Extension: Newton's Third Law, Action and Reaction
You'll be able to identify the action and reaction force in any scenario.
Forces always come in pairs. Work out what pushes back, and you can explain walking, swimming and rockets.
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 on either object
- 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.
The reason there are no exceptions is that a force is not a thing one object owns. A force is an interaction between two objects, and you cannot have half an interaction any more than you can have one end of a tug-of-war rope. When you push the wall, your hand and the wall are pressing into each other, and describing that pressing from your side gives one force while describing it from the wall's side gives the other. They are two accounts of a single event, which is exactly why the reaction can never be missing, delayed, or a different size.
Because there are two accounts, every force in this topic has to be named with three pieces of information rather than one: what is pushing, what is being pushed, and which way. Say "my hands push the wall, to the north" and you have named the action force, and the object it lands on is the wall. Say "the wall pushes my hands, to the south" and you have named the reaction force, and the object it lands on is you. Get into the habit of writing that arrow, pusher → pushed, because almost every mistake in this lesson comes from leaving one end off.
Try it on the skateboard. The force on you is the wall's push, southward, and there is nothing else pushing you north, so you accelerate south and roll away. Notice that the wall does not move even though an equal force lands on it, and that is not a failure of the law: the wall is bolted to a building that is attached to the Earth, so the same size force is being asked to shift an enormous mass and produces no acceleration you could measure. Same force, different masses, different outcomes, which is Newton's Second Law doing the rest of the work.
Text description
- Rocket launch: the rocket pushes gas down (action); the gas pushes the rocket up (reaction).
- Swimmer push-off: the swimmer pushes the wall (action); the wall pushes the swimmer back (reaction).
- You and the floor: you push down on the floor (action); the floor pushes up on you (reaction).
- Newton's 3rd Law: an action force on object A comes with an equal and opposite reaction on object B. Each pair acts on different objects and does not cancel on either individual object. If both objects form one system, the pair is internal and cancels in the system’s total force.
This is the one place in the lesson where the "different objects" point does real work, so it is worth doing properly rather than repeating. Cancelling is not something forces do to each other in mid-air. It is an operation you carry out on a free-body diagram, and a free-body diagram, as the net force lesson set out, shows only the forces acting on one chosen object. Two forces can only cancel if they both appear on the same diagram.
Now look at what that means for a Third Law pair. The action lands on one object and the reaction lands on the other, so the two forces are drawn on two different diagrams. Neither single-object diagram contains both forces, so the pair does not cancel when calculating the net force on either object separately. If both objects are treated as one combined system, the pair is internal and cancels in that system's force sum.
The contrast that makes this usable. Put a book on a table. Gravity pulls the book down with 5 N and the table pushes the book up with 5 N, and those two do cancel, which is why the book sits still. Equal, opposite, cancelling — and they are not a Newton's Third Law pair. Check them against the pusher-and-pushed test: gravity is Earth pulling the book, and its partner is the book pulling Earth. The table's push is the table pushing the book, and its partner is the book pushing down on the table. Two separate pairs, and the two forces that cancel came one from each. So the test is not "are they equal and opposite?", because both situations pass that. The test is which object is each force acting on. Forces on the same object can balance. A Third Law pair must come from the same interaction: object A pushes object B and object B pushes object A. Merely acting on different objects is not enough to make two forces a pair.
With that test in hand the three examples below stop being things to remember and become things to check:
- 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, the pair does not cancel on either object because the two forces act 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?
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, so they do not cancel when calculating the net force on either object separately.
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. Forces from the wall's supports and foundation balance your push, keeping the wall stationary.
MCQ 4
C The two forces act on different objects, so they do not balance each other on either object. Each object's acceleration depends on all forces acting on that object; other forces may balance the pair member.
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. The interaction forces are equal. The accelerations need not be equal because each depends on the object's mass and the total net force acting on it.
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). When calculating the net force on one object, include only forces acting on that object (1 mark). Each object accelerates only if all the forces acting on it add to a non-zero net force; other forces may balance its member of the pair (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.
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.
- Newton's Third Law: whenever one object pushes another, the second pushes back equally hard in the opposite direction, at the same instant.
- The two forces in a pair always act on different objects, so they do not cancel when finding the net force on either object separately. Other forces can still balance the force on each object.
- In a truck-and-car collision both vehicles feel the same size force (Third Law), and the car is damaged more because its smaller mass gives it a much larger acceleration (Second Law). Two different laws, one event.