Year 7 Science · Unit 3 · Lesson 7
Extension · Apply 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. In your own words, what does Newton's Third Law say? What single check tells you whether two forces are partners under this law?
Match each event to its force pair
Write the letter of the correct force pair next to each event. Each letter is used once.
| Event | Letter | Force pair |
|---|---|---|
| A kookaburra flying | A. Feet push the boat backward; the boat pushes the person forward. | |
| A fast bowler releasing the ball | B. Feet push the block backward; the block pushes the swimmer forward. | |
| Jumping from a boat onto a jetty | C. A hand pushes the ball forward; the ball pushes back on the hand. | |
| A swimmer diving off the starting block | D. Feet push the floor down; the floor pushes the feet up. | |
| Standing still on the floor | E. Wings push air down; the air pushes the bird up. |
Order the steps: a rocket launch
Number the events from 1 (first) to 5 (last).
| Step number | Event |
|---|---|
| At the same instant, the engine pushes hot gas down out of the nozzle and the gas pushes the rocket up. | |
| Fuel in the rocket engine starts to burn. | |
| The upward push on the rocket is bigger than its weight, so the rocket speeds up off the launch pad. | |
| The rocket clears the launch tower and keeps speeding up while the fuel burns. | |
| The burning makes a large amount of hot gas that expands inside the engine. |
2. At take-off, a netball player's feet push down on the court with a force of 900 N. Describe the reaction force: its size, its direction and the object it acts on. What happens to the player as a result?
Read the data
Scenario: pushing apart on skateboards
Ella sits on a skateboard facing a partner on another skateboard. They push palm to palm for the same short time, and force sensors on their hands measure the push on each person. The class then measures how fast each person rolls away.
| Pair | Masses (kg) | Force on each person (N) | Speed after the push (m/s) |
|---|---|---|---|
| 1 | Ella 40, Sam 40 | Ella 80, Sam 80 | Ella 1.0, Sam 1.0 |
| 2 | Ella 40, Dad 80 | Ella 80, Dad 80 | Ella 1.0, Dad 0.5 |
| 3 | Ella 40, Mia 50 | Ella 80, Mia 80 | Ella 1.0, Mia 0.8 |
Source: made-up results for this worksheet.
3. Look at the middle column of Table 1. What do you notice about the two readings in each row, and which of Newton's laws explains it?
4. In pair 2, Dad rolls away at half Ella's speed. Use Table 1 to explain why, even though the forces are equal.
5. Ella pushes apart in the same way from her 20 kg cousin. Predict the speed of each of them, and give your reason.
6. Lachlan swims laps. At the end of the pool he tucks, puts his feet flat on the wall and pushes off. His friend asks: “What pushes you forward, the wall or the water?” Write Lachlan a full answer using Newton's Third Law.
Question 1
For every action force there is an equal and opposite reaction force (1). Ask: which object does each force act on? A pair acts on two different objects (1). [2 marks]
Match each event to its force pair
A kookaburra flying → E; A fast bowler releasing the ball → C; Jumping from a boat onto a jetty → A; A swimmer diving off the starting block → B; Standing still on the floor → D.
Order the steps
Reading down the table as printed, the correct step numbers are 3, 1, 4, 5, 2. In order: fuel burns; hot gas expands; engine pushes gas down and gas pushes rocket up at the same instant; rocket speeds up off the pad; rocket clears the tower.
Question 2
900 N (1), upward, acting on the player (1). The upward push is bigger than her weight, so she speeds up upward and leaves the court (1). [3 marks]
Question 3
The forces are equal in every pair, 80 N on each person (1). Newton's Third Law (1). [2 marks]
Question 4
Both feel 80 N (1). Dad's mass, 80 kg, is twice Ella's 40 kg (1). The same force on a bigger mass gives a smaller acceleration, so he ends up slower (Second Law) (1). [3 marks]
Question 5
Ella 1.0 m/s, as in every pair (1). Cousin 2.0 m/s: half Ella's mass, so twice her speed (1). [2 marks]
Question 6
Marking guide: 1 the wall; 1 feet push the wall backward (on the wall); 1 the wall pushes his feet forward (on Lachlan); 1 equal size, opposite direction, same instant; 1 nothing cancels the wall's push on him, so he speeds up; 1 the wall is fixed to the Earth, a huge mass, so it does not noticeably move. [6 marks]
Model answer: The wall. My feet push the wall backward and, at the same instant, the wall pushes my feet forward just as hard. The forces act on different objects, so nothing cancels the wall's push on me and I speed up. The wall is fixed to the Earth, a huge mass, so it does not noticeably move.
Common error: treating the reaction as coming after the action.