing texvc executable; please see math/README to configure.): m

moving in the Failed to parse (Missing texvc executable; please see math/README to configure.): x
direction is:

Failed to parse (Missing texvc executable; please see math/README to configure.): F_x = \gamma^3 m a_x \,

Failed to parse (Missing texvc executable; please see math/README to configure.): F_y = \gamma m a_y \,

Failed to parse (Missing texvc executable; please see math/README to configure.): F_z = \gamma m a_z \,

where the Lorentz factor

Failed to parse (Missing texvc executable; please see math/README to configure.): \gamma = \frac{1}{\sqrt{1 - v^2/c^2}}.

^[23]

In the early history of relativity, the expressions Failed to parse (Missing texvc executable; please see math/README to configure.): \gamma^3 m

and Failed to parse (Missing texvc executable; please see math/README to configure.): \gamma m
were called longitudinal and transverse mass. Relativistic force does not produce a constant acceleration, but an ever decreasing acceleration as the object approaches the speed of light. Note that Failed to parse (Missing texvc executable; please see math/README to configure.):  \gamma
is undefined for an object with a non-zero rest mass at the speed of light, and the theory yields no prediction at that speed.

One can, however, restore the form of

Failed to parse (Missing texvc executable; please see math/README to configure.): F^\mu = mA^\mu \,

for use in relativity through the use of four-vectors. This relation is correct in relativity when Failed to parse (Missing texvc executable; please see math/README to configure.): F^\mu

is the four-force, Failed to parse (Missing texvc executable; please see math/README to configure.): m
is the invariant mass, and Failed to parse (Missing texvc executable; please see math/README to configure.): A^\mu
is the four-acceleration.[24]

Feynman diagrams[link]

A Feynman diagram for the decay of a neutron into a proton. The W boson is between two vertices indicating a repulsion.

In modern particle physics, forces and the acceleration of particles are explained as a mathematical by-product of exchange of momentum-carrying gauge bosons. With the development of quantum field theory and general relativity, it was realized that force is a redundant concept arising from conservation of momentum (4-momentum in relativity and momentum of virtual particles in quantum electrodynamics). The conservation of momentum, can be directly derived from homogeneity (=shift symmetry) of space and so is usually considered more fundamental than the concept of a force. Thus the currently known fundamental forces are considered more accurately to be "fundamental interactions".^[6] When particle A emits (creates) or absorbs (annihilates) virtual particle B, a momentum conservation results in recoil of particle A making impression of repulsion or attraction between particles A A' exchanging by B. This description applies to all forces arising from fundamental interactions. While sophisticated mathematical descriptions are needed to predict, in full detail, the accurate result of such interactions, there is a conceptually simple way to describe such interactions through the use of Feynman diagrams. In a Feynman diagram, each matter particle is represented as a straight line (see world line) traveling through time which normally increases up or to the right in the diagram. Matter and anti-matter particles are identical except for their direction of propagation through the Feynman diagram. World lines of particles intersect at interaction vertices, and the Feynman diagram represents any force arising from an interaction as occurring at the vertex with an associated instantaneous change in the direction of the particle world lines. Gauge bosons are emitted away from the vertex as wavy lines (similar to waves) and, in the case of virtual particle exchange, are absorbed at an adjacent vertex.^[25]

The utility of Feynman diagrams is that other types of physical phenomena that are part of the general picture of fundamental interactions but are conceptually separate from forces can also be described using the same rules. For example, a Feynman diagram can describe in succinct detail how a neutron decays into an electron, proton, and neutrino, an interaction mediated by the same gauge boson that is responsible for the weak nuclear force.^[25]

Fundamental models[link]

All the forces in the universe are based on four fundamental interactions. The strong and weak forces act only at very short distances, and are responsible for the interactions between subatomic particles including nucleons and compound nuclei. The electromagnetic force acts between electric charges and the gravitational force acts between masses. All other forces are based on the existence of the four fundamental interactions. For example, friction is a manifestation of the electromagnetic force acting between the atoms of two surfaces, and the Pauli Exclusion Principle,^[26] which does not allow atoms to pass through each other. The forces in springs, modeled by Hooke's law, are also the result of electromagnetic forces and the Exclusion Principle acting together to return the object to its equilibrium position. Centrifugal forces are acceleration forces which arise simply from the acceleration of rotating frames of reference.^[3]

