4cNewton's Third Law
Explore action-reaction pairs — why every force has an equal and opposite partner, and why those forces never cancel each other out.
Newton's Third Law explains how rockets reach space, how we walk, how fish swim, and why collisions affect both objects involved. It is fundamental to understanding every interaction between objects in the universe.
Lesson Overview
Newton's Third Law states that for every action force there is an equal and opposite reaction force. These action-reaction pairs always act on different objects, which is why they do not cancel each other. This lesson explores action-reaction pairs, why they don't cancel, and real-world examples including rocket propulsion, walking, and swimming.
Key Concepts
Action-Reaction Pairs
Every force has an equal and opposite partner force
Equal Magnitude
Both forces in a pair have the same size
Opposite Direction
The two forces point in exactly opposite directions
Different Objects
Each force acts on a different object — they never cancel
Rocket Propulsion
Exhaust pushed back → rocket pushed forward
Walking & Swimming
Push ground/water backward → ground/water pushes you forward
A person pushes a wall with 50 N to the right. Identify the action-reaction pair.
A rocket expels exhaust gases downward with a force of 200,000 N. What force propels the rocket upward?
A 60 kg person pushes a 10 kg skateboard backward with 30 N. What force does the skateboard exert on the person, and what is each object's acceleration?
Why don't action-reaction pairs cancel each other out, even though they are equal and opposite?
A swimmer pushes water backward with her hands. Explain how this propels her forward.
A horse pulls a cart forward with 500 N. What force does the cart exert on the horse, and in which direction?
Hint: Apply Newton's Third Law: identify the action force and describe the reaction.
A 5 kg ball rests on the floor. Earth pulls the ball down with 50 N. What is the reaction force to gravity?
Hint: The reaction to Earth pulling the ball is the ball pulling _____ with _____ N in which direction?
Two ice skaters push off each other. Skater A has mass 50 kg and Skater B has mass 80 kg. If they push with 40 N, find each skater's acceleration.
Hint: Both experience the same force magnitude. Use a = F/m for each separately.
A gun fires a bullet. The bullet experiences a large forward force. What happens to the gun, and why?
Hint: Think about the reaction force — what object does it act on, and in which direction?
A bird pushes air downward with its wings. What reaction force allows the bird to fly?
Hint: Identify what the air pushes on and in which direction.
Key Vocabulary
Newton's Third Law
For every action force, there is an equal in magnitude and opposite in direction reaction force.
Example: A foot pushes the ground backward; the ground pushes the foot forward, allowing walking.
Action-Reaction Pair
Two forces that are equal in magnitude, opposite in direction, and act on two different objects.
Example: Earth pulls a ball downward (action); the ball pulls Earth upward (reaction) — an action-reaction pair.
Rocket Propulsion
Motion produced when a rocket expels exhaust gases; the reaction force to the expelled gas pushes the rocket in the opposite direction.
Example: A rocket engine burns fuel and expels hot gas downward, and the reaction force pushes the rocket upward.
Normal Force Pair
When an object rests on a surface, the object pushes down on the surface (action) and the surface pushes up on the object (reaction).
Example: A book on a table pushes down on the table; the table pushes up on the book — these are Newton's Third Law pairs.
Interactive Practice — 5 Questions
A person pushes a box with 40 N to the right. What force does the box exert on the person?
Why don't action-reaction forces cancel each other?
A rocket in space fires its engines, expelling gas backward. What happens to the rocket?
Two students push against each other with equal force. Student A has twice the mass of Student B. Which student accelerates more?
When you walk, your foot pushes the ground backward. What is the reaction force?
Independent Practice
State Newton's Third Law and give three examples of action-reaction pairs from everyday life.
A 70 kg astronaut in space pushes a 200 kg satellite with 50 N. Find the acceleration of both the astronaut and the satellite.
Explain why a rocket can accelerate in the vacuum of space where there is nothing to "push against."
Two ice skaters (mass 60 kg and 90 kg) push off each other. If the 60 kg skater accelerates at 3 m/s², what is the acceleration of the 90 kg skater?
★ A gun of mass 2 kg fires a 0.01 kg bullet at 400 m/s. (a) Find the recoil velocity of the gun using Newton's Third Law and conservation of momentum. (b) Explain why the gun's recoil speed is much less than the bullet's speed.
ChallengeCommon Mistakes
Thinking action-reaction forces cancel because they are equal and opposite.
They act on different objects, so they cannot cancel. Cancellation only happens when two forces act on the same object.
Believing the larger/heavier object exerts a bigger force on the smaller one.
Newton's Third Law forces are always equal in magnitude regardless of mass. A truck and a bug exert equal forces on each other during a collision.
Confusing Newton's Third Law pairs with balanced forces (Newton's First Law equilibrium).
Third Law pairs always involve two different objects. Balanced forces (equilibrium) involve two forces on the same object.
Math Tips
To identify a Newton's Third Law pair, ask: (1) Are the forces equal in magnitude? (2) Are they opposite in direction? (3) Do they act on two different objects? If all three are yes, it's a Third Law pair.