Newton's Third Law, Action and Reaction
In 2023, SpaceX's Starship produced 72.5 MN of thrust by expelling exhaust downward, proving Newton's Third Law at 120 m tall and 5,000 tonnes.
When you walk forward, what force actually pushes you forward? Think carefully, your legs push backward on the ground, but what pushes you in the forward direction?
A rocket fires its engines in space where there is nothing to push against, no ground, no air. How can it possibly move? Try to explain the mechanism before the lesson.
● Know
- Newton's third law of motion
- Action-reaction pairs
- Examples of Newton's third law
● Understand
- Why action and reaction forces act on DIFFERENT objects
- Why action and reaction do NOT cancel out
- How rockets work in space
● Can do
- Identify action-reaction pairs in everyday situations
- Explain how walking, swimming and rockets use Newton's third law
- Distinguish between forces that cancel and action-reaction pairs
Stand on a skateboard and push your foot backward against the ground: the ground pushes your foot forward and the skateboard rolls. You applied a force to the ground; the ground applied an equal force back to you. Newton's Third Law of Motion states that if object A exerts a force on object B, then object B simultaneously exerts a force on object A that is equal in magnitude and opposite in direction, acting on different objects, which is why they don't cancel.
Key properties of action-reaction pairs:
- They are equal in magnitude (same size).
- They are opposite in direction.
- They act on different objects.
- They are the same type of force (both contact, both gravitational, etc.).
- They occur simultaneously - one does not cause the other in a temporal sense.
Because action and reaction forces act on different objects, they do not cancel out. The action force on object A affects object A motion; the reaction force on object B affects object B motion.
When you walk, your foot pushes backward on the ground (action). The ground pushes forward on your foot with an equal force (reaction). It is this reaction force that accelerates you forward. Without friction between your shoes and the ground, the ground could not push forward, and you would slip in place (like trying to walk on ice). Rockets work the same way: the rocket pushes exhaust gases backward (action), and the gases push the rocket forward (reaction). Rockets do not push against the air or the ground - they push against their own exhaust. This is why rockets work in vacuum.
Australian rocket propulsion: Southern Launch operates orbital launch facilities at Whalers Way in South Australia, leveraging Australia southern latitude for polar orbit launches. Rocket propulsion is pure Newton Third Law: hot gas expelled backward at high velocity produces forward thrust. Australian company Gilmour Space is developing hybrid rockets that combine solid and liquid propellants. Understanding action-reaction dynamics is essential for calculating thrust, specific impulse, and trajectory. Australia space industry growth depends on engineers who can apply these fundamental principles.
Action-reaction forces cancel out, so they have no effect. This is false. Action and reaction forces do not cancel because they act on different objects. If they acted on the same object, they would cancel and nothing would ever move. But since they act on different objects, each force contributes to the acceleration of its respective object. When you push a wall, the wall pushes back on you, accelerating you backward. The wall does not accelerate noticeably because it is attached to Earth enormous mass (a = F/m, and m for Earth is huge).
You stand on a skateboard and push against a wall. Predict what happens.
You accelerate away from the wall. The wall pushes back on you with a force equal to your push (Newton Third Law). Since the wall is attached to Earth, it does not move noticeably.
Use these terms in your explanation: action · reaction · equal · opposite
Identifying action-reaction pairs correctly is essential for applying Newton Third Law.
Method: For any force acting on object A due to object B, the reaction is a force of the same type acting on object B due to object A, equal in magnitude and opposite in direction.
Examples:
- Book on table: Table pushes up on book (normal force). Reaction: Book pushes down on table (normal force).
- Gravity on book: Earth pulls book down. Reaction: Book pulls Earth up (gravitational force).
- Rocket thrust: Rocket pushes gas backward. Reaction: Gas pushes rocket forward (contact force during expansion).
- Magnet attracting nail: Magnet pulls nail. Reaction: Nail pulls magnet (magnetic force).
Notice that action-reaction pairs are always the same type of force. You cannot pair a normal force with a gravitational force.
A horse pulls a cart. The horse exerts a forward force on the cart through the harness. The cart exerts a backward force on the horse through the harness (Newton Third Law). So why does the system move forward? Because the horse also pushes backward on the ground with its hooves, and the ground pushes forward on the horse (another Third Law pair). The forward ground force on the horse exceeds the backward cart force on the horse, giving the horse a net forward force. Meanwhile, the forward harness force on the cart accelerates the cart. Both horse and cart accelerate forward because the ground provides an external force on the horse-cart system. Without the ground (in space), the horse could not pull the cart forward.
