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Secondary 2 Science Physical Sciences Quiz
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Secondary 2 Science Quiz - Physical Sciences (Answer Key)
Total Marks: 50
Section A: Multiple Choice Questions (10 marks)
1. B — Gravitational potential energy is converted to kinetic energy only.
Explanation: With no air resistance, mechanical energy is conserved. GPE converts entirely to KE as the ball falls. No heat is produced.
2. C — 100 J
Working: Work done = Force × Distance = 25 N × 4 m = 100 J.
3. B — 108 000 J
Working: Energy = Power × Time = 60 W × (30 × 60 s) = 60 × 1800 = 108 000 J.
4. B — 24 000 W
Working: KE gained = ½mv² = ½ × 1200 × 20² = 240 000 J.
Average power = Work/Time = 240 000 J / 10 s = 24 000 W.
5. A — 0.125 J
Working: Compression x = F/k = 10/200 = 0.05 m.
Elastic PE = ½kx² = ½ × 200 × (0.05)² = 100 × 0.0025 = 0.25 J.
Wait, recalculating: ½ × 200 × 0.0025 = 0.25 J. That's option B. Let me recheck.
F = kx → x = F/k = 10/200 = 0.05 m. EPE = ½kx² = ½ × 200 × 0.05² = 100 × 0.0025 = 0.25 J.
Correct answer: B — 0.25 J
6. A — Work
Explanation: Work is a process of energy transfer, not a form of energy itself. Heat, kinetic energy, and gravitational potential energy are all forms of energy.
7. B — Position B only
Explanation: At the lowest point (B), gravitational potential energy is minimum, so kinetic energy is maximum (by conservation of energy).
8. D — 83.3%
Working: Work output = Load × Load distance = 500 N × 2 m = 1000 J.
Work input = Effort × Effort distance = 200 N × 6 m = 1200 J.
Efficiency = (Output/Input) × 100% = (1000/1200) × 100% = 83.3%.
9. B — 144 J
Working: Work done = Change in KE = ½m(v² - u²) = ½ × 2 × (13² - 5²) = 1 × (169 - 25) = 144 J.
10. C — Gravitational potential → Electrical
Explanation: Water at height has GPE, which converts to KE as it falls, then to electrical energy via turbines and generators.
Section B: Structured Questions (24 marks)
11. (a) Principle of conservation of energy: Energy cannot be created or destroyed. It can only be converted from one form to another, and the total amount of energy in a closed system remains constant. [2]
Marking points: (1) Energy cannot be created or destroyed [1]; (2) Energy can be converted/transformed from one form to another OR total energy remains constant [1].
(b) Speed at 10 m height:
Loss in GPE = Gain in KE
mg(h₁ - h₂) = ½mv²
v² = 2g(h₁ - h₂) = 2 × 10 × (40 - 10) = 20 × 30 = 600
v = √600 = 24.5 m/s (or 10√6 m/s) [2]
Marking: Correct formula/substitution [1]; Correct answer with unit [1].
12. (a) Work done by student:
Work = Force × Distance × cosθ = 60 N × 10 m × cos 30° = 600 × 0.866 = 519.6 J ≈ 520 J [2]
Marking: Correct use of cos 30° [1]; Correct calculation and unit [1].
(b) Friction force (constant velocity → net force = 0):
Horizontal component of pull = F cos 30° = 60 × 0.866 = 51.96 N
Friction = Horizontal component = 52.0 N (or 51.96 N) [2]
Marking: Recognise constant velocity → balanced forces [1]; Correct value with unit [1].
13. (a) Initial KE:
KE = ½mv² = ½ × 0.2 × 20² = 0.1 × 400 = 40 J [1]
(b) Maximum height:
Initial KE = Final GPE (at max height, v = 0)
40 = mgh = 0.2 × 10 × h = 2h
h = 20 m [2]
Marking: Equating KE to GPE [1]; Correct answer with unit [1].
(c) Energy conversion: Kinetic energy → Gravitational potential energy [1]
14. (a) Energy required:
Q = mcΔθ = 1.5 × 4200 × (100 - 25) = 1.5 × 4200 × 75 = 472 500 J [2]
Marking: Correct formula and substitution [1]; Correct answer with unit [1].