The development of fundamental theories for forces proceeded along the lines of unification of disparate ideas. For example, Isaac Newton unified the force responsible for objects falling at the surface of the Earth with the force responsible for the orbits of celestial mechanics in his universal theory of gravitation. Michael Faraday and James Clerk Maxwell demonstrated that electric and magnetic forces were unified through one consistent theory of electromagnetism. In the 20th century, the development of quantum mechanics led to a modern understanding that the first three fundamental forces (all except gravity) are manifestations of matter (fermions) interacting by exchanging virtual particles called gauge bosons.^[27] This standard model of particle physics posits a similarity between the forces and led scientists to predict the unification of the weak and electromagnetic forces in electroweak theory subsequently confirmed by observation. The complete formulation of the standard model predicts an as yet unobserved Higgs mechanism, but observations such as neutrino oscillations indicate that the standard model is incomplete. A grand unified theory allowing for the combination of the electroweak interaction with the strong force is held out as a possibility with candidate theories such as supersymmetry proposed to accommodate some of the outstanding unsolved problems in physics. Physicists are still attempting to develop self-consistent unification models that would combine all four fundamental interactions into a theory of everything. Einstein tried and failed at this endeavor, but currently the most popular approach to answering this question is string theory.^[6]

Gravity[link]

An initially stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. An image was taken 20 flashes per second. During the first 1/20th of a second the ball drops one unit of distance (here, a unit is about 12 mm); by 2/20ths it has dropped a total of 4 units; by 3/20ths, 9 units and so on.

What we now call gravity was not identified as a universal force until the work of Isaac Newton. Before Newton, the tendency for objects to fall towards the Earth was not understood to be related to the motions of celestial objects. Galileo was instrumental in describing the characteristics of falling objects by determining that the acceleration of every object in free-fall was constant and independent of the mass of the object. Today, this acceleration due to gravity towards the surface of the Earth is usually designated as Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{g}

and has a magnitude of about 9.81 meters per second squared (this measurement is taken from sea level and may vary depending on location), and points toward the center of the Earth.[28] This observation means that the force of gravity on an object at the Earth's surface is directly proportional to the object's mass. Thus an object that has a mass of Failed to parse (Missing texvc executable; please see math/README to configure.): m
will experience a force:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = m\vec{g}

In free-fall, this force is unopposed and therefore the net force on the object is its weight. For objects not in free-fall, the force of gravity is opposed by the reactions of their supports. For example, a person standing on the ground experiences zero net force, since his weight is balanced by a normal force exerted by the ground.^[3]

Newton's contribution to gravitational theory was to unify the motions of heavenly bodies, which Aristotle had assumed were in a natural state of constant motion, with falling motion observed on the Earth. He proposed a law of gravity that could account for the celestial motions that had been described earlier using Kepler's Laws of Planetary Motion.^[29]

Newton came to realize that the effects of gravity might be observed in different ways at larger distances. In particular, Newton determined that the acceleration of the Moon around the Earth could be ascribed to the same force of gravity if the acceleration due to gravity decreased as an inverse square law. Further, Newton realized that the acceleration due to gravity is proportional to the mass of the attracting body.^[29] Combining these ideas gives a formula that relates the mass (Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle M_\oplus ) and the radius (Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle R_\oplus ) of the Earth to the gravitational acceleration:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{g}=-\frac{GM_\oplus}{{R_\oplus}^2} \hat{r}

where the vector direction is given by Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \hat{r} , the unit vector directed outward from the center of the Earth.^[10]

In this equation, a dimensional constant Failed to parse (Missing texvc executable; please see math/README to configure.): G

is used to describe the relative strength of gravity. This constant has come to be known as Newton's Universal Gravitation Constant,[30] though its value was unknown in Newton's lifetime. Not until 1798 was Henry Cavendish able to make the first measurement of Failed to parse (Missing texvc executable; please see math/README to configure.): G
using a torsion balance; this was widely reported in the press as a measurement of the mass of the Earth since knowing Failed to parse (Missing texvc executable; please see math/README to configure.): G
could allow one to solve for the Earth's mass given the above equation. Newton, however, realized that since all celestial bodies followed the same laws of motion, his law of gravity had to be universal. Succinctly stated, Newton's Law of Gravitation states that the force on a spherical object of mass Failed to parse (Missing texvc executable; please see math/README to configure.): m_1
due to the gravitational pull of mass Failed to parse (Missing texvc executable; please see math/README to configure.): m_2
is