Australian biomechanics: The Australian Institute of Sport studies ground reaction forces in athletics using force plates. When a sprinter pushes against the starting blocks, the blocks push back with an equal force (Third Law). The magnitude and direction of this ground reaction force determine acceleration out of the blocks. Australian sprint coaches analyse force-time curves to optimise starting technique. Similarly, swimming propulsion depends on the swimmer pushing water backward (action) and water pushing the swimmer forward (reaction). Australian swim coaches use underwater video and force analysis to maximise this propulsive force.
The action force is always the larger or more important one. This is false. The words action and reaction are arbitrary labels - neither force is privileged. Which you call action and which reaction is a matter of convenience, not physics. The forces are exactly equal in magnitude. If you push a truck with 100 N, the truck pushes back on you with exactly 100 N. The truck does not move much because its mass is huge (a = F/m is tiny), not because the reaction force is smaller. The equality of action and reaction is absolute, regardless of the masses involved.
Newton three laws form a complete framework for analysing motion. In practice, all three are often needed together.
First Law tells us that objects maintain their state of motion unless acted upon by a net force. It defines inertia and establishes that force is needed to change motion, not to maintain it.
Second Law quantifies how motion changes under force: F = ma. It allows us to calculate acceleration from known forces, or forces from observed acceleration.
Third Law tells us that forces always come in pairs. It helps identify all forces in a system and ensures we do not miss interactions.
Combined analysis: To solve a mechanics problem, use Third Law to identify all forces, draw free-body diagrams, then apply Second Law to calculate motion. First Law is a special case of Second Law when F=0.
Analysing a person jumping:
Third Law: The person legs push down on the ground; the ground pushes up on the person.
Second Law: During the push-off phase, the ground reaction force exceeds weight, producing upward acceleration. F_net = N - mg = ma.
First Law: After leaving the ground, no ground force acts (only gravity), so the person continues upward with constant deceleration until velocity reaches zero at the peak, then accelerates downward.
All three laws are needed: Third Law identifies the ground reaction force, Second Law calculates the acceleration during push-off, and First Law describes the free-fall motion after leaving the ground.
Australian physics heritage: While Newton was English, Australian physics has made significant contributions to mechanics and related fields. The ANU Department of Quantum Science applies Newtonian principles to precision measurement. Australian engineers used classical mechanics to design the Snowy Mountains Hydro-Electric Scheme, calculating forces on tunnels, dams, and turbines. The Sydney Harbour Bridge design required detailed force analysis using Newton laws. These applications show that 17th-century physics remains essential for 21st-century Australian engineering.
Newton laws are outdated and have been replaced by relativity and quantum mechanics. This is false for everyday scales. Newtonian mechanics is perfectly accurate for objects moving much slower than light, with masses much larger than atoms, and in weak gravitational fields. This covers virtually all engineering, most of astronomy, and all everyday experiences. Einstein relativity and quantum mechanics are needed for extreme conditions (near light speed, strong gravity, atomic scales), but they reduce to Newton laws in the everyday limit. Newton laws are not wrong; they are approximations with an enormous domain of validity.
Rockets are a spectacular example of Newton's third law. A rocket engine expels hot gases backward at high speed. The gases push the rocket forward with an equal and opposite force.
This works even in the vacuum of space because the rocket is pushing against its own exhaust gases, it does not need air or any external surface to push against. This is a common misconception: rockets do NOT push against the air.
The Australian Space Agency and companies like Gilmour Space are developing rockets that use this same principle to launch satellites from Australian soil.
Why can a rocket accelerate in the vacuum of space where there is no air?
Action-reaction pairs are everywhere:
- Swimming: You push water backward; water pushes you forward.
- Jumping: You push down on the ground; the ground pushes you up.
- Recoil of a gun: The bullet is pushed forward; the gun is pushed backward.
- Birds flying: Wings push air downward; air pushes the bird upward.
- Rowing a boat: Oars push water backward; water pushes the boat forward.
When a swimmer pushes water backward with their hand, what is the reaction force?
Wrong: "Action and reaction forces cancel each other out." No, they act on different objects, so they cannot cancel. Only forces on the same object can cancel.
Right: Action and reaction forces act on different objects, so they can never cancel each other. To find the net force on an object, you only consider forces acting ON that object. A horse pulling a cart forward and the cart pulling the horse backward are real equal and opposite forces, but they act on different objects and produce separate accelerations.
Wrong: "Rockets push against the air to move." No, rockets push exhaust gases backward; the gases push the rocket forward. This works even in the vacuum of space.