(b) Minimum time:
Energy = Power × Time → Time = Energy / Power = 472 500 / 2000 = 236.25 s ≈ 236 s (or 3 min 56 s) [2]
Marking: Correct rearrangement [1]; Correct answer with unit [1].
15. (a) Elastic PE stored:
EPE = ½kx² = ½ × 150 × (0.1)² = 75 × 0.01 = 0.75 J [2]
Marking: Correct formula [1]; Correct answer with unit [1].
(b) Maximum speed of block:
EPE → KE (smooth surface, no losses)
0.75 = ½mv² = ½ × 0.5 × v² = 0.25v²
v² = 0.75 / 0.25 = 3
v = √3 = 1.73 m/s [2]
Marking: Equating EPE to KE [1]; Correct answer with unit [1].
16. (a) Work done by crane:
Work = Force × Distance = Weight × Height = mg × h = 800 × 10 × 15 = 120 000 J [2]
Marking: Correct formula (Work = mgh) [1]; Correct answer with unit [1].
(b) Average power output:
Power = Work / Time = 120 000 / 20 = 6000 W [2]
Marking: Correct formula [1]; Correct answer with unit [1].
Section C: Longer Structured Questions (16 marks)
17. (a) Speed at point B (h = 0 m):
Loss in GPE from A to B = Gain in KE
mg(h_A - h_B) = ½mv_B²
v_B² = 2g(h_A - h_B) = 2 × 10 × (0.5 - 0) = 10
v_B = √10 = 3.16 m/s [2]
Marking: Correct energy conservation equation [1]; Correct answer with unit [1].
(b) Speed at point C (h = 0.3 m):
Loss in GPE from A to C = Gain in KE
mg(h_A - h_C) = ½mv_C²
v_C² = 2g(h_A - h_C) = 2 × 10 × (0.5 - 0.3) = 20 × 0.2 = 4
v_C = 2 m/s [2]
Marking: Correct height difference used [1]; Correct answer with unit [1].
(c) Normal reaction at top of loop:
At the top of the loop, the centripetal force required is provided by weight + normal reaction: mg + N = mv²/r. For the car to maintain contact with the track, N ≥ 0. If N < 0, the car would lose contact and fall. [1]
Key idea: Contact with track requires N ≥ 0.
18. (a) Initial KE of car:
KE = ½mv² = ½ × 1200 × 25² = 600 × 625 = 375 000 J [2]
Marking: Correct formula [1]; Correct answer with unit [1].
(b) Average braking force:
Work done by brakes = Loss in KE (Work-Energy Theorem)
F × d = 375 000
F × 50 = 375 000
F = 7500 N [2]
Marking: Work-energy principle applied [1]; Correct answer with unit [1].
(c) Energy conversion during braking:
Kinetic energy is converted to heat energy (and sound) due to friction between brake pads and discs, and between tyres and road. [1]
19. (a) GPE lost per second:
Mass per second = 500 kg/s
GPE lost per second = mgh = 500 × 10 × 80 = 400 000 J/s = 400 000 W [2]
Marking: Correct interpretation of rate [1]; Correct answer with unit [1].
(b) Electrical power output:
Output power = Efficiency × Input power = 0.80 × 400 000 = 320 000 W = 320 kW [2]
Marking: Correct use of efficiency [1]; Correct answer with unit [1].
(c) Forms of "lost" energy:
Heat energy (due to friction in turbines, generators, and water turbulence) and sound energy. [2]
Marking: Any two valid forms: heat, sound, kinetic energy of splashing water, etc. [1 each].
20. (a) Expression for maximum height h:
Elastic PE at max compression = GPE at max height (conservation of energy)
½kx² = mgh
h = kx² / (2mg) [2]
Marking: Correct energy equation [1]; Correct rearrangement for h [1].
(b) Height for x = 0.08 m:
h = (250 × 0.08²) / (2 × 0.05 × 10) = (250 × 0.0064) / 1 = 1.6 / 1 = 1.6 m [2]
Marking: Correct substitution [1]; Correct answer with unit [1].
(c) Reasons for lower measured height:
- Air resistance acts on the toy during upward motion, converting some mechanical energy to heat.
- Friction between the toy and the launcher guide/spring converts some energy to heat.
- The spring has mass, so some elastic PE becomes KE of the spring itself.
- Energy losses as sound during launch.
(Any two valid reasons) [2]
Marking: Each valid reason with brief explanation [1 each].
End of Answer Key