Year 7 Science · Unit 3 · Lesson 7
Extension · Foundation 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.
Fill the gap
Choose the correct word or phrase from the word bank. Each one is used at most once. Two are not needed: cross them out.
Newton's Third Law says that forces always come in a . When you push on a wall, the wall pushes back on you with an force in the direction. The two forces happen at the : there is no delay between them. The two forces act on objects, so they can never each other out.
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. The reaction force comes a moment after the action force.
Correct it:
2. When a tennis racket hits a ball, the racket pushes on the ball harder than the ball pushes on the racket.
Correct it:
3. If you push a brick wall with a 50 N force, the wall pushes back on you with a 50 N force.
Correct it:
4. Your push on a wall and the wall's push on you both act on you.
Correct it:
5. A rocket can speed up in space, even though there is no air there to push against.
Correct it:
1. Mia pushes a skateboard ramp with a 30 N force. How big is the force the ramp puts on Mia, and which way does it point? On which object does each of the two forces act?
Sort it: which object does the force act on?
For each force, ask “what is being pushed or pulled?” Then write the force in the correct box. The boxes do not hold the same number of items.
Acts on a person
Acts on something else
Read the data
Scenario: two spring scales hooked together
Ava holds scale A and Jin holds scale B. The scales are hooked together and the two students pull in opposite directions. Each scale shows the force on it.
| Pull | What they did | Scale A (N) | Scale B (N) |
|---|---|---|---|
| 1 | Both pull gently | 4 | 4 |
| 2 | Jin tries to pull harder | 9 | 9 |
| 3 | Ava tries to pull harder | 15 | 15 |
| 4 | Jin lets go; scale B is hooked on a door handle | 12 | 12 |
Source: sample class results.
2. Describe the pattern in Table 1. Which part of Newton's Third Law does it show?
3. After pull 2, Jin says: “I pulled harder, so my scale should show a bigger number.” Use Table 1 to explain why Jin is wrong.
4. In pull 4 nobody holds scale B and the door handle does not move. Explain how scale B can still show 12 N.
5. Oliver says: “If every force has an equal and opposite partner, the two should cancel out, so nothing could ever start moving.” Kai stands on a skateboard and pushes a wall, and he rolls away. Use Kai's push to explain why Oliver is wrong.
Fill the gap
pair; equal; opposite; same instant; different; cancel. Not used: same object, later.
True or false
1. False: they happen at the same instant. 2. False: the ball pushes the racket with an equal force; it flies off as its mass is small. 3. True. 4. False: your push acts on the wall and the wall's push acts on you: different objects. 5. True: exhaust gas pushes the rocket forward; no air needed.
Sort it
Acts on a person: wall pushes Kai's hands; block pushes swimmer; boat pushes jumper; ball pushes bowler's hand. Acts on something else: swimmer's feet push block; bowler's hand pushes ball; Kai's hands push wall.
Question 1
30 N, opposite to Mia's push (1). Mia's push acts on the ramp; the ramp's push on Mia (1). [2 marks]
Question 2
Both scales always read the same (1). Paired forces are always equal (1). [2 marks]
Question 3
In pull 2 both scales read 9 N (1). The scales pull on each other, a force pair, so the forces are equal whoever tries harder (1). [2 marks]
Question 4
The door handle pulls back on scale B with an equal 12 N (1). A fixed object pulls back without moving, like the lesson's wall (1). [2 marks]
Question 5
Marking guide: 1 Oliver is wrong; 1 Kai pushes the wall; 1 the wall pushes Kai back equally; 1 forces cancel only on one object, so nothing cancels the push on Kai and he speeds up; 1 wall fixed to Earth: huge mass, no noticeable acceleration. [5 marks]
Model answer: Oliver is wrong. Kai and the wall push each other equally hard, on different objects, so nothing cancels the wall's push and Kai rolls away; the wall, fixed to the Earth, has an enormous mass, so the same force gives it no noticeable acceleration.
Common error: thinking the stronger or heavier object puts on the bigger force.