Newton's Third Law Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Newton's Third Law.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

For every action force, there is an equal in magnitude and opposite in direction reaction force.

When you push something, it pushes back on you just as hard.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Forces always come in paired interactions — each force has a matching opposite force on the other object.

Common stuck point: The action and reaction forces act on DIFFERENT objects, so they don't cancel out.

Sense of Study hint: When applying Newton's third law, first identify the two interacting objects. Then name both the action force and reaction force, specifying which object each acts on. Finally, remember that the two forces are equal in magnitude, opposite in direction, and act on different objects — they never cancel each other.

Worked Examples

Example 1

easy
A person pushes against a wall with a force of 50 \text{ N}. What force does the wall exert on the person?

Solution

  1. 1
    Identify the action force: the person pushes on the wall with 50 \text{ N}.
  2. 2
    Apply Newton's third law: every action force has a reaction force equal in magnitude and opposite in direction.
  3. 3
    The wall therefore exerts a reaction force of 50 \text{ N} back on the person, directed away from the wall.

Answer

F_{\text{wall on person}} = 50 \text{ N (opposite direction)}
Newton's third law states that forces always come in action-reaction pairs. The wall pushes back on the person with the same magnitude but opposite direction.

Example 2

medium
A 70 \text{ kg} skater pushes a 50 \text{ kg} skater with a force of 100 \text{ N}. What is the acceleration of each skater?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
A hammer strikes a nail with 200 \text{ N} of force. What force does the nail exert on the hammer?

Example 2

easy
A rocket pushes exhaust gases backward with a force of 4.0 \times 10^4 \text{ N}. What force do the gases exert on the rocket?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

forcenewtons first law