How does a rocket launch into space when there is nothing to push against? How do you walk across a room? The answer lies in Newton's third law — one of the most powerful and misunderstood ideas in physics.
Think about jumping off a small boat onto a dock.
Write down your answers before reading on:
For every action there is an equal and opposite reaction
Newton's third law of motion states:
For every action, there is an equal and opposite reaction.
This means that forces always come in pairs. When object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A.
Importantly, the two forces in an action-reaction pair:
The crucial detail that students often miss
Students often wonder: "If the forces are equal and opposite, why don't they cancel out?"
The answer is simple but crucial: action and reaction forces act on different objects.
For example, when you push on a wall, the wall pushes back on you with equal force. These forces do not cancel because one acts on the wall and one acts on you. You feel the wall pushing back — that is why your hand does not go through it.
How every step you take uses physics
When you walk, you push backward on the ground with your foot. The ground pushes forward on you with an equal and opposite force. This forward force is what propels you.
On slippery ice, it is hard to walk because there is not enough friction for you to push backward effectively. The action force is weak, so the reaction force is also weak.
How rockets work where there is nothing to push against
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.
Action-reaction pairs are everywhere
Action-reaction pairs are everywhere:
"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.
"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.
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.
1. What does Newton's third law state?
2. Why do action and reaction forces NOT cancel out?
3. How does a rocket move in space?
4. When you walk, what is the action-reaction pair?
5. A swimmer pushes water backward with a force of 200 N. What force does the water exert on the swimmer?
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
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
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
Go back to your Think First answer. Has your understanding changed?
C — Newton's third law states that for every action force there is an equal and opposite reaction force.
B — Action and reaction forces do not cancel because they act on different objects. Only forces on the same object can cancel.
B — A 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.
B — When you walk, you push backward on the ground (action). The ground pushes forward on you (reaction). This forward force propels you forward.
D — According to Newton's third law, the water exerts an equal and opposite force of 200 N forward on the swimmer.
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.
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.
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.
Blast through levels using action-reaction physics! Every move you make has an equal and opposite reaction — use it to your advantage.
Tick when you have finished all activities and checked your answers.