Year 7 Science · Unit 3 · Lesson 7
Extension · Challenge Worksheet
Learning Goals
Extension · optional
Newton's Third Law sits in Stage 5 (SC5-WAM-01), beyond the Stage 4 Forces dot points this unit is built on. Nothing on this sheet is assessed as part of Year 7 Forces, so it is enrichment rather than required practice. Do it if you are interested, and skip it without losing any marks if you are not.
1. An action and its reaction always share some features. List three of them.
Pair or not?
For each row, decide: are these two forces action and reaction partners? On the lines, write the row number, Yes or No, and a reason.
| Row | The two forces |
|---|---|
| 1 | A dog pulls its lead forward; the owner pulls the same lead backward, and the lead does not move. |
| 2 | A hammer pushes a nail into wood; the nail pushes back on the hammer. |
| 3 | Earth pulls an apple down; the apple pulls Earth up. |
| 4 | The wind pushes a kite up; the kite string pulls the kite down. |
Find the mistakes
A student wrote this answer
“When a netball player catches a fast pass, the ball pushes her hands back. That is the action force. A moment later her hands push back on the ball, which is the reaction force. Because the player is much heavier than the ball, her push on the ball is bigger than the ball's push on her. The two forces cancel, and that is why the ball stops.”
2. The answer contains three different errors. For each one, quote the error and write the correct science.
3. You stand still on a footpath. Your weight pulls you down, and the footpath pushes you up with an equal force. Are these two forces a Third Law pair? Explain, and name the true Third Law partner of each force.
Read the data
Scenario: trolley collisions
In a lab, two trolleys X and Y roll toward each other and collide. A force sensor on each bumper records the biggest force that trolley feels. The class also measures how much each trolley's speed changes in the collision.
| Collision | Masses X, Y (kg) | Biggest force on X, Y (N) | Change in speed of X, Y (m/s) |
|---|---|---|---|
| 1 | 1.0, 1.0 | 6.0, 6.0 | 0.8, 0.8 |
| 2 | 1.0, 2.0 | 8.0, 8.0 | 1.2, 0.6 |
| 3 | 0.5, 2.0 | 7.5, 7.5 | 1.6, 0.4 |
Source: made-up results for this worksheet.
4. Riley claims: “In collision 3, trolley Y hit harder, because it is four times heavier.” Use Table 1 to evaluate Riley's claim.
5. In collision 3, trolley X's speed changed four times as much as trolley Y's. Explain this using both the Second and the Third Laws.
6. A bug hits the windscreen of a moving car. Priya says: “The windscreen hits the bug much harder than the bug hits the windscreen. That is why the bug is squashed and the car does not even slow down.” Evaluate Priya's claim using Newton's Second and Third Laws.
Question 1
Equal in size; opposite in direction; act on different objects (also accept: happen at the same instant). Three correct = 2; two = 1. [2 marks]
Pair or not?
1 each. 1. No: both forces act on the lead. 2. Yes: one acts on the nail, one on the hammer. 3. Yes: one acts on the apple, one on Earth. 4. No: both act on the kite. [4 marks]
Find the mistakes: Question 2
1 each: “a moment later”: both forces happen at the same instant; “her push is bigger”: they are equal whatever the masses; “the two forces cancel”: they act on different objects; the ball stops as her hands' push on it is unbalanced. [3 marks]
Question 3
No: both forces act on you, so they cancel and are not a pair (1). Partner of your weight: you pull Earth up (1). Partner of the footpath's push: your feet push down on the footpath (1). [3 marks]
Question 4
Wrong: the biggest forces on X and Y were both 7.5 N (1). By the Third Law the forces are equal whatever the masses (1). [2 marks]
Question 5
Third Law: both feel the same 7.5 N (1). X's mass, 0.5 kg, is a quarter of Y's 2.0 kg (1). Second Law: the same force on a smaller mass gives a larger acceleration, so X's speed changes more (1). [3 marks]
Question 6
Marking guide: 1 judgement (right about the effects, wrong about the forces); 1 Third Law: equal forces; 1 opposite, on different objects; 1 Second Law: the bug's tiny mass gets a huge acceleration; 1 the car's huge mass gets a tiny, unnoticeable one; 1 conclusion: damage is not evidence of a bigger force. [6 marks]
Model answer: Priya is right about the effects but wrong about the forces. The windscreen and the bug push each other equally hard, in opposite directions, one force on the bug and one on the windscreen. The same force gives the bug's tiny mass a huge acceleration but barely changes the car's motion: equal forces, very different effects. The squashed bug is not evidence of a bigger force.
Common error: using the size of the damage as proof of a bigger force.