Secondary 1 Science Semestral Assessment 2 (End of Year) Paper 4

TuitionGoWhere Practice Paper - Science Secondary 1

SA2 Practice – Version 4 of 5: Answer Key & Marking Scheme

Section A: Multiple Choice Questions [10 marks]

Section B: Structured Response Questions [25 marks]

11. [5 marks]

(a) [2 marks]

Work done = Force × Distance (in the direction of force) Work done = 40 × 3.0 = 120 J

• 1 mark for correct formula/substitution
• 1 mark for correct answer with unit (J)

(b) [1 mark]

Chemical energy → Gravitational potential energy

• Award 1 mark for correct energy conversion. Accept "muscle energy" in place of "chemical energy".

(c) [2 marks]

No work is done because there is no displacement of the bag in the direction of the applied force. Work is defined as force multiplied by distance moved in the direction of the force. Since the bag is stationary, the distance moved is zero, so the work done is zero.

• 1 mark for stating that there is no displacement / distance moved is zero
• 1 mark for linking this to the definition of work (W = F × d, where d = 0)

Common mistake: Students may say "no force is applied" — this is incorrect. A force IS applied (to hold the bag up), but no displacement occurs.

12. [5 marks]

(a) [2 marks]

Position P has the greatest gravitational potential energy. This is because gravitational potential energy depends on height above the reference level (GPE = mgh), and P is at the highest point.

• 1 mark for identifying P
• 1 mark for correct explanation linking height to GPE

(b) [2 marks]

Position Q has the greatest kinetic energy. This is because as the car descends from P to Q, gravitational potential energy is converted to kinetic energy. At Q (the lowest point), the car has the greatest speed and therefore the greatest kinetic energy.

• 1 mark for identifying Q
• 1 mark for correct explanation linking lowest point / greatest speed to KE

(c) [1 mark]

The principle of conservation of energy. Energy is converted from gravitational potential energy to kinetic energy (and vice versa), but the total mechanical energy remains constant (in the absence of friction).

• Accept "conservation of energy" or "law of conservation of energy".

13. [5 marks]

(a) [1 mark]

Weight = mass × g = 50 × 10 = 500 N

• 1 mark for correct answer with unit

(b) [2 marks]

GPE = mgh = 50 × 10 × 6.0 = 3000 J

• 1 mark for correct substitution
• 1 mark for correct answer with unit (J)

(c) [2 marks]

Power = Work done / Time = 3000 / 12 = 250 W

• 1 mark for correct formula or substitution
• 1 mark for correct answer with unit (W)

Common mistake: Students may use the horizontal distance instead of vertical height for GPE calculation.

14. [5 marks]

(a) [1 mark]

Gravitational potential energy

• Accept "potential energy" or "GPE". Do NOT accept "kinetic energy" — the bob is momentarily at rest.

(b) [2 marks]

As the bob swings from A to B, gravitational potential energy is converted to kinetic energy. At A, the bob has maximum GPE and zero KE. At B (the lowest point), the bob has maximum KE and minimum GPE.

• 1 mark for identifying the two energy forms involved
• 1 mark for describing the conversion direction (GPE → KE)

(c) [2 marks]

In real life, some of the mechanical energy is converted to thermal energy (heat) and sound energy due to air resistance and friction at the pivot point. Therefore, the total mechanical energy gradually decreases, and the bob cannot reach the same height at C as at A.

• 1 mark for identifying energy loss (to heat/sound)
• 1 mark for linking energy loss to the reduced height

15. [6 marks]

(a) [2 marks]

Useful work output = Force (load) × Distance moved by load = 300 × 2.0 = 600 J

• 1 mark for correct substitution
• 1 mark for correct answer with unit (J)

(b) [2 marks]

Total work input = Effort × Distance moved by effort = 150 × 5.0 = 750 J

• 1 mark for correct substitution
• 1 mark for correct answer with unit (J)

(c) [2 marks]

Efficiency = (Useful work output / Total work input) × 100% = (600 / 750) × 100% = 80%

• 1 mark for correct formula/substitution
• 1 mark for correct answer (%)

Common mistake: Students may invert the fraction (input/output) — this gives an efficiency > 100%, which is not physically possible.

16. [4 marks]

(a) Work done [2 marks]

Work done is defined as the product of the force applied on an object and the distance moved by the object in the direction of the force. The SI unit of work is the Joule (J).

• 1 mark for correct definition (force × distance in direction of force)
• 1 mark for correct unit (Joule / J)

(b) Power [2 marks]

Power is defined as the rate of doing work (or the amount of work done per unit time). The SI unit of power is the Watt (W).

• 1 mark for correct definition (rate of work / work per unit time)
• 1 mark for correct unit (Watt / W)

17. [4 marks]

(a) [2 marks]

Work done by the boy = Applied Force × Distance = 60 × 4.0 = 240 J

• 1 mark for correct substitution
• 1 mark for correct answer with unit (J)

(b) [2 marks]

Net force = Applied force − Frictional force = 60 − 20 = 40 N

Net work done = Net force × Distance = 40 × 4.0 = 160 J

• 1 mark for calculating net force correctly
• 1 mark for correct net work with unit (J)

Alternative method: Work done against friction = 20 × 4.0 = 80 J. Net work = 240 − 80 = 160 J. Award full marks for this approach.

18. [2 marks]

Advantage (1 mark): A machine can reduce the effort needed (provide a mechanical advantage) / change the direction of the applied force / allow work to be done more conveniently.

Disadvantage (1 mark): A machine is never 100% efficient — some energy is always lost to friction (as heat) / machines can be expensive to build and maintain.

• Award 1 mark for a valid advantage and 1 mark for a valid disadvantage.

Section C: Data-Based / Application Question [15 marks]

19. Investigation — Mass and Work Done [8 marks]

(a) [1 mark]

The mass of the object (in kg).

• Accept "mass".

(b) [1 mark]

The work done (in J).

• Accept "work done in lifting the object".

(c) [2 marks]

Weight of 5.0 kg object = 5.0 × 10 = 50 N

Work done = Force × Distance = 50 × 2.0 = 100 J

• 1 mark for calculating weight (or using g = 10 N/kg)
• 1 mark for correct answer (100 J)

Note: The table uses g = 10 N/kg (Weight = Mass × 10).

(d) [1 mark]

The work done is directly proportional to the mass of the object. As the mass increases, the work done increases proportionally.

• Accept "work done increases as mass increases" or "directly proportional".

(e) [2 marks]

Using any row from the table (e.g., first row):

Work done = Force × Height 20 = 10 × Height Height = 20 / 10 = 2.0 m

• 1 mark for correct formula/substitution
• 1 mark for correct answer (2.0 m)

(f) [1 mark]

Any one of the following:

• The height to which the objects were lifted
• The value of g (gravitational field strength)
• The surface/level from which objects were lifted (same starting point)

20. Hydroelectric Power Station [7 marks]

(a) [3 marks]

Step 1: Calculate weight of water Weight = mass × g = 500 000 × 10 = 5 000 000 N

Step 2: Calculate GPE lost GPE = Weight × Height = 5 000 000 × 80 = 400 000 000 J (or 4.0 × 10⁸ J)

• 1 mark for calculating weight correctly
• 1 mark for correct formula (GPE = mgh or Weight × h)
• 1 mark for correct answer with unit (J)

Alternative: GPE = mgh = 500 000 × 10 × 80 = 400 000 000 J. Award 3 marks for correct single-step calculation.

(b) [2 marks]

Useful electrical energy output = 80% × 400 000 000 = 320 000 000 J (or 3.2 × 10⁸ J)

• 1 mark for correct method (80% of input energy)
• 1 mark for correct answer with unit (J)

(c) [2 marks]

Two reasons why efficiency is less than 100%:

1. Friction between moving parts of the turbine and generator converts some mechanical energy into thermal energy (heat).
2. Air resistance / friction in the penstocks (pipes) causes some energy to be lost as heat.
3. Sound energy is produced by the moving water and machinery.
4. Some energy is used to overcome friction in the bearings and other mechanical components.

• Award 1 mark for each valid reason, up to 2 marks.

(d) [1 mark]

Any one of the following:

• It is a renewable energy source (water cycle is continuous).
• It does not produce greenhouse gases / pollution during operation.
• It is reliable as water flow can be controlled.

Mark Summary

Common Mistakes to Watch For

1. Confusing mass and weight: Weight = mass × g. Always convert mass (kg) to weight (N) before using in work and energy calculations.
2. Work done when stationary: If there is no displacement in the direction of the force, no work is done — even if a force is applied.
3. Efficiency formula: Efficiency = (Useful output / Total input) × 100%. Never invert the fraction.
4. Power vs. Energy: Power is the rate of doing work (Energy / Time). Students often confuse the two.
5. Energy conversions: Always state the initial and final energy forms. Do not skip intermediate steps in multi-step conversions.
6. Units: Always include units in final answers. Work and energy in Joules (J), Power in Watts (W), Force in Newtons (N).