Secondary 1 Science Semestral Assessment 2 (End of Year) Paper 3

TuitionGoWhere Practice Paper - Science Secondary 1

SA2 Physical Sciences – Version 3: Answer Key and Marking Scheme

SECTION A: Multiple Choice and Short Response

1 Answer: C [1]

Explanation:

• Vertically: F1 (12 N up) = F2 (12 N down), so vertical forces balance → equilibrium.
• Horizontally: F3 (8 N right) ≠ F4 (5 N left), so net force = 3 N to the right → not in equilibrium horizontally.
• The block accelerates to the right, but C is the only statement about equilibrium conditions. D is incorrect because horizontal forces don't balance; A is wrong because acceleration is to the right; B describes the motion but the question asks about equilibrium statement.

2 Answer: [2]

Marking points:

• States Hooke's Law: extension is directly proportional to load/force (within elastic limit) [1]
• Identifies that data up to 300 g shows direct proportionality (constant ratio or straight line through origin) [0.5]
• Identifies that 400 g gives extension 8.5 cm, which does NOT fit pattern (would expect 8.0 cm) [0.5]

Full answer: Hooke's Law states that the extension of a spring is directly proportional to the applied force (or mass), provided the elastic limit is not exceeded. For the first four readings, the extension doubles as the mass doubles (constant ratio of 0.02 cm/g). However, at 400 g, the extension is 8.5 cm instead of the expected 8.0 cm, so the spring does not obey Hooke's Law over the entire range. The elastic limit was exceeded between 300 g and 400 g.

3 (a) Answer: Chemical energy → gravitational potential energy [1]

(b) Answer: [2]

Marking points:

• Work is done against gravity / force is applied upward through a distance [1]
• Energy is transferred/converted even though speed (and hence kinetic energy) stays constant [1]

Full answer: As the student climbs, she exerts an upward force through a vertical distance. Work done against gravity transfers chemical energy from her muscles into gravitational potential energy. Although her kinetic energy stays constant (constant speed), her height increases, so gravitational potential energy ($E_p = mgh$) increases.

4 Answer: [3]

Working:

• Clockwise moment = Anticlockwise moment for balance
• Mass A × distance A = Mass B × distance B
• 300 g × 20 cm = Mass B × 20 cm
• Mass B = 300 g × 20 / 20 = 300 g [2 for working, 1 for answer]

Explanation: The principle of moments states that for a balanced object, the sum of clockwise moments equals the sum of anticlockwise moments. Both masses are at equal perpendicular distances from the pivot (20 cm), so they must have equal masses for balance. Common mistake: Using weight in newtons is also valid (2.94 N each) and earns full credit if consistent.

5 (a) Answer: [2]

Working: $a = \frac{v-u}{t} = \frac{20-0}{10} = 2 \text{ m/s}^2$

• Correct formula [1], correct substitution and answer with unit [1]

(b) Answer: [2]

Working: $F = ma = 1200 \times 2 = 2400 \text{ N}$

• Correct formula [1], correct substitution and answer with unit [1]

6 Answer: [2]

Advantage (1 mark): Provides reliable/safe stopping; allows control of speed; essential for safety Disadvantage (1 mark): Causes wear of brake pads and rim; generates heat which can damage components; reduces efficiency

Example answers:

• Advantage: Friction between brake pads and rim allows the cyclist to slow down and stop safely when needed.
• Disadvantage: Friction causes wear on both the brake pads and the wheel rim, requiring regular maintenance and replacement.

7 (a) Answer: [2]

Working: $\text{Speed} = \frac{\text{distance}}{\text{time}} = \frac{50}{10} = 5 \text{ m/s}$

• Correct formula [1], correct substitution and answer with unit [1]

(b) Answer: The cyclist is stationary / at rest / not moving [1]

(c) Answer: [2]

Working: $\text{Average speed} = \frac{\text{total distance}}{\text{total time}} = \frac{100}{35} = 2.86 \text{ m/s} \text{ (or } 2\frac{6}{7} \text{ m/s or } \approx 2.9 \text{ m/s)}$

• Correct total distance and total time identified [1]
• Correct calculation and unit [1]

8 (a) Answer: [2]

Working: $P = IV \Rightarrow I = \frac{P}{V} = \frac{2000}{240} = 8.33 \text{ A}$

• Rearrangement of formula [1], correct substitution and answer with unit [1]

(b) Answer: [3]

Working: $E = Pt = 2000 \times (30 \times 60) = 2000 \times 1800 = 3\,600\,000 \text{ J}$ $= 3600 \text{ kJ}$

• Convert time to seconds: 30 min = 1800 s [1]
• Correct substitution into $E = Pt$ [1]
• Correct answer in kilojoules with unit [1]

9 Answer: [3]

Working: Total resistance: $R_{total} = R_1 + R_2 = 4 + 8 = 12 \, \Omega$ [1]

Current: $I = \frac{V}{R} = \frac{12}{12} = 1.0 \text{ A}$ [2 for correct formula, substitution, answer]

Explanation: In a series circuit, total resistance equals the sum of individual resistances. With the switch closed, the complete circuit has 12 Ω resistance and 12 V EMF, giving 1.0 A current by Ohm's Law.

