Acceleration - How do things speed up or slow down?
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Kid on bike

Acceleration is a way to measure how fast something is speeding up. Suppose you are riding your bike. You start out going very slowly, hardly pedaling at all. Now you begin to pedal as hard as you can, to speed up - you are accelerating. Now that you are going at a normal speed, you stop pedaling so hard, and just pedal normally. You're still going, but you're not getting any faster, just going along at your normal speed. You're not accelerating anymore.

If you stop pedaling now, friction will work on your bike tires (and you'll have friction from the air, too), and you'll soon start to go slower. That's negative acceleration, or deceleration. You're still going, but you are slowing down.

Cliff diving

One important cause of acceleration is gravity. Suppose you dive off a cliff or a high diving board. You will start off falling slowly, but as gravity pulls on you, you will speed up (accelerate) until you are going very fast.

The acceleration of Earth's gravity will speed you up at about 9.8 meters per second per second (9.8 m/s2, or 9.8 meters per second squared). That means that for every second you fall, you'll be going 9.8 meters/second faster.

Standing on the cliff before you jump, you're going zero meters/second. One second after you jump, you'll be going 9.8 meters/second. Two seconds after you jump, you'll be going 19.6 meters/second. Three seconds after you jump, you'll be going 29.4 meters/second.

You can use acceleration to find out the mass of an object, because force = mass x acceleration. This is how we find out the mass of other stars and planets far away from us.

Learn by doing - Using bikes to work with the physics of motion

Bibliography and further reading about acceleration:

Or check out this link to the Encyclopedia Britannica's article on acceleration.

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Copyright 2012-2015 Karen Carr, Portland State University. This page last updated September 2015.

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