Practice Newton's Third Law in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick 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.

Showing a random 20 of 50 problems.

Example 1

easy

When you jump, you push down on Earth. What pushes you up?

Example 2

easy

A hammer strikes a nail with

200 \text{ N}

of force. What force does the nail exert on the hammer?

Example 3

medium

A horse pulls a cart. If forces are equal and opposite, why does the system accelerate?

Example 4

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?

Example 5

easy

A rocket pushes gas backward. What pushes the rocket forward?

Example 6

challenge

3\text{ kg}

astronaut throws a

1\text{ kg}

wrench at

6\text{ m/s}

in deep space. Find the astronaut's recoil speed.

Example 7

medium

2\text{ kg}

cart pushes a

6\text{ kg}

cart with

24\text{ N}

. By the third law, what force does the

6\text{ kg}

cart push back, and find its acceleration.

2 kg cart pushes 6 kg cart with 24 N; reaction: 6 kg cart pushes 2 kg cart with 24 N. a₆ = 4 m/s²

Example 8

hard

50 \text{ kg}

swimmer pushes off a pool wall with a force of

250 \text{ N}

for

0.4 \text{ s}

. Find the swimmer's speed after pushing off (start from rest, ignore drag).

Example 9

easy

State Newton's third law.

Example 10

medium

2\text{ kg}

ball exerts

19.6\text{ N}

down on Earth (

g=9.8

). What force does Earth exert on the ball, and what is its acceleration?

Example 11

hard

80 \text{ kg}

person and a

4000 \text{ kg}

truck experience the same collision force. Whose acceleration is larger and by what factor?

Example 12

easy

A person pushes against a wall with a force of

50 \text{ N}

. What force does the wall exert on the person?

Example 13

medium

Why can you not lift yourself by pulling up on your own belt?

Example 14

easy

A swimmer pushes water backward with a force of

80 \text{ N}

. What force pushes the swimmer forward?

Example 15

hard

40 \text{ kg}

child stands on a

5 \text{ kg}

skateboard on frictionless ice. The child jumps forward off the board at

3 \text{ m/s}

relative to the ground. Find the board's speed afterward.

Example 16

medium

5 \text{ kg}

box rests on the floor. Earth pulls it down with

49 \text{ N}

. Name the reaction partner and the object on which it acts.

Example 17

medium

Two ice skaters (

80 \text{ kg}

and

60 \text{ kg}

) push off each other. If the

80 \text{ kg}

skater moves at

1.5 \text{ m/s}

, how fast does the other move?

80 kg skater moves at 1.5 m/s; by momentum conservation 60 kg skater moves at 80×1.5/60 = 2 m/s

Example 18

challenge

On a frictionless floor a

50 \text{ kg}

student stands on a

10 \text{ kg}

cart and walks at

1 \text{ m/s}

relative to the ground. Find the cart's velocity (assume system started at rest).

Example 19

medium

60 \text{ kg}

person on roller skates pushes a

30 \text{ kg}

wagon with a force of

90 \text{ N}

. Find the magnitude of the person's acceleration. (Friction is negligible.)

Person pushes 30 kg wagon right with 90 N; by third law wagon pushes person left 90 N → a = 1.5 m/s²

Example 20

medium

Two carts,

1\text{ kg}

and

4\text{ kg}

, push apart via a spring. If the

1\text{ kg}

cart accelerates at

8\text{ m/s}^2

, find the

4\text{ kg}

cart's acceleration.

Spring exerts equal 8 N on both carts. 4 kg cart: a = 8/4 = 2 m/s²; 1 kg cart: a = 8/1 = 8 m/s²

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