# Scientists Say: Acceleration

## This word describes a change in speed or direction

#### Acceleration (noun, “ack-SELL-er-AY-shun”)

This is the rate of change in velocity over time. Velocity is the speed of something in a given direction. Acceleration is when velocity changes. Because velocity is both speed and direction, acceleration can involve speed and direction as well.

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. 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.

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. 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.

Acceleration is often used when scientists calculate force. That’s the F in the equation F = ma (force equals mass times acceleration). 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. 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). The negative acceleration as the car crashes to a stop will be much, much less at the slower velocity.

#### In a sentence

Insects called sharpshooters fling pee as they feed — at up to six times the acceleration an astronaut feels on takeoff.

### About Bethany Brookshire

Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.