Lesson 5 of 14
Action and reaction
Show on a big screenA bat hits a ball. Does the ball push back on the bat?
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Step 1 of 2
- Two carts sit together with a squeezed spring between them.
- Released: 125 N on cart A and 125 N on cart B, opposite ways.
Yes. The ball pushes the bat with an equal force, the other way.
The rule
If A pushes B, B pushes A equally the other way. The two forces act on different objects.
This is Newton's third law. Forces come in pairs. This holds for pushes and pulls, and for touching and non-touching forces. Two magnets pull each other equally.
| One force | Its partner |
|---|---|
| Your foot pushes the ground back | The ground pushes you forward |
| A paddle pushes water back | Water pushes the paddle forward |
| A garden hose shoots water forward | The water pushes the hose back |
| A swimmer pushes the pool wall | The wall pushes the swimmer away |
You walk because friction from the ground pushes your foot forward. On wet, polished floors that friction is small, so you slip.
Watch out!
The push-back does not arrive later. Both forces of a pair exist at the same moment.
Try the lab to finish this lesson.