# 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.

Physics (university) · Part 02: Newton's laws and forces · Card 4 of 10 · Last updated 2026-10-19

## Analogy

Pushing a wall while on roller skates: you roll backward.

## Example

**Force pair**

Swimmer pushes wall: 200 N
Wall pushes swimmer: 200 N
Opposite 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.

## Related concepts

- [Newton's second law](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-3)
- [Newton's first law](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-1)
- [Net force](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-2)
- [Weight](https://studyvisually.com/learn/science/physics-university/physics-university-part-2/physics-university-part-2-concept-5)

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