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S4 Physics: Newton’s Laws of Motion Paper 3 (third law, momentum and tension)
S4 Physics: Newton’s Laws of Motion Paper 3 (third law, momentum and tension)
Paper 3 works with force pairs: action and reaction, then momentum in collisions and tension in a rope.
Quick answer Equal and opposite on two different bodies; the oar pushes water; each reads 20 N; the pair acts on different bodies; 1.2 m/s; 4 m/s; 72 N; 8 N; 4.5 m/s; the pair never cancels.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | 25 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Physics: Newton’s Laws of Motion |
| Level | S4 |
02 Questions (10 items)
- FoundationWhat three features does Newton third law give to an action and reaction pair?
- FoundationA rower pushes water backward with the oar and the boat moves forward. Explain with the third law.
- FoundationTwo spring scales are pulled against each other. Are their readings equal, and why?
- FoundationWhen a person pushes a wall the wall pushes back. Why does the person move instead of staying still?
- IntermediateA 2 kg trolley at 3 m/s hits a stationary 3 kg trolley and they stick together. Find the common velocity.
- IntermediateA 4 kg trolley at 5 m/s hits a stationary 1 kg trolley and they stick. Find the final velocity.
- IntermediateA 0.2 kg ball hits a wall at 10 m/s and rebounds at 8 m/s in 0.05 s. Find the average force from the wall.
- ChallengeA 20 N horizontal force pulls a 2 kg and a 3 kg block joined by a rope on a smooth floor. Find the acceleration and the tension.
- ChallengeA 60 kg person jumps off a stationary 40 kg trolley at 3 m/s. Find the recoil speed of the trolley.
- Error redoA student writes that action and reaction cancel, so a pushed cart cannot move. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | Equal in size, opposite in direction and acting on two different bodies | All three are needed, and the third explains why the pair never cancels out. |
| 2 | The water pushes the oar forward with an equal force, carrying the boat along | The oar on water and water on oar form an action and reaction pair of equal size and opposite direction. |
| 3 | Yes, both read the same size of force | An action and reaction pair is equal in size, so the two readings match. |
| 4 | The two forces act on different bodies, so only the wall force acts on the person and he accelerates backward | Cancellation needs both forces on one body; here each body gets its own acceleration. |
| 5 | 1.2 m/s | Momentum conservation gives 6 as 5 times v, so v is 1.2 m/s in the original direction. |
| 6 | 4 m/s | The total momentum is 20, the combined mass is 5, so the speed is 4 m/s. |
| 7 | 72 N | The momentum change is 3.6 units and dividing by 0.05 s gives 72 N. |
| 8 | 4 m/s² with tension 8 N | The pair accelerates at 4 m/s2, and for the 2 kg block the rope tension alone is the net force, so it is 8 N. |
| 9 | 4.5 m/s | Total momentum starts at zero, so 180 equals 40 times v and the trolley recoils at 4.5 m/s. |
| 10 | The pair acts on different bodies and never cancels, so the cart feels only the push and friction. | Only forces on one body can be added and cancelled, and adding a pair across two bodies is a conceptual error. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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