Secondary 3 Combined Science Practice Paper 2

TuitionGoWhere Practice Paper — Answer Key

Combined Science (Secondary 3) — Physical Sciences Focus

Practice Paper Version 2 of 5

Section A: Multiple Choice Questions (10 marks)

1. (b) Energy cannot be created or destroyed, only converted from one form to another.

• [1 mark] for correct answer.
• Marking note: This is the standard statement of the Principle of Conservation of Energy. Option (a) and (c) are incorrect because energy cannot be created or destroyed. Option (d) is incorrect because energy is not "lost" — it is converted to other forms.

2. (c) 100 J

• [1 mark] for correct answer.
• Working: GPE = mgh = 2 × 10 × 5 = 100 J

3. (c) Elastic potential energy

• [1 mark] for correct answer.
• Marking note: A stretched spring stores elastic potential energy due to its deformation.

4. (c) At the bottom of the slope

• [1 mark] for correct answer.
• Marking note: As the ball descends, gravitational potential energy is converted to kinetic energy. At the bottom, all the lost GPE has been converted to KE, so KE is maximum.

5. (c) Natural gas

• [1 mark] for correct answer.
• Marking note: Natural gas is a fossil fuel and is non-renewable. Solar, wind, and hydroelectric are renewable energy sources.

6. (b) 300 J

• [1 mark] for correct answer.
• Working: Useful energy output = 60% × 500 = 0.60 × 500 = 300 J

7. (b) Gravitational potential → Kinetic → Electrical

• [1 mark] for correct answer.
• Marking note: Water held at a height has gravitational potential energy. As it falls, this is converted to kinetic energy, which turns turbines to generate electrical energy.

8. (b) 2.8 m/s

• [1 mark] for correct answer.
• Working: GPE at top = mgh = 0.5 × 10 × 0.4 = 2.0 J. By conservation of energy, KE at bottom = 2.0 J. Using KE = ½mv²: 2.0 = ½ × 0.5 × v² → v² = 8.0 → v = √8.0 ≈ 2.8 m/s

9. (b) Some energy is converted to unwanted forms such as thermal energy due to friction.

• [1 mark] for correct answer.
• Marking note: Energy is conserved overall, but in real machines, some input energy is always converted to thermal energy (e.g., through friction, air resistance, or electrical resistance), reducing the useful output.

10. (b) 36 J

• [1 mark] for correct answer.
• Working: Net force = Applied force − Frictional force = 20 − 8 = 12 N. Net work done = Net force × distance = 12 × 3 = 36 J. Alternative: Work done by applied force = 20 × 3 = 60 J; Work done against friction = 8 × 3 = 24 J; Net work = 60 − 24 = 36 J.

Section B: Structured Response Questions (20 marks)

11. State the Principle of Conservation of Energy. [2]

Answer:

• Energy cannot be created or destroyed. [1]
• It can only be converted from one form to another (or transferred from one object to another). [1]
• Alternative acceptable phrasing: The total energy in a closed system remains constant. [1] Energy can be transformed from one form to another but the total energy remains the same. [1]
• Marking note: Award 1 mark for each correct component. The statement must convey both that energy is not created/destroyed AND that it is converted/transferred. A statement such as "energy is conserved" alone is too vague for full marks at this level.

12. A 3 kg steel ball is dropped from a height of 8 m.

(a) Calculate the gravitational potential energy at the top. [2]

Answer:

• GPE = mgh [1]
• GPE = 3 × 10 × 8 = 240 J [1]
• Marking note: Award 1 mark for correct formula (or correct substitution) and 1 mark for correct answer with unit. Accept g = 9.8 m/s² giving 235.2 J.

(b) Calculate the kinetic energy just before it hits the ground. [1]

Answer:

• By the Principle of Conservation of Energy, KE at bottom = GPE at top = 240 J [1]
• Marking note: The student must state or imply conservation of energy. Simply writing "240 J" without justification may not receive the mark unless part (a) is correct and the link is clear.

(c) Calculate the speed of the ball just before it hits the ground. [2]

Answer:

• KE = ½mv² [1]
• 240 = ½ × 3 × v²
• v² = 160
• v = √160 ≈ 12.6 m/s [1]
• Marking note: Award 1 mark for correct substitution into KE formula and 1 mark for correct final answer. Accept 12.65 m/s or 12.6 m/s. Unit (m/s) is required for the second mark.

13. Roller coaster car on a frictionless track from point A (height 12 m) to point B (ground level).

(a) State the type of energy the car has at point A. [1]

Answer:

• Gravitational potential energy [1]
• Marking note: The car starts from rest, so it has no kinetic energy at A. Accept "GPE" as abbreviation.

(b) Explain why the speed at point B depends only on the height difference and not on the path taken. [3]

Answer:

• At point A, the car has gravitational potential energy (and zero kinetic energy since it starts from rest). [1]
• By the Principle of Conservation of Energy, as the car moves to point B, gravitational potential energy is converted to kinetic energy. [1]
• Since the track is frictionless, no energy is lost to thermal energy. Therefore, the total kinetic energy gained depends only on the total height lost (mgh), not on the shape or length of the path. Hence, the speed at B depends only on the height difference. [1]
• Marking note: Award marks for: (1) identifying the energy at A, (2) applying conservation of energy, (3) linking the height difference to the speed and explaining why the path shape does not matter. The explanation must reference the frictionless condition.

14. Motor lifting a 50 kg load vertically to 6 m in 10 s. Total electrical energy supplied = 4000 J.

(a) Calculate the useful work done in lifting the load. [2]

Answer:

• Useful work done = Gain in GPE = mgh [1]
• = 50 × 10 × 6 = 3000 J [1]
• Marking note: Award 1 mark for correct formula/substitution, 1 mark for correct answer with unit.

(b) Calculate the efficiency of the motor. [2]

Answer:

• Efficiency = (Useful energy output / Total energy input) × 100% [1]
• = (3000 / 4000) × 100% = 75% [1]
• Marking note: Award 1 mark for correct formula or substitution, 1 mark for correct answer. Accept answer as a decimal (0.75) or fraction (3/4) but percentage form is preferred.

(c) Explain what happens to the energy that is not converted to useful work. [1]

Answer:

• The remaining energy (1000 J) is converted to thermal energy (heat) in the motor due to resistance in the coils and friction in the moving parts. [1]
• Marking note: Accept any reasonable explanation involving thermal/heat energy. "Lost as heat" is acceptable. Do not accept "energy is lost" without specifying where it goes.

15. Pendulum bob: mass = 0.2 kg, raised to 0.15 m above lowest point.

(a) Calculate the gravitational potential energy at the highest point. [2]

Answer:

• GPE = mgh [1]
• = 0.2 × 10 × 0.15 = 0.30 J [1]
• Marking note: Award 1 mark for correct formula/substitution, 1 mark for correct answer with unit.

(b) State the kinetic energy at the lowest point. Explain your answer. [2]

Answer:

• KE at lowest point = 0.30 J [1]
• Explanation: By the Principle of Conservation of Energy, the gravitational potential energy at the highest point is entirely converted to kinetic energy at the lowest point (assuming no energy losses to air resistance). [1]
• Marking note: The explanation must reference conservation of energy. Simply stating the value without explanation receives 1 mark only.

(c) Calculate the maximum speed of the bob. [2]

Answer:

• KE = ½mv² [1]
• 0.30 = ½ × 0.2 × v²
• v² = 3.0
• v = √3.0 ≈ 1.73 m/s [1]
• Marking note: Award 1 mark for correct substitution, 1 mark for correct answer with unit. Accept 1.7 m/s or 1.73 m/s.

16. Coal-fired power station: Chemical → Thermal → Kinetic → Electrical energy.

(a) Explain why the overall efficiency is typically only about 35%. [2]

Answer:

• During each energy conversion, some energy is converted to unwanted thermal energy (waste heat). [1]
• Significant thermal energy is lost to the surroundings through the exhaust gases, cooling systems, and friction in the turbines and generators. Therefore, not all the chemical energy in the coal is converted to useful electrical energy. [1]
• Marking note: Award 1 mark for identifying that energy is "lost" as thermal energy during conversions, and 1 mark for explaining where or how this loss occurs. The answer must go beyond simply restating that efficiency is low.

(b) Suggest one way to reduce the environmental impact of using coal. [1]

Answer: (Any one of the following)

• Use renewable energy sources (e.g., solar, wind) instead of coal. [1]
• Install scrubbers/filters to remove sulfur dioxide and other pollutants from exhaust gases. [1]
• Use the waste heat for district heating (combined heat and power) to improve overall efficiency. [1]
• Switch to cleaner fuels such as natural gas. [1]
• Marking note: Accept any reasonable suggestion that addresses environmental impact. "Reduce coal use" alone is too vague — the student should suggest an alternative or a mitigation method.

Section C: Data-Based / Application Question (10 marks)

17. Calculate the gravitational potential energy at the drop height of 2.0 m. [2]

Answer:

• GPE = mgh [1]
• = 0.4 × 10 × 2.0 = 8.0 J [1]
• Marking note: Award 1 mark for correct formula/substitution, 1 mark for correct answer with unit.

18. Calculate the gravitational potential energy at the rebound height of 1.5 m. [2]

Answer:

• GPE = mgh [1]
• = 0.4 × 10 × 1.5 = 6.0 J [1]
• Marking note: Award 1 mark for correct formula/substitution, 1 mark for correct answer with unit.

19. Determine the energy lost during the bounce. Explain where this energy has gone. [3]

Answer:

• Energy lost = GPE at drop height − GPE at rebound height [1]
• = 8.0 − 6.0 = 2.0 J [1]
• The 2.0 J of energy has been converted to thermal energy (and some sound energy) during the deformation of the ball and the floor at the point of impact. [1]
• Marking note: Award 1 mark for correct calculation method, 1 mark for correct value, 1 mark for explanation. Accept "heat energy" for thermal energy. Sound energy may also be mentioned but is not required.

20. Evaluate the student's claim that the Principle of Conservation of Energy has been violated. [3]

Answer:

• The student's claim is incorrect / not valid. [1]
• The Principle of Conservation of Energy states that energy cannot be created or destroyed, only converted from one form to another. The total energy in the system is still conserved. [1]
• The "missing" 2.0 J of energy has not disappeared — it has been converted to thermal energy (and sound energy) during the bounce. If we account for all forms of energy, the total energy before and after the bounce remains the same. [1]
• Marking note: Award 1 mark for stating the claim is wrong, 1 mark for correctly stating the principle, 1 mark for explaining that the energy was converted to other forms (thermal/sound) and was not destroyed. The answer must clearly address the misconception.

END OF ANSWER KEY

Total Marks: 40