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S4 Physics work, energy and power paper 2: ten items on energy transfer
S4 Physics work, energy and power paper 2: ten items on energy transfer
Ten items built on the work-energy relation, covering sliding, braking, hoists and angled pulls.
Direct answers Items 01 to 03 convert work into energy, 04 to 06 cover dissipation and efficiency, 07 to 09 test angled force and squares. Allow 25 minutes.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 Physics Work, Energy and Power |
| Level | S4 |
02 Questions (10 items)
- FoundationA net force of 18 N acts on a 3 kg trolley at rest over 3 m. Find its final speed.
- FoundationA 2 kg ball slides down from 1.8 m with no friction. Find its speed at the bottom (g = 10 N/kg).
- FoundationA motor rated at 200 W input lifts 20 kg at a steady 0.25 m/s. Find its efficiency (g = 10 N/kg).
- IntermediateA block with 90 J of kinetic energy slides 6 m on rough ground and stops. Find the average friction.
- IntermediateA hoist lifts 15 kg at a steady 0.4 m/s against 40 N of friction. Find its output power (g = 10 N/kg).
- IntermediateA 0.2 kg ball falls freely from 5 m. Find its landing speed (g = 10 N/kg, no air resistance).
- ChallengeA 50 N force at 60 degrees to the horizontal pulls an object 4 m along the horizontal. Find the work done.
- ChallengeTwo cars of mass 1000 kg travel at 10 m/s and 20 m/s. Find the ratio of their kinetic energies.
- ChallengeA 2000 W kettle runs for 90 s. Find the electrical energy it uses.
- Error redoA pupil claims the work done is greatest when the force is perpendicular to the motion. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 6 m/s | Work = 18 x 3 = 54 J, and 54 = 1/2 x 3 x v squared, so v = 6 m/s. |
| 2 | 6 m/s | Potential energy 2 x 10 x 1.8 = 36 J becomes kinetic energy, so v = 6 m/s. |
| 3 | 25% | Useful power = mgv = 20 x 10 x 0.25 = 50 W, and 50 / 200 x 100% = 25%. |
| 4 | 15 N | All the kinetic energy goes into overcoming friction: 90 = f x 6, so f = 15 N. |
| 5 | 76 W | Total pull = 150 + 40 = 190 N, so power = 190 x 0.4 = 76 W. |
| 6 | 10 m/s | v = square root of 2gh = 10 m/s; the mass cancels out. |
| 7 | 100 J | W = F x s x cos 60 = 50 x 4 x 0.5 = 100 J. |
| 8 | 1 : 4 | Energy depends on the square of speed, and 100 : 400 reduces to 1 : 4. |
| 9 | 180 000 J | Energy = power x time = 2000 x 90 = 180 000 J. |
| 10 | A perpendicular force does no work at all, because cos 90 is zero. | W = F x s x cos 90 = 0; a perpendicular force only changes direction, never the energy. |
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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