Newton's Third Law — Action and Reaction
At the 2000 Sydney Olympics, Cathy Freeman's spikes pushed backward on the track with around 1,800 N each stride — Newton's Third Law pushed her forward to gold in 49.11 seconds.
Printable Worksheets
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Q1 · When you push off a wall in a pool, what pushes you forward — the wall or the water?
Q2 · If a truck and a small car collide head-on, which experiences a larger force — the truck or the car?
● Know
- Newton's Third Law: for every action force there is an equal and opposite reaction force
- Force pairs always act on DIFFERENT objects
- Force pairs are equal in size and opposite in direction
● Understand
- Why Newton's Third Law force pairs do NOT cancel each other
- Why rockets fly (exhaust gas pushed back → rocket pushed forward)
- Why you don't fall through the floor (you push down → floor pushes up)
● Can do
- Identify the action and reaction force in any scenario
- Name which object each force acts on
- Explain the truck-car collision result using Newton's 2nd and 3rd Laws combined
- Newton's Third Law
- Action force
- Reaction force
- Force pair
- Equal and opposite
- The two forces in Newton's Third Law — action and reaction together
- For every force applied, an equal and opposite force acts back
- Same magnitude, but pointing in opposite directions
- The force that pushes back in response to the action force
- The initial force applied by one object on another
Stand on a skateboard and push a wall. The wall pushes you BACK. Newton's Third Law: for every force you apply, something applies an equal and opposite force back on you. Always. No exceptions.
Two things to keep straight:
- Action force: you push the wall (your hands → wall). The force is on the wall.
- Reaction force: the wall pushes back on you (wall → your hands). The force is on you.
The forces are equal in size and opposite in direction. But here's the critical point — they act on different objects. The action is on the wall; the reaction is on you. They can never cancel each other because cancelling requires forces on the same object.
This is why you slide backwards when you push the wall — the reaction force on you has nothing opposing it, so you accelerate.
A very common misconception: "If the forces are equal and opposite, won't they cancel and nothing moves?" The answer is no — they act on different objects.
Forces only cancel when they act on the same object. Gravity pulling you down and the floor pushing you up cancel on you — so you don't accelerate. But in a Newton's Third Law pair, each force is on a different object:
- Jumping off a boat: You push the boat back (action on boat). Boat pushes you forward (reaction on you). You go forward, boat goes backward — they never cancel because the forces are on different objects.
- Rocket launch: Exhaust gas is pushed backward at high speed (action on gas). Gas pushes rocket forward (reaction on rocket). Both move — in opposite directions.
- Swimmer pushes starting block: Feet push block backward (action on block). Block pushes swimmer forward (reaction on swimmer). Swimmer accelerates into the pool.
Newton's Third Law is everywhere in nature and sport:
- Kookaburra in flight: The bird's wings push air downward (action on air). The air pushes the bird upward (reaction on bird). That's how birds — and aeroplanes — stay aloft.
- Australian cricket fast bowler: The bowler's hand pushes the ball forward at release (action on ball). The ball pushes back on the bowler's hand (reaction on hand). That's why fast bowling is hard on fingers and shoulders.
- Sailing: The wind pushes the sail (action on sail). The sail pushes back on the wind — but the sail is attached to the boat, so the boat moves forward.
The truck-car puzzle answered: In a head-on collision, both vehicles experience the same magnitude of force (Newton's Third Law — the car pushes the truck, the truck pushes the car equally). But the car has far less mass, so by Newton's Second Law, the car's acceleration (and therefore damage) is much greater. Same force, vastly different consequences.
Wrong: "The truck exerts a bigger force on the car in a collision." No — by Newton's Third Law, both vehicles exert the same force on each other. The difference in damage comes from Newton's Second Law (different masses → different accelerations).
Right: Both exert equal forces (3rd Law). The car experiences greater acceleration because it has less mass (2nd Law) — that's why it suffers more damage.
Wrong: "Since the forces are equal and opposite, nothing ever moves." This confuses same-object balance with Third Law pairs on different objects. They are on different objects — both can accelerate.
Right: Force pairs act on different objects, so each object can accelerate freely in its own direction.
Wrong: "The reaction force comes after the action force — like a delay." They are simultaneous. The moment you push, the push-back is there. You cannot have one without the other.
Right: Action and reaction happen at exactly the same instant — there is no delay. They exist together or not at all.
You push a solid brick wall with 50 N of force. The wall doesn't move at all. Predict: does the wall exert any force on you? If so, how much force and in which direction?
How close was your prediction?
Earlier you were asked: If a truck and a small car collide head-on, which experiences a larger force?
Write a fuller answer using the words Newton's Third Law, equal, and acceleration at least once each.
Q1. Describe the action-reaction force pair when a swimmer pushes off a starting block. Identify (a) the action force and the object it acts on, and (b) the reaction force and the object it acts on. (3 marks)
Q2. A student says "Newton's Third Law force pairs cancel, so nothing ever moves." Explain why this reasoning is incorrect. (3 marks)
Q3. Describe two real-world examples of Newton's Third Law (other than rockets). For each, identify the action force, the reaction force, and the objects they act on. (4 marks)
Answers
▾MCQ 1
B — Newton's Third Law force pairs always act on different objects. That's what prevents them from cancelling.
MCQ 2
C — The rocket pushes exhaust gas backward (action on gas). By Newton's Third Law, the gas pushes the rocket forward (reaction on rocket). No air is needed — rockets work in the vacuum of space.
MCQ 3
B — By Newton's Third Law, the reaction force is equal in magnitude to the action force. You push 50 N; the wall pushes back 50 N. The wall's mass keeps it stationary.
MCQ 4
C — Forces cancel only when they act on the SAME object. Newton's Third Law pairs act on DIFFERENT objects, so each object experiences a net force and can accelerate.
MCQ 5
C — Newton's Third Law: the ball exerts a force on the racket equal and opposite to the racket's force on the ball. Both forces are equal — only the consequences (accelerations) differ because the masses differ.
Short Answer 1
Model answer: (a) Action force: the swimmer's feet push the starting block backward (force acts on the starting block) — 1 mark. (b) Reaction force: the starting block pushes the swimmer forward (force acts on the swimmer) — 1 mark. Both forces are equal in size and opposite in direction — 1 mark.
Short Answer 2
Model answer: The reasoning is wrong because Newton's Third Law force pairs act on different objects, not the same object (1 mark). Forces only cancel when they act on the same object (1 mark). Because the action and reaction are on different objects, each object can experience a net force and accelerate — that's why things move (1 mark).
Short Answer 3
Model answer: Example 1 — Swimmer pushing off a pool wall: feet push wall backward (action on wall), wall pushes swimmer forward (reaction on swimmer). Example 2 — Jumping off a boat: person pushes boat backward (action on boat), boat pushes person forward (reaction on person). 1 mark per example for naming both forces and both objects correctly, up to 4 marks total.