Right: Rockets propel themselves by expelling exhaust gases backward at high speed. By Newton's third law, those gases exert an equal and opposite force pushing the rocket forward. No air or external surface is needed, this is why rockets work perfectly in the vacuum of space.
Wrong: "The action force is always bigger than the reaction force." No, Newton's third law states they are ALWAYS equal in size. The effects may differ because the objects have different masses.
Right: Newton's third law guarantees that action and reaction forces are always exactly equal in magnitude and opposite in direction. A mosquito hitting a bus exerts the same force on the bus as the bus exerts on it, but because the mosquito has far less mass, it experiences a much greater acceleration (a = F/m).
Newton's Third Law in Australian Context
Australian Space Agency: Australia is developing a domestic space launch capability. Newton's third law is the fundamental principle behind every rocket launch. Companies like Gilmour Space and Equatorial Launch Australia are building launch sites in Queensland and the Northern Territory to send satellites into orbit using the same physics Newton described in 1687.
Swimming and athletics: Australian Olympic swimmers and sprinters use Newton's third law to maximise their performance. Swimmers push water backward with their hands and feet; sprinters push the starting blocks and track backward. The harder the action, the greater the reaction.
Indigenous hunting: Traditional Aboriginal spear-throwers (woomeras) use action-reaction principles. The woomera acts as a lever, allowing the thrower to push the spear forward with greater force. The forward force on the spear is matched by a backward force on the thrower, demonstrating Newton's third law.
✍ Copy Into Your Books
▾Newton's Third Law
- For every action there is an equal and opposite reaction
- Action and reaction forces are equal in size, opposite in direction
- They ALWAYS act on different objects
Action-Reaction Pairs
- Walking: push ground backward, ground pushes you forward
- Swimming: push water backward, water pushes you forward
- Rockets: push exhaust backward, exhaust pushes rocket forward
Common Mistakes
- Action and reaction do NOT cancel, they act on different objects
- Rockets do NOT push against air
- The forces are ALWAYS equal, even if effects differ
Action-Reaction Hunt
Rockets and Propulsion
At the start of this lesson you were shown a rocket in empty space with nothing obvious to push against still accelerating, because it pushes exhaust gases backward, and those gases push the rocket forward with equal and opposite force.
Now that you've worked through the lesson, how has your thinking shifted? Can you explain that hook idea more precisely using what you've learned today?
Q1. 1. Explain Newton's third law. Why don't action and reaction forces cancel each other out? Use a specific example in your answer. 4 MARKS
Q2. 2. Explain how a rocket can accelerate in the vacuum of space where there is nothing to push against. Refer to Newton's third law. 4 MARKS
Q3. 3. Describe the action-reaction pairs involved when a person jumps off a small boat onto a dock. Explain why the boat moves backward. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain why action and reaction forces do not cancel out?
- How would you explain to someone why rockets work in space?
Model answers (click to reveal)
Answers
▾MCQ 1
CNewton's third law states that for every action force there is an equal and opposite reaction force.
MCQ 2
BAction and reaction forces do not cancel because they act on different objects. Only forces on the same object can cancel.
MCQ 3
BA rocket moves by expelling exhaust gases backward at high speed. The gases push the rocket forward with an equal and opposite force. This works in space because the rocket pushes against its own exhaust, not the air.
MCQ 4
BWhen you walk, you push backward on the ground (action). The ground pushes forward on you (reaction). This forward force propels you forward.
MCQ 5
DAccording to Newton's third law, the water exerts an equal and opposite force of 200 N forward on the swimmer.
Short Answer 1
Model answer: Newton's third law states that for every action force there is an equal and opposite reaction force. Action and reaction forces do not cancel because they act on different objects. For example, when you walk, your foot pushes backward on the ground (action) and the ground pushes forward on you (reaction). The action force acts on the ground, while the reaction force acts on you. Since they act on different objects, they cannot cancel each other.
Short Answer 2
Model answer: A rocket accelerates in space by expelling hot exhaust gases backward at high speed. According to Newton's third law, as the rocket pushes the exhaust backward (action), the exhaust pushes the rocket forward with an equal and opposite force (reaction). The rocket does not need air or any external surface to push against, it pushes against its own exhaust gases. This is why rockets work perfectly well in the vacuum of space.
Short Answer 3
Model answer: When a person jumps off a boat, they push backward on the boat with their legs (action force). According to Newton's third law, the boat pushes forward on the person with an equal and opposite force (reaction force). The boat moves backward because the action force acts on the boat, pushing it in the opposite direction to the person's jump. The two forces are equal in size but act on different objects, the person and the boat, which is why they do not cancel and both objects accelerate in opposite directions.