Push on a wall and you feel it push back on your hands. That is not a coincidence. Every force is half of an interaction between two objects, and the other half is always there.
Key idea
Newton's third law: when object A pushes on object B, B pushes on A with a force of the same size in the opposite direction. The two forces act on different objects.
Why it works
A force is an interaction, like a handshake: you can't shake hands without the other hand shaking yours. When a bug hits a car's windshield, the windshield pushes on the bug and the bug pushes on the windshield, with equal force. The results look different only because the same force does much more to a tiny bug than to a heavy car.
Worked example: Pushing off
A skater pushes a wall with 90 N. What force does the wall exert on the skater?
- The pair to "skater pushes wall" is "wall pushes skater."
- Same size, opposite direction: 90 N, away from the wall.
- That push is what sends the skater gliding backward.
Answer: 90 N, pushing the skater away from the wall
Worked example: A rocket
What pushes a rocket upward when it takes off?
- The rocket pushes hot exhaust gas down out of its nozzle.
- By the third law, the gas pushes the rocket up just as hard.
- No air is needed, which is why rockets work in space.
Answer: The exhaust gas, pushing back on the rocket
Common mistake
Thinking the bigger or faster object pushes harder in a collision. In a crash between a truck and a small car, each pushes the other with exactly the same force. The small car is damaged more because the same force gives its smaller mass a bigger acceleration. Check: third-law pairs are always equal, whatever the masses or speeds.
Try the skaters in the Science Lab: The Third Law: Action and Reaction.