Acceleration

1. This is the rate of change in velocity over time. Velocity is the speed of something in a given direction. 
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3. Acceleration is when velocity changes. Because velocity is both speed and direction, acceleration can involve speed and direction as well.
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5. Speeding up is accelerating. Turning left is accelerating, too. Even slowing down is technically accelerating! How does that work? It’s still a change in velocity — but in this case, the acceleration is negative. 
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7. Some people might call that deceleration. But deceleration refers only to a decrease in speed. Negative acceleration can be a decrease in speed, but it could also be a change in direction — moving backward instead of forward.
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9. Keep in mind that acceleration and velocity aren’t the same thing. Something can have a very high velocity — like a jet flying in the air — and speed up or slow down only a little. In other words, the plane has a high velocity and a low acceleration. 
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11. And a car can be stopped at a stop sign and then speed very quickly along the street. That’s a small velocity — the car is stopped, so velocity is zero — and a large acceleration, or change in speed.
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13. Acceleration is often used when scientists calculate force. That’s the F in the equation F = ma (force equals mass times acceleration). 
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15. Say a glass falls and hits the ground. The force it hits with is equal to the mass of the glass times the acceleration as it fell. 
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17. This is also why a car crash at 8 kilometers per hour (5 miles per hour) will have far less force than a crash at 80 kph (50 mph). 
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19. The negative acceleration as the car crashes to a stop will be much, much less at the slower velocity.

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