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}=-\frac{Gm_{1}m_{2}}{r^2} \hat{r}

where Failed to parse (Missing texvc executable; please see math/README to configure.): r

is the distance between the two objects' centers of mass and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \hat{r}
is the unit vector pointed in the direction away from the center of the first object toward the center of the second object.[10]

This formula was powerful enough to stand as the basis for all subsequent descriptions of motion within the solar system until the 20th century. During that time, sophisticated methods of perturbation analysis^[31] were invented to calculate the deviations of orbits due to the influence of multiple bodies on a planet, moon, comet, or asteroid. The formalism was exact enough to allow mathematicians to predict the existence of the planet Neptune before it was observed.^[32]

It was only the orbit of the planet Mercury that Newton's Law of Gravitation seemed not to fully explain. Some astrophysicists predicted the existence of another planet (Vulcan) that would explain the discrepancies; however, despite some early indications, no such planet could be found. When Albert Einstein finally formulated his theory of general relativity (GR) he turned his attention to the problem of Mercury's orbit and found that his theory added a correction which could account for the discrepancy. This was the first time that Newton's Theory of Gravity had been shown to be less correct than an alternative.^[33]

Since then, and so far, general relativity has been acknowledged as the theory which best explains gravity. In GR, gravitation is not viewed as a force, but rather, objects moving freely in gravitational fields travel under their own inertia in straight lines through curved space-time – defined as the shortest space-time path between two space-time events. From the perspective of the object, all motion occurs as if there were no gravitation whatsoever. It is only when observing the motion in a global sense that the curvature of space-time can be observed and the force is inferred from the object's curved path. Thus, the straight line path in space-time is seen as a curved line in space, and it is called the ballistic trajectory of the object. For example, a basketball thrown from the ground moves in a parabola, as it is in a uniform gravitational field. Its space-time trajectory (when the extra ct dimension is added) is almost a straight line, slightly curved (with the radius of curvature of the order of few light-years). The time derivative of the changing momentum of the object is what we label as "gravitational force".^[3]

Electromagnetic forces[link]

The electrostatic force was first described in 1784 by Coulomb as a force which existed intrinsically between two charges.^[34] The properties of the electrostatic force were that it varied as an inverse square law directed in the radial direction, was both attractive and repulsive (there was intrinsic polarity), was independent of the mass of the charged objects, and followed the superposition principle. Coulomb's Law unifies all these observations into one succinct statement.^[35]

Subsequent mathematicians and physicists found the construct of the electric field to be useful for determining the electrostatic force on an electric charge at any point in space. The electric field was based on using a hypothetical "test charge" anywhere in space and then using Coulomb's Law to determine the electrostatic force.^[36] Thus the electric field anywhere in space is defined as

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{E} = {\vec{F} \over{q}}

where Failed to parse (Missing texvc executable; please see math/README to configure.): q

is the magnitude of the hypothetical test charge.

Meanwhile, the Lorentz force of magnetism was discovered to exist between two electric currents. It has the same mathematical character as Coulomb's Law with the proviso that like currents attract and unlike currents repel. Similar to the electric field, the magnetic field can be used to determine the magnetic force on an electric current at any point in space. In this case, the magnitude of the magnetic field was determined to be

Failed to parse (Missing texvc executable; please see math/README to configure.): B = {F \over{I \ell}}

where Failed to parse (Missing texvc executable; please see math/README to configure.): I

is the magnitude of the hypothetical test current and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \ell
is the length of hypothetical wire through which the test current flows. The magnetic field exerts a force on all magnets including, for example, those used in compasses. The fact that the Earth's magnetic field is aligned closely with the orientation of the Earth's axis causes compass magnets to become oriented because of the magnetic force pulling on the needle.

