Why do challenge items take two steps?

Find the energy change first, then divide by distance to get force, or reverse it; neither step can be skipped.

What does efficiency above 100% mean?

Nothing real: it means an energy source was missed or a measurement is wrong.

S4 Physics work, energy and power paper 3: ten multi-step items

2026-11-10

Tutor Prep Guide · S4 Physics Work, Energy and Power

Ten items in DSE long-question style, linking kinetic energy change, net force, braking distance and efficiency.

Direct answers　Items 01 and 02 turn an energy difference into force or speed, 03 to 05 mix power with braking, and 08 links momentum to energy. 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 1200 kg car speeds up from 10 m/s to 25 m/s. Find the energy needed, and the average net force if it covers 150 m.
FoundationAn 800 kg coaster drops 12.8 m without friction. Find the potential energy lost and the speed at the lowest point (g = 10 N/kg).
FoundationA pump raises 600 kg of water through 8 m each minute. Find its average output power (g = 10 N/kg).
IntermediateAn engine of 30% efficiency takes 360 000 J and accelerates a 1500 kg car from rest. Find the final speed, ignoring other losses.
IntermediateA 900 kg car travelling at 20 m/s brakes with 3000 N. Find the stopping distance.
IntermediateA cyclist and bike of total mass 80 kg climb 5 m in 4 s. Find the average power (g = 10 N/kg).
ChallengeA 2 kg ball falls from 3.2 m. Find its landing speed (g = 10 N/kg, no air resistance).
ChallengeTwo objects have the same momentum and masses in the ratio 4 : 9. Find the ratio of their kinetic energies.
ChallengeA motor drawing 300 W lifts 40 kg at a steady 0.5 m/s. Find its efficiency (g = 10 N/kg).
Error redoA pupil claims a machine can reach 120% efficiency. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	315 000 J; 2100 N	Gain in kinetic energy = 1/2 x 1200 x (625 – 100) = 315 000 J, and the net force is 315 000 / 150 = 2100 N.
2	102 400 J; 16 m/s	mgh = 800 x 10 x 12.8 = 102 400 J, and v = square root of 2 x 10 x 12.8 = 16 m/s.
3	800 W	Work per minute = 600 x 10 x 8 = 48 000 J, divided by 60 s gives 800 W.
4	12 m/s	Useful energy = 0.30 x 360 000 = 108 000 J, so v = square root of (2 x 108 000 / 1500) = 12 m/s.
5	60 m	Kinetic energy = 1/2 x 900 x 400 = 180 000 J, so the distance is 180 000 / 3000 = 60 m.
6	1000 W	Work = 80 x 10 x 5 = 4000 J, divided by 4 s gives 1000 W.
7	8 m/s	v = square root of 2 x 10 x 3.2 = 8 m/s.
8	9 : 4	Kinetic energy = p squared / 2m, so at equal momentum energy is inversely proportional to mass.
9	about 67%	Useful power = 40 x 10 x 0.5 = 200 W, and 200 / 300 is about 66.7%, that is about 67%.
10	Efficiency above 100% is impossible because energy cannot be created.	Output energy can never exceed input energy; any excess means another source or a measurement 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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