Year 7 Science · Unit 3 · Lesson 7

Newton's Third Law — Action-Reaction Pairs

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Learning Goals

Apply Newton's Third Law to explain how a rocket launch works.
Sequence the cause-and-effect steps of a rocket launching using force pairs.
Name which object each force in the launch sequence acts on.

Warm Up

Real-world context

Australia's Arnhem Space Centre, located near Nhulunbuy in the Northern Territory, became the first commercial spaceport in the Southern Hemisphere to launch a NASA rocket in July 2022. When a rocket launches, its engines burn fuel to produce expanding gases. These gases are forced out of the rocket's engine nozzles at extremely high speed — directly downward. The rocket itself then rises up off the launch pad. This all happens in a fraction of a second, and Newton's Third Law is entirely responsible for the motion.

(a) Using Newton's Third Law, identify the action force and the reaction force during the rocket launch. For each force, name the object it acts on.

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(b) The exhaust gas and the rocket move in opposite directions. Explain why this happens using Newton's Third Law, and state whether this makes the rocket launch a good or a poor example of the law.

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Your Turn

Order the steps

Number the events from 1 to 6 to show the correct order of a rocket launch explained using Newton's Third Law. Event 1 = what happens first. The steps are shuffled — put them in the right sequence.

Order	Event
	The equal and opposite reaction force pushes the rocket upward with the same size force.
	Fuel inside the rocket engine begins to burn rapidly.
	The rocket accelerates upward off the launch pad, gaining speed.
	Hot gases expand and are pushed out of the engine nozzles downward at high speed (action force on the gas).
	The rocket clears the launch tower and continues to accelerate as long as fuel burns.
	The burning produces a large amount of rapidly expanding gas inside the engine chamber.

Show What You Know

1. A swimmer pushes off the starting block with a force of 200 N. Describe the reaction force: its size, its direction, and which object it acts on. What happens to the swimmer as a result?

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2. In a head-on collision, a large truck and a small car hit each other. Using Newton's Third Law, compare the forces the truck and car exert on each other. Why does the car suffer more damage?

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Wrap Up

In one sentence, what was the main idea of this lesson?