Year 7 Science · Unit 3 · Lesson 7

Newton's Third Law, Action-Reaction Pairs

Challenge Worksheet

Name
Date
Class

Learning Goals

Go Deeper

Explain it to a classmate

Your classmate is confused: "If the action force and the reaction force are equal and opposite, surely they cancel out, so nothing can ever move! Newton's Third Law doesn't make sense." Use the sentence starters below to explain clearly why action-reaction pairs do not cancel each other out. Use real examples from the lesson in your answers.

Starter 1: "Forces only cancel each other when they both act on..."

Starter 2: "In a Newton's Third Law pair, the two forces act on different objects, for example, when a rocket launches..."

Starter 3: "This means that even though the forces are the same size, each object can still accelerate because..."

Starter 4: "A way to remember this is to ask: 'The force acts on which object?' because..."

Now use your explanation from page 1 to answer these challenge questions.

Show What You Know

1. You are standing still on a footpath. Your weight pulls you down, and the footpath pushes you up with an equal normal force. Are these two forces a Newton's Third Law force pair? Explain your reasoning carefully, hint: think about which objects the forces act on, and whether there is a true action-reaction relationship.

Challenge 3 marks

2. A student argues: "In the truck-car collision, the truck must push harder on the car, otherwise why does the car get so much more damage?" Evaluate this student's reasoning using both Newton's Second and Third Laws. Write a response that corrects any errors and explains what actually causes the difference in damage.

Challenge 4 marks

Wrap Up

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

HSCScience · Year 7 Science · Unit 3 · Lesson 7 · Challenge Worksheet