Pressure (the symbol: p) is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure (also spelled gage pressure) is the pressure relative to the local atmospheric or ambient pressure.

Pressure is the effect of a force applied to a surface. Pressure is the amount of force acting per unit area. The symbol of pressure is p.

Mathematically:

where:

Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:

The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outward.

It is incorrect (although rather usual) to say "the pressure is directed in such or such direction". The pressure, as a scalar, has no direction. It is the force given by the previous relationship to the quantity that has a direction, not the pressure. If we change the orientation of the surface element, the direction of the normal force changes accordingly, but the pressure remains the same.

Nadia Ali (Urdu: نادیہ علی; born August 3, 1980) is a Pakistani American singer-songwriter, born in Libya and raised in Queens, New York. She gained attention as the frontwoman and songwriter of the band iiO, after their 2001 hit "Rapture" reached No. 2 on the UK Singles Chart. The song also charted across several countries in Europe. Their 2006 single, "Is It Love?", reached the top of the Billboard Hot Dance Club Play Chart.

After embarking on a solo career in 2005, Ali became successful as an oft-requested vocalist in electronic dance music. She released her debut album Embers in 2009. Critics praised her unique songwriting, which combined electronica with ballads, Eastern and acoustic music. Three singles from the album reached the top-ten of the Billboard Hot Dance Club Play Chart, including the No. 1 hit, "Love Story". The song was also nominated at the 25th International Dance Music Awards at the Winter Music Conference, while "Fantasy" was nominated for a Grammy award.

In 2010, she released a remix compilation titled Queen of Clubs Trilogy to mark her decade-long career as a singer-songwriter. "Rapture" was re-released as the only single from the compilation and the song was once again a chart success in Europe. Ali released the single "Pressure" with Starkillers and Alex Kenji in 2011, which became a club and festival anthem and received an International Dance Music Award. She has also collaborated with several notable producers and DJs, such as Armin van Buuren, Schiller, BT and John Creamer & Stephane K. Ali is currently working on her second studio album titled Phoenix, which is due to be released in 2012.

William Martin "Billy" Joel (born May 9, 1949) is an American pianist, singer-songwriter, and composer. Since releasing his first hit song, "Piano Man," in 1973, Joel has become the sixth best-selling recording artist and the third best-selling solo artist in the United States, according to the RIAA.

Joel had Top 40 hits in the 1970s, 1980s, and 1990s, achieving 33 Top 40 hits in the United States, all of which he wrote himself. He is also a six-time Grammy Award winner, a 23-time Grammy nominee and has sold over 150 million records worldwide. He was inducted into the Songwriter's Hall of Fame (1992), the Rock and Roll Hall of Fame (1999), the Long Island Music Hall of Fame (2006), and the Hit Parade Hall of Fame (2009). In 2008, Billboard magazine released a list of the Hot 100 All-Time Top Artists to celebrate the US singles chart's 50th anniversary, with Billy Joel positioned at No. 23. With the exception of the 2007 songs "All My Life" and "Christmas in Fallujah," Joel stopped recording pop/rock material after 1993's River of Dreams, but he continued to tour extensively until 2010.

Ann "Annie" Lennox, OBE (born 25 December 1954), is a Scottish singer-songwriter, political activist and philanthropist. After achieving minor success in the late 1970s as part of the New Wave band The Tourists, she and fellow musician David A. Stewart went on to achieve major international success in the 1980s as Eurythmics. Lennox is the most recognized female artist at the Brit Awards, winning a total of eight awards. She has also been named the "Brits Champion of Champions".

Lennox embarked on a solo career in the 1990s with her debut album, Diva (1992), which produced several hit singles including "Why" and "Walking on Broken Glass". To date, she has released five solo studio albums and a compilation album, The Annie Lennox Collection (2009). She is the recipient of eight Brit Awards, four Grammy Awards and an MTV Video Music Award. In 2002, Lennox received a Billboard Century Award; the highest accolade from Billboard Magazine. In 2004, she won both the Golden Globe and the Academy Award for Best Original Song for "Into the West", written for the soundtrack to the feature film The Lord of the Rings: The Return of the King.

David Bowie ( /ˈboʊ.i/ BOH-ee; born David Robert Jones on 8 January 1947) is an English musician, actor, record producer and arranger. A major figure for over four decades in the world of popular music, Bowie is widely regarded as an innovator, particularly for his work in the 1970s. He is known for his distinctive voice and the intellectual depth and eclecticism of his work.

Bowie first caught the eye and ear of the public in July 1969, when his song "Space Oddity" reached the top five of the UK Singles Chart. After a three-year period of experimentation he re-emerged in 1972 during the glam rock era with the flamboyant, androgynous alter ego Ziggy Stardust, spearheaded by the hit single "Starman" and the album The Rise and Fall of Ziggy Stardust and the Spiders from Mars. Bowie's impact at that time, as described by biographer David Buckley, "challenged the core belief of the rock music of its day" and "created perhaps the biggest cult in popular culture." The relatively short-lived Ziggy persona proved merely one facet of a career marked by continual reinvention, musical innovation and striking visual presentation.