Through combining the definition of electric current as the time rate of change of electric charge, a rule of vector multiplication called Lorentz's Law describes the force on a charge moving in a magnetic field.^[36] The connection between electricity and magnetism allows for the description of a unified electromagnetic force that acts on a charge. This force can be written as a sum of the electrostatic force (due to the electric field) and the magnetic force (due to the magnetic field). Fully stated, this is the law:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = q(\vec{E} + \vec{v} \times \vec{B})

where Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{F}

is the electromagnetic force, Failed to parse (Missing texvc executable; please see math/README to configure.): q
is the magnitude of the charge of the particle, Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{E}
is the electric field, Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{v}
is the velocity of the particle which is crossed with the magnetic field (Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{B}

).

The origin of electric and magnetic fields would not be fully explained until 1864 when James Clerk Maxwell unified a number of earlier theories into a set of 20 scalar equations, which were later reformulated into 4 vector equations by Oliver Heaviside and Willard Gibbs.^[37] These "Maxwell Equations" fully described the sources of the fields as being stationary and moving charges, and the interactions of the fields themselves. This led Maxwell to discover that electric and magnetic fields could be "self-generating" through a wave that traveled at a speed which he calculated to be the speed of light. This insight united the nascent fields of electromagnetic theory with optics and led directly to a complete description of the electromagnetic spectrum.^[38]

However, attempting to reconcile electromagnetic theory with two observations, the photoelectric effect, and the nonexistence of the ultraviolet catastrophe, proved troublesome. Through the work of leading theoretical physicists, a new theory of electromagnetism was developed using quantum mechanics. This final modification to electromagnetic theory ultimately led to quantum electrodynamics (or QED), which fully describes all electromagnetic phenomena as being mediated by wave-particles known as photons. In QED, photons are the fundamental exchange particle which described all interactions relating to electromagnetism including the electromagnetic force.^[39]

It is a common misconception to ascribe the stiffness and rigidity of solid matter to the repulsion of like charges under the influence of the electromagnetic force. However, these characteristics actually result from the Pauli Exclusion Principle. Since electrons are fermions, they cannot occupy the same quantum mechanical state as other electrons. When the electrons in a material are densely packed together, there are not enough lower energy quantum mechanical states for them all, so some of them must be in higher energy states. This means that it takes energy to pack them together. While this effect is manifested macroscopically as a structural force, it is technically only the result of the existence of a finite set of electron states.

Nuclear forces[link]

There are two "nuclear forces" which today are usually described as interactions that take place in quantum theories of particle physics. The strong nuclear force^[40] is the force responsible for the structural integrity of atomic nuclei while the weak nuclear force^[41] is responsible for the decay of certain nucleons into leptons and other types of hadrons.^[3]

The strong force is today understood to represent the interactions between quarks and gluons as detailed by the theory of quantum chromodynamics (QCD).^[42] The strong force is the fundamental force mediated by gluons, acting upon quarks, antiquarks, and the gluons themselves. The (aptly named) strong interaction is the "strongest" of the four fundamental forces.

The strong force only acts directly upon elementary particles. However, a residual of the force is observed between hadrons (the best known example being the force that acts between nucleons in atomic nuclei) as the nuclear force. Here the strong force acts indirectly, transmitted as gluons which form part of the virtual pi and rho mesons which classically transmit the nuclear force (see this topic for more). The failure of many searches for free quarks has shown that the elementary particles affected are not directly observable. This phenomenon is called color confinement.

The weak force is due to the exchange of the heavy W and Z bosons. Its most familiar effect is beta decay (of neutrons in atomic nuclei) and the associated radioactivity. The word "weak" derives from the fact that the field strength is some 10¹³ times less than that of the strong force. Still, it is stronger than gravity over short distances. A consistent electroweak theory has also been developed which shows that electromagnetic forces and the weak force are indistinguishable at a temperatures in excess of approximately 10¹⁵ kelvins. Such temperatures have been probed in modern particle accelerators and show the conditions of the universe in the early moments of the Big Bang.

Non-fundamental forces[link]

Some forces are consequences of the fundamental ones. In such situations, idealized models can be utilized to gain physical insight.

Normal force[link]

F_N represents the normal force exerted on the object.

