Acceleration is a change in the velocity of an object. We often think of it as “speeding up”, but there is also negative acceleration, known as deceleration. Acceleration is measured in metres per second per second (m/s2). A change of direction is also acceleration, as the velocity is changed.
A sprinter accelerates from zero velocity by pushing against the blocks. This propels her forward, but she will still be accelerating for several metres before she reaches her fastest velocity. Her momentum carries her forward even after she has passed the finish line, but as she stops using energy to push her feet against the ground, she gradually slows and stops.
Acceleration is the name we give to any process where the velocity changes. Since velocity is a speed and a direction, there are only two ways for you to accelerate: change your speed or change your direction—or change both.
If you’re not changing your speed and you’re not changing your direction, then you simply cannot be accelerating—no matter how fast you’re going. So, a jet moving with a constant velocity at 800 miles per hour along a straight line has zero acceleration, even though the jet is moving really fast, since the velocity isn’t changing. When the jet lands and quickly comes to a stop, it will have acceleration since it’s slowing down.
Or, you can think about it this way. In a car you could accelerate by hitting the gas or the brakes, either of which would cause a change in speed. But you could also use the steering wheel to turn, which would change your direction of motion. Any of these would be considered acceleration since they change velocity.
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