Newton's third law
When object A pushes on object B, B pushes back on A with an equal and opposite force. The two forces act on different objects.
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Analogy
Pushing a wall while on roller skates: you roll backward.
Example
Swimmer pushes wall: 200 NWall pushes swimmer: 200 NOpposite directions.
In practice
A swimmer pushes the pool wall backward and the wall pushes her forward with the same size force.
Practice
Flashcards
- Do third-law force pairs cancel each other?No. They act on different objects, so they never cancel.
Interview questions
- Explain Newton's third law using a rocket in space.
- Engine pushes gas backward
- Gas pushes rocket forward with equal force
- Forces act on different objects, so rocket accelerates
Questions people ask
If forces are equal and opposite, how does anything move?
The two forces act on different objects. When you step forward, you push the ground back and the ground pushes you forward. That forward force acts on you, so you accelerate. Forces only cancel when they act on the same object.
Does a truck hitting a car exert more force on the car?
No. Both feel forces of the same size, as the third law says. The car suffers more damage and a bigger change in speed because it has much less mass. By F = ma, the same force gives the lighter car a much larger acceleration.