- published: 13 Jun 2011
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In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. For example, a car driving away (from standstill) is increasing its speed and is thus accelerating. Similarly, a car braking to stop in front of a traffic light is still said (in physics) to undergo acceleration, although now a negative one. In common speech, it is said to be decelerating.
However, velocity has both a magnitude and direction (i.e. it is a vector), and thus acceleration is also a vector. As such, it describes the rate of change of both the magnitude (the speed) and the direction of velocity. This means that an object moving in a circular motion—such as a satellite orbiting the earth—is also accelerating, even though it may be moving at a constant speed. When an object is executing such a motion where it changes direction, but not speed, it is said to be undergoing centripetal (directed towards the center) acceleration. Oppositely, a change in the speed of an object, but not its direction of motion, is a tangential acceleration.
Lionel Andrés Messi (Spanish pronunciation: [ljoˈnel anˈdɾes ˈmesi], born 24 June 1987) is an Argentine footballer who plays in La Liga for FC Barcelona and is the captain of the Argentina national team, mainly as a forward.
By the age of 21, Messi had received Ballon d'Or and FIFA World Player of the Year nominations. The following year in 2009 he won both the Ballon d'Or and FIFA World Player of the Year, and won the inaugural FIFA Ballon d'Or in 2010 and 2011. He also won the 2010–11 UEFA Best Player in Europe Award. Messi is the fourth football player to win three Ballons d'Or, after Johan Cruyff, Michel Platini and Marco van Basten and the second player to win three consecutive Ballons d'Or, after Michel Platini. His playing style and small stature have drawn comparisons to Diego Maradona, who himself declared Messi as his "successor".
Messi has won five La Liga titles and three Champions League titles. In 2012, Messi made UEFA Champions League history by becoming the first player to score five goals in one match, in a 7–1 win against Bayer Leverkusen. With two goals in the next round against A.C. Milan, he matched José Altafini's record of 14 goals in a single UEFA Champions League/European Cup season. He also became the first player to top-score in four successive European Champion Clubs' Cup campaigns. He set the world record for most goals scored in a season during the 2011–12 season, with 73 goals.