Newton's Third Law of Motion

​Newton's Third Law of Motion

By Kevin Goettge

The initial push or pull exerted on an object.

​​Action Force

The equal and opposite force that occurs in response.

​​Reaction Force

Newton's Third Law of Motion

For every action, there is an equal and opposite reaction.

Action-Reaction Pairs

Action-reaction forces are equal in strength and opposite in direction.

Forces always act in pairs, known as action-reaction pairs.

Newton's Third Law is key to understanding collisions between objects.

During a collision, objects exert equal and opposite forces on each other.

In a truck-car crash, the forces they exert on each other are equal.

​

Act on the same object, canceling each other out.

​​Balanced Forces

Act on different objects and do not cancel out.

​​Action-Reaction Forces

​What's the Difference?

Forces always come in pairs — if object A pushes object B, object B pushes back equally on A.

Multiple Choice

When you accidentally stub your toe on a table leg, your toe exerts a force on the table.
According to Newton’s Third Law, what else happens?

• The table exerts an equal and opposite force on your toe.

• The table absorbs all the force and doesn’t push back.

• The table pushes harder than your toe.

• There is no reaction force because the table doesn’t move.

Multiple Choice

When a moving cue ball hits a stationary eight ball, what happens according to Newton’s Third Law?

• The cue ball pushes on the eight ball, but the eight ball doesn’t push back.

• The eight ball pushes back on the cue ball with an equal and opposite force

• Both balls push in the same direction.

• The cue ball stops because friction cancels its force.

Multiple Choice

A baseball bat hits a ball with 600 N of force. According to Newton’s Third Law:

• The ball pushes on the bat with 300 N.

• The ball pushes on the bat with 600 N.

• The ball pushes on the bat with 1200 N.

• The bat exerts force, but the ball doesn’t.

Multiple Choice

When a car’s tires push backward on the road, what does the road do?

• Push forward on the tires with equal force.

• Push backward on the car with greater force.

• Stay still with no reaction.

• Reduce the car’s acceleration.

Drag and Drop

When you jump forward off a small boat, the boat moves ​

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because you push it with an ​

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and opposite force.

Drag these tiles and drop them in the correct blank above

backward

equal

unequal

forward

sideways

zigzag

Drag and Drop

When an astronaut pushes on the wall of the space station, the wall pushes back on the astronaut with ​

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force in the ​

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direction.

Drag these tiles and drop them in the correct blank above

equal

opposite

unequal

greater

less

same

Multiple Choice

The rocket engines push exhaust gases downward with a force of 1,000,000 N.
According to Newton’s Third Law, what other force must be happening?

• The Earth is pushing down on the gases.

• The gases are pushing upward on the rocket with a force of 1,000,000 N.

• The rocket is pulling the gases downward.

• Gravity cancels out the exhaust’s force.

Multiple Choice

Consider the following force:
A powerful magnet on the International Space Station pulls a floating metal tool toward it.
According to Newton’s Third Law, what other force must be happening?

• The metal tool pushes on the magnet.

• The magnet pulls on itself.

• The tool moves without exerting any force.

• The magnet pushes on the space station walls.

Multiple Choice

Due to gravity, Earth pulls downward on a satellite with a force of 2,000 N.
According to Newton’s Third Law, what other force must be happening?

• The satellite is pulling upward on the Earth with a force of 2,000 N.

• The satellite pushes down on Earth with a smaller force.

• Earth is pushing upward on the satellite.

• There is no reaction force in space.

Multiple Choice

An astronaut pushes a 20 kg tool bag away from her during a spacewalk.
What happens next?

• Only the tool bag moves away because it has less mass.

• Both the astronaut and the tool bag move in opposite directions.

• The astronaut stays still because she’s heavier.

• Neither moves because there’s no gravity.

Match

Match each action force with its reaction force.

• Action: Rocket engines push gases downward

• Action: Astronaut pushes wall

• Action: Earth pulls satellite downward

Reaction: Gases push rocket upward

Reaction: Wall pushes astronaut

Reaction: Satellite pulls Earth upward

• Reaction: Gases push rocket upward

• Reaction: Wall pushes astronaut

• Reaction: Satellite pulls Earth upward

Drag and Drop

When a space probe fires its thrusters, gases are pushed ​

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, and the probe moves ​

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.

Drag these tiles and drop them in the correct blank above

backward

forward

sideways

zigzag

Multiple Choice

Two satellites bump into each other.
According to Newton’s Third Law, how do their forces compare?

• The larger satellite exerts more force.

• Both satellites exert equal and opposite forces on each other.

• The smaller satellite exerts the stronger force.

• The faster satellite exerts no force.

Multiple Choice

If a small spacecraft pushes against the space station while docking, what happens to the station?

• It doesn’t move because it’s massive.

• It moves slightly in the opposite direction.

• It speeds forward faster than the craft.

• It stays motionless because there’s no friction.

Multiple Choice

A lunar lander fires its downward thrusters to slow its descent.
According to Newton’s Third Law, what force causes the lander to slow down?

• The Moon’s gravity pulling up on it

• The exhaust gases pushing upward on the lander

• The air pushing downward

• Friction between the air and the lander