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In contrast, instantaneous acceleration is measured over a "short" time interval. Average acceleration is a quantity calculated from two velocity measurements. The velocity at the beginning of this interval is called the initial velocity, represented by the symbol v 0 (vee nought), and the velocity at the end is called the final velocity, represented by the symbol v (vee). The word long in this context means finite - something with a beginning and an end. Average acceleration is determined over a "long" time interval. Much like velocity, there are two kinds of acceleration: average and instantaneous. Acceleration occurs anytime an object's speed increases or decreases, or it changes direction. Thus, a falling apple accelerates, a car stopping at a traffic light accelerates, and the moon in orbit around the Earth accelerates. That's because acceleration depends on the change in velocity and velocity is a vector quantity - one with both magnitude and direction. Yes, that's right, a change in the direction of motion results in an acceleration even if the moving object neither sped up nor slowed down.
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This example illustrates acceleration as it is commonly understood, but acceleration in physics is much more than just increasing speed.Īny change in the velocity of an object results in an acceleration: increasing speed (what people usually mean when they say acceleration), decreasing speed (also called deceleration or retardation), or changing direction (called centripetal acceleration). When a commercial shouts "zero to sixty in six point seven seconds" what they're saying here is that this particular car takes 6.7 s to reach a speed of 60 mph starting from a complete stop. For many Americans, their only experience with acceleration comes from car ads. In everyday English, the word acceleration is often used to describe a state of increasing speed. Acceleration is the rate of change of velocity with time. When the velocity of an object changes it is said to be accelerating.