Newton's Third Law

Newton's Third Law

Middle School

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Learning Objectives

• Define Newton's Third Law of Motion and identify action-reaction force pairs.

• Explain why action-reaction forces do not cancel each other out.

• Distinguish between action-reaction forces and balanced forces.

• Describe real-world examples of Newton's Third Law, including thrust.

Key Vocabulary

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Newton's Third Law

This law of motion states that for every action, there is an equal and opposite reaction.

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Action Force

The initial force one object exerts on another object during an interaction between the two objects.

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Reaction Force

The force a second object exerts back on the first, equal in strength and opposite in direction.

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Force Pair

The action and reaction forces that occur together in any interaction between two different objects.

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Thrust

A reaction force that propels an object forward when a mass is accelerated in the opposite direction.

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What is Newton's Third Law?

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

• This means that forces always come in pairs called action-reaction forces.

• When an object exerts a force, the second object pushes back equally.

• A skateboarder pushes on the ground, so the ground pushes them forward.

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Multiple Choice

According to Newton's Third Law, what is the relationship between an action force and a reaction force?

• The reaction force is stronger than the action force.

• The reaction force is equal in strength and opposite in direction to the action force.

• The action and reaction forces act in the same direction.

• The reaction force happens a few seconds after the action force.

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Action-Reaction vs. Balanced Forces

Action-Reaction Forces

• These are forces that two different objects apply to each other.

• The forces are equal and opposite, but act on different objects.

• They do not cancel out and can cause a change in motion.

Balanced Forces

• These are equal and opposite forces that act on a single object.

• Because they act on the same object, the forces cancel each other out.

• This results in no change to the object's state of motion.

Multiple Choice

Why do action-reaction force pairs not cancel each other out?

• Because one force is always stronger than the other.

• Because they act on two different objects.

• Because they act on the same object.

• Because they are not always opposite in direction.

Examples of Force Pairs

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Walking

• ​Your foot pushes backward on the Earth (action).

• ​​The Earth pushes your foot forward (reaction).

• ​This pair of forces is what allows you to walk.

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Racket and Ball

• ​A tennis racket applies a forward force to the ball (action).

• ​​The ball exerts an equal backward force on the racket (reaction).

• ​You can feel this reaction force on your hand and arm.

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Gun and Bullet

• ​The gun pushes the bullet forward with great force (action).

• ​​The bullet pushes back on the gun with equal force (reaction).

• ​This backward push on the gun is known as recoil.

Multiple Choice

When a bowling ball hits a pin, the ball exerts a force on the pin. What is the reaction force in this scenario?

• The force of gravity pulling the pin down.

• The force the pin exerts back on the bowling ball.

• The force of the floor on the pin.

• The force of the air slowing the ball down.

Thrust: A Reaction Force

• Thrust is a reaction force that pushes objects like rockets and planes forward.

• It is a real-world example of Newton's Third Law.

• A rocket's downward exhaust creates an equal upward push, launching it into space.

Multiple Choice

How does a rocket engine generate the thrust needed for liftoff?

• By using wings to push air downwards, creating lift.

• By being lighter than the air around it.

• By pushing hot gases downward, which creates an equal and opposite upward force on the rocket.

• By pushing against the launchpad to spring upwards.

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Common Misconceptions

Misconception	Correction
Action-reaction forces cancel each other out.	They act on different objects, so they don't cancel out.
The reaction force is weaker or happens after the action force.	The reaction force is equal in strength and happens at the same time.
Only moving objects create action-reaction forces.	Force pairs exist even for stationary objects.

Multiple Choice

A student pushes against a wall with a force of 10 Newtons. What is the force exerted by the wall on the student?

• 0 Newtons, because the wall does not move.

• 10 Newtons, in the same direction as the student's push.

• 10 Newtons, in the opposite direction of the student's push.

• Less than 10 Newtons, because the wall is stationary.

Multiple Choice

Which of the following scenarios is the best example of Newton's Third Law?

• A book sliding to a stop due to friction.

• A rocket accelerating in space by expelling gas.

• A car coasting along a flat road at a constant speed.

• An apple falling from a tree due to gravity.

Multiple Choice

If a small car and a large truck collide, which statement is true about the forces they exert on each other during the collision?

• The truck exerts a greater force on the car because it is larger.

• The car exerts a greater force on the truck because it changes speed more rapidly.

• The truck and the car exert equal and opposite forces on each other.

• The forces are only equal if they were traveling at the same speed before the collision.

Multiple Choice

In a tug-of-war, the winning team pushes off the ground with their feet. How does this help them win?

• It makes the rope stronger.

• It increases the force of gravity on the team.

• The ground pushes the team forward with a force that helps overcome the pull from the opposing team.

• It reduces the friction between their shoes and the ground.

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Summary

• Forces always occur in equal and opposite action-reaction pairs.

• Action-reaction forces act on different objects, so they don’t cancel out.

• Balanced forces act on the same object, resulting in no change in motion.

• Thrust is a reaction force that propels objects like rockets forward.

Poll

On a scale of 1-4, how confident are you about applying Newton's Third Law to different scenarios?

• 1

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• 3

• 4