Year 7 Science · Unit 3 · Lesson 7
Challenge Worksheet
Learning Goals
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.
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.
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.
Wrap Up
In one sentence, what was the main idea of this lesson?