The normal force is due to repulsive forces of interaction between atoms at close contact. When their electron clouds overlap, Pauli repulsion (due to fermionic nature of electrons) follows resulting in the force which acts in a direction normal to the surface interface between two objects.^[43] The normal force, for example, is responsible for the structural integrity of tables and floors as well as being the force that responds whenever an external force pushes on a solid object. An example of the normal force in action is the impact force on an object crashing into an immobile surface.^[3]

Friction[link]

Friction is a surface force that opposes relative motion. The frictional force is directly related to the normal force which acts to keep two solid objects separated at the point of contact. There are two broad classifications of frictional forces: static friction and kinetic friction.

The static friction force (Failed to parse (Missing texvc executable; please see math/README to configure.): F_{\mathrm{sf}} ) will exactly oppose forces applied to an object parallel to a surface contact up to the limit specified by the coefficient of static friction (Failed to parse (Missing texvc executable; please see math/README to configure.): \mu_{\mathrm{sf}} ) multiplied by the normal force (Failed to parse (Missing texvc executable; please see math/README to configure.): F_N ). In other words the magnitude of the static friction force satisfies the inequality:

Failed to parse (Missing texvc executable; please see math/README to configure.): 0 \le F_{\mathrm{sf}} \le \mu_{\mathrm{sf}} F_\mathrm{N}

.

The kinetic friction force (Failed to parse (Missing texvc executable; please see math/README to configure.): F_{\mathrm{kf}} ) is independent of both the forces applied and the movement of the object. Thus, the magnitude of the force equals:

Failed to parse (Missing texvc executable; please see math/README to configure.): F_{\mathrm{kf}} = \mu_{\mathrm{kf}} F_\mathrm{N}

,

where Failed to parse (Missing texvc executable; please see math/README to configure.): \mu_{\mathrm{kf}}

is the coefficient of kinetic friction. For most surface interfaces, the coefficient of kinetic friction is less than the coefficient of static friction.[3]

Tension[link]

Tension forces can be modeled using ideal strings which are massless, frictionless, unbreakable, and unstretchable. They can be combined with ideal pulleys which allow ideal strings to switch physical direction. Ideal strings transmit tension forces instantaneously in action-reaction pairs so that if two objects are connected by an ideal string, any force directed along the string by the first object is accompanied by a force directed along the string in the opposite direction by the second object.^[44] By connecting the same string multiple times to the same object through the use of a set-up that uses movable pulleys, the tension force on a load can be multiplied. For every string that acts on a load, another factor of the tension force in the string acts on the load. However, even though such machines allow for an increase in force, there is a corresponding increase in the length of string that must be displaced in order to move the load. These tandem effects result ultimately in the conservation of mechanical energy since the work done on the load is the same no matter how complicated the machine.^[3][45]

Elastic force[link]

F_k is the force that responds to the load on the spring.

An elastic force acts to return a spring to its natural length. An ideal spring is taken to be massless, frictionless, unbreakable, and infinitely stretchable. Such springs exert forces that push when contracted, or pull when extended, in proportion to the displacement of the spring from its equilibrium position.^[46] This linear relationship was described by Robert Hooke in 1676, for whom Hooke's law is named. If Failed to parse (Missing texvc executable; please see math/README to configure.): \Delta x

is the displacement, the force exerted by an ideal spring equals:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}=-k \Delta \vec{x}

where Failed to parse (Missing texvc executable; please see math/README to configure.): k

is the spring constant (or force constant), which is particular to the spring. The minus sign accounts for the tendency of the force to act in opposition to the applied load.[3]

Continuum mechanics[link]

When the drag force (Failed to parse (Missing texvc executable; please see math/README to configure.): F_d

) associated with air resistance becomes equal in magnitude to the force of gravity on a falling object (Failed to parse (Missing texvc executable; please see math/README to configure.): F_g

), the object reaches a state of dynamic equilibrium at terminal velocity.

Newton's laws and Newtonian mechanics in general were first developed to describe how forces affect idealized point particles rather than three-dimensional objects. However, in real life, matter has extended structure and forces that act on one part of an object might affect other parts of an object. For situations where lattice holding together the atoms in an object is able to flow, contract, expand, or otherwise change shape, the theories of continuum mechanics describe the way forces affect the material. For example, in extended fluids, differences in pressure result in forces being directed along the pressure gradients as follows:

Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{\vec{F}}{V} = - \vec{\nabla} P

where Failed to parse (Missing texvc executable; please see math/README to configure.): V

is the volume of the object in the fluid and Failed to parse (Missing texvc executable; please see math/README to configure.): P
is the scalar function that describes the pressure at all locations in space. Pressure gradients and differentials result in the buoyant force for fluids suspended in gravitational fields, winds in atmospheric science, and the lift associated with aerodynamics and flight.[3]

A specific instance of such a force that is associated with dynamic pressure is fluid resistance: a body force that resists the motion of an object through a fluid due to viscosity. For so-called "Stokes' drag" the force is approximately proportional to the velocity, but opposite in direction:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}_\mathrm{d} = - b \vec{v} \,

where:

Failed to parse (Missing texvc executable; please see math/README to configure.): b

is a constant that depends on the properties of the fluid and the dimensions of the object (usually the cross-sectional area), and

Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{v}

is the velocity of the object.[3]

More formally, forces in continuum mechanics are fully described by a stress-tensor with terms that are roughly defined as

Failed to parse (Missing texvc executable; please see math/README to configure.): \sigma = \frac{F}{A}

where Failed to parse (Missing texvc executable; please see math/README to configure.): A

is the relevant cross-sectional area for the volume for which the stress-tensor is being calculated. This formalism includes pressure terms associated with forces that act normal to the cross-sectional area (the matrix diagonals of the tensor) as well as shear terms associated with forces that act parallel to the cross-sectional area (the off-diagonal elements). The stress tensor accounts for forces that cause all deformations including also tensile stresses and compressions.

Fictitious forces[link]

There are forces which are frame dependent, meaning that they appear due to the adoption of non-Newtonian (that is, non-inertial) reference frames. Such forces include the centrifugal force and the Coriolis force.^[47] These forces are considered fictitious because they do not exist in frames of reference that are not accelerating.^[3]

In general relativity, gravity becomes a fictitious force that arises in situations where spacetime deviates from a flat geometry. As an extension, Kaluza-Klein theory and string theory ascribe electromagnetism and the other fundamental forces respectively to the curvature of differently scaled dimensions, which would ultimately imply that all forces are fictitious.

Rotations and torque[link]

Relationship between force (F), torque (τ), and momentum vectors (p and L) in a rotating system.

Forces that cause extended objects to rotate are associated with torques. Mathematically, the torque of a force Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{F}

is defined relative to an arbitrary reference point as the cross-product:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{\tau} = \vec{r} \times \vec{F}

where

Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{r}

is the position vector of the force application point relative to the reference point.

Torque is the rotation equivalent of force in the same way that angle is the rotational equivalent for position, angular velocity for velocity, and angular momentum for momentum. As a consequence of Newton's First Law of Motion, there exists rotational inertia that ensures that all bodies maintain their angular momentum unless acted upon by an unbalanced torque. Likewise, Newton's Second Law of Motion can be used to derive an analogous equation for the instantaneous angular acceleration of the rigid body:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{\tau} = I\vec{\alpha}

where

Failed to parse (Missing texvc executable; please see math/README to configure.): I

is the moment of inertia of the body

Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{\alpha}

is the angular acceleration of the body.

This provides a definition for the moment of inertia which is the rotational equivalent for mass. In more advanced treatments of mechanics, where the rotation over a time interval is described, the moment of inertia must be substituted by the tensor that, when properly analyzed, fully determines the characteristics of rotations including precession and nutation.

Equivalently, the differential form of Newton's Second Law provides an alternative definition of torque:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{\tau} = \frac{\mathrm{d}\vec{L}}{\mathrm{dt}},

^[48]

where Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{L}

is the angular momentum of the particle.

Newton's Third Law of Motion requires that all objects exerting torques themselves experience equal and opposite torques,^[49] and therefore also directly implies the conservation of angular momentum for closed systems that experience rotations and revolutions through the action of internal torques.

Centripetal force[link]

For an object accelerating in circular motion, the unbalanced force acting on the object equals:^[50]

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = - \frac{mv^2 \hat{r}}{r}

where Failed to parse (Missing texvc executable; please see math/README to configure.): m

is the mass of the object, Failed to parse (Missing texvc executable; please see math/README to configure.): v
is the velocity of the object and Failed to parse (Missing texvc executable; please see math/README to configure.): r
is the distance to the center of the circular path and Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \hat{r}
is the unit vector pointing in the radial direction outwards from the center. This means that the unbalanced centripetal force felt by any object is always directed toward the center of the curving path. Such forces act perpendicular to the velocity vector associated with the motion of an object, and therefore do not change the speed of the object (magnitude of the velocity), but only the direction of the velocity vector. The unbalanced force that accelerates an object can be resolved into a component that is perpendicular to the path, and one that is tangential to the path. This yields both the tangential force which accelerates the object by either slowing it down or speeding it up and the radial (centripetal) force which changes its direction.[3]

Kinematic integrals[link]

Forces can be used to define a number of physical concepts by integrating with respect to kinematic variables. For example, integrating with respect to time gives the definition of impulse:^[51]

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{I}=\int_{t_1}^{t_2}{\vec{F} \mathrm{d}t}

which, by Newton's Second Law, must be equivalent to the change in momentum (yielding the Impulse momentum theorem).

Similarly, integrating with respect to position gives a definition for the work done by a force:^[52]

Failed to parse (Missing texvc executable; please see math/README to configure.): W=\int_{\vec{x}_1}^{\vec{x}_2}{\vec{F} \cdot{\mathrm{d}\vec{x}}}

which is equivalent to changes in kinetic energy (yielding the work energy theorem).^[52]

Power P is the rate of change dW/dt of the work W, as the trajectory is extended by a position change Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle {d}\vec{x}

in a time interval dt:[53]

Failed to parse (Missing texvc executable; please see math/README to configure.): \text{d}W\, =\, \frac{\text{d}W}{\text{d}\vec{x}}\, \cdot\, \text{d}\vec{x}\, =\, \vec{F}\, \cdot\, \text{d}\vec{x}, \qquad \text{ so } \quad P\, =\, \frac{\text{d}W}{\text{d}\vec{x}}\, \cdot\, \frac{\text{d}\vec{x}}{\text{d}t}\, =\, \vec{F}\, \cdot\, \vec{v},

with Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle{\vec{v}\text{ }=\text{ d}\vec{x}/\text{d}t}

the velocity.

Potential energy[link]

Instead of a force, often the mathematically related concept of a potential energy field can be used for convenience. For instance, the gravitational force acting upon an object can be seen as the action of the gravitational field that is present at the object's location. Restating mathematically the definition of energy (via the definition of work), a potential scalar field Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle{U(\vec{r})}

is defined as that field whose gradient is equal and opposite to the force produced at every point:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F}=-\vec{\nabla} U.

Forces can be classified as conservative or nonconservative. Conservative forces are equivalent to the gradient of a potential while nonconservative forces are not.^[3]

Conservative forces[link]

A conservative force that acts on a closed system has an associated mechanical work that allows energy to convert only between kinetic or potential forms. This means that for a closed system, the net mechanical energy is conserved whenever a conservative force acts on the system. The force, therefore, is related directly to the difference in potential energy between two different locations in space,^[54] and can be considered to be an artifact of the potential field in the same way that the direction and amount of a flow of water can be considered to be an artifact of the contour map of the elevation of an area.^[3]

Conservative forces include gravity, the electromagnetic force, and the spring force. Each of these forces has models which are dependent on a position often given as a radial vector Failed to parse (Missing texvc executable; please see math/README to configure.): \scriptstyle \vec{r}

emanating from spherically symmetric potentials.[55] Examples of this follow:

For gravity:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = - \frac{G m_1 m_2 \vec{r}}{r^3}

where Failed to parse (Missing texvc executable; please see math/README to configure.): G

is the gravitational constant, and Failed to parse (Missing texvc executable; please see math/README to configure.): m_n
is the mass of object n.

For electrostatic forces:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = \frac{q_{1} q_{2} \vec{r}}{4 \pi \epsilon_{0} r^3}

where Failed to parse (Missing texvc executable; please see math/README to configure.): \epsilon_{0}

is electric permittivity of free space, and Failed to parse (Missing texvc executable; please see math/README to configure.): q_n
is the electric charge of object n.

For spring forces:

Failed to parse (Missing texvc executable; please see math/README to configure.): \vec{F} = - k \vec{r}

where Failed to parse (Missing texvc executable; please see math/README to configure.): k

is the spring constant.[3]

Nonconservative forces[link]

For certain physical scenarios, it is impossible to model forces as being due to gradient of potentials. This is often due to macrophysical considerations which yield forces as arising from a macroscopic statistical average of microstates. For example, friction is caused by the gradients of numerous electrostatic potentials between the atoms, but manifests as a force model which is independent of any macroscale position vector. Nonconservative forces other than friction include other contact forces, tension, compression, and drag. However, for any sufficiently detailed description, all these forces are the results of conservative ones since each of these macroscopic forces are the net results of the gradients of microscopic potentials.^[3]

The connection between macroscopic nonconservative forces and microscopic conservative forces is described by detailed treatment with statistical mechanics. In macroscopic closed systems, nonconservative forces act to change the internal energies of the system, and are often associated with the transfer of heat. According to the Second Law of Thermodynamics, nonconservative forces necessarily result in energy transformations within closed systems from ordered to more random conditions as entropy increases.^[3]

Units of measurement[link]

The SI unit of force is the newton (symbol N), which is the force required to accelerate a one kilogram mass at a rate of one meter per second squared, or kg·m·s⁻².^[56] The corresponding CGS unit is the dyne, the force required to accelerate a one gram mass by one centimeter per second squared, or g·cm·s⁻². A newton is thus equal to 100,000 dynes.

The gravitational foot-pound-second English unit of force is the pound-force (lbf), defined as the force exerted by gravity on a pound-mass in the standard gravitational field of 9.80665 m·s⁻².^[56] The pound-force provides an alternative unit of mass: one slug is the mass that will accelerate by one foot per second squared when acted on by one pound-force.^[56]

An alternative unit of force in a different foot-pound-second system, the absolute fps system, is the poundal, defined as the force required to accelerate a one pound mass at a rate of one foot per second squared.^[56] The units of slug and poundal are designed to avoid a constant of proportionality in Newton's Second Law.

The pound-force has a metric counterpart, less commonly used than the newton: the kilogram-force (kgf) (sometimes kilopond), is the force exerted by standard gravity on one kilogram of mass.^[56] The kilogram-force leads to an alternate, but rarely used unit of mass: the metric slug (sometimes mug or hyl) is that mass which accelerates at 1 m·s⁻² when subjected to a force of 1 kgf. The kilogram-force is not a part of the modern SI system, and is generally deprecated; however it still sees use for some purposes as expressing jet thrust, bicycle spoke tension, torque wrench settings and engine output torque. Other arcane units of force include the sthène which is equivalent to 1000 N and the kip which is equivalent to 1000 lbf.

See also[link]

Underwater diving portal

• Nonlinear system

Notes[link]

References[link]

• Corbell, H.C.; Philip Stehle (1994). Classical Mechanics p 28,. New York: Dover publications. ISBN 0-486-68063-0. 
• Cutnell, John d.; Johnson, Kenneth W. (2004). Physics, Sixth Edition. Hoboken, NJ: John Wiley & Sons Inc.. ISBN 041-44895-8. 
• Feynman, R. P., Leighton, R. B., Sands, M. (1963). Lectures on Physics, Vol 1. Addison-Wesley. ISBN 0-201-02116-1. 
• Halliday, David; Robert Resnick; Kenneth S. Krane (2001). Physics v. 1. New York: John Wiley & Sons. ISBN 0-471-32057-9. 
• Parker, Sybil (1993). Encyclopedia of Physics, p 443,. Ohio: McGraw-Hill. ISBN 0-07-051400-3. 
• Sears F., Zemansky M. & Young H. (1982). University Physics. Reading, MA: Addison-Wesley. ISBN 0-201-07199-1. 
• Serway, Raymond A. (2003). Physics for Scientists and Engineers. Philadelphia: Saunders College Publishing. ISBN 0-534-40842-7. 
• Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.). W. H. Freeman. ISBN 0-7167-0809-4. 
• Verma, H.C. (2004). Concepts of Physics Vol 1. (2004 Reprint ed.). Bharti Bhavan. ISBN 81-7709-187-5. 

External links[link]

Wikimedia Commons has media related to: Forces

Look up force in Wiktionary, the free dictionary.

• Video lecture on Newton's three laws by Walter Lewin from MIT OpenCourseWare
• A Java simulation on vector addition of forces