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.