10 Answer: [2]

Marking points:

• Heating element heats water at the bottom [1]
• Hot water rises (by convection), cold water sinks, setting up convection current that heats all water efficiently [1]

Full answer: The heating element is placed at the bottom because when water is heated, it expands, becomes less dense, and rises. Cooler, denser water sinks to take its place and gets heated in turn. This creates a convection current that distributes heat throughout the kettle efficiently. If the element were at the top, only the top layer would be heated.

SECTION B: Structured Response

11 (a) Answer: [2]

Marking points:

• Trolley starts from rest (initial velocity $u = 0$) [1]
• Constant acceleration assumed (or neglecting friction/air resistance) / equation of motion for uniform acceleration applies [1]

11 (b) Answer: [3]

Working: $s = \frac{1}{2}at^2 \Rightarrow a = \frac{2s}{t^2}$

$a = \frac{2 \times 0.80}{(0.63)^2} = \frac{1.60}{0.3969} = 4.03 \approx 4.0 \text{ m/s}^2$

• Correct rearrangement [1]
• Correct substitution [1]
• Correct answer with unit (2–4.1 m/s² acceptable) [1]

11 (c) Answer: [2]

Any two from:

• Repeat measurements and calculate mean/average time
• Use a narrower card on the trolley to more precisely define when it passes the gate
• Ensure the trolley starts from the exact same point each time (use a release mechanism)
• Use a more precise electronic timer
• Reduce friction in the wheel bearings / lubricate wheels

[1 mark each, max 2]

12 (a) Answer: 150 N [1]

12 (b) Answer: [2]

Marking points:

• Single fixed pulley only changes direction of force, does not multiply force (no mechanical advantage) [1]
• Useful because it allows the person to apply downward force (using body weight) rather than lifting upward; more convenient/comfortable/allows use of gravity [1]

12 (c) Answer: [3]

Working: $\text{Work done} = \text{force} \times \text{distance} = 150 \times 3.0 = 450 \text{ J}$

$\text{Power} = \frac{\text{work done}}{\text{time}} = \frac{450}{5.0} = 90 \text{ W}$

• Correct work calculation [1]
• Correct power formula and substitution [1]
• Correct answer with unit [1]

13 (a) Answer: 0°C [1]

13 (b) Answer: [2]

Marking points:

• The substance is melting / changing from solid to liquid [1]
• Heat energy is being used to break intermolecular bonds (increase potential energy) rather than raise kinetic energy / temperature [1]

13 (c) Answer: [3]

Working: $E = ml = 200 \times 334 = 66\,800 \text{ J} = 66.8 \text{ kJ}$

• Correct formula $E = ml$ [1]
• Correct substitution [1]
• Correct answer with unit [1]

13 (d) Answer: [2]

Marking points:

• During boiling, energy supplied is used as latent heat of vaporization [1]
• Energy breaks intermolecular bonds / separates molecules rather than increasing kinetic energy, so temperature stays constant during phase change [1]

14 (a) Answer: [2]

Working: $R_{total} = R_1 + R_2 = 6 + 6 = 12 \, \Omega$

• Correct formula for series resistors [1], correct answer with unit [1]

14 (b) Answer: [3]

Working: For two equal resistors in parallel: $\frac{1}{R_{total}} = \frac{1}{6} + \frac{1}{6} = \frac{2}{6} = \frac{1}{3}$ $R_{total} = 3 \, \Omega$ [1]

$I = \frac{V}{R} = \frac{12}{3} = 4.0 \text{ A}$ [2]

14 (c) Answer: [3]

Order: P (12 Ω) < R (6 Ω) < Q (3 Ω) [1 for correct order]

Reasoning:

• Power $P = \frac{V^2}{R}$ (same voltage across all circuits) [1]
• Power is inversely proportional to total resistance; lower resistance means higher current, therefore higher power [1]
• P has highest resistance → lowest power; Q has lowest resistance → highest power; R is intermediate

15 (a) Answer: [4]

Marking points (any three well-explained features):

[Max 4 marks]

15 (b) Answer: [3]

Working and explanation:

• Temperature difference = 85 − 25 = 60°C [1]
• If surrounding temperature rises to 35°C, new temperature difference = 85 − 35 = 50°C [1]
• Rate of thermal energy transfer decreases because the temperature gradient/difference driving heat transfer is smaller; less steep gradient means slower energy flow by conduction/convection [1]

SECTION C: Data Analysis and Extended Response

16 (a) Answer: [2]

P × V for 100 kPa: 6000 [1] P × V for 300 kPa: 6000 [1]

(All values should be approximately 6000; slight rounding variations accepted)

16 (b) Answer: [2]

Marking points:

• As pressure increases, volume decreases / pressure and volume are inversely proportional [1]
• Product P × V is constant (within experimental error), which is Boyle's Law [1]

16 (c) Answer: [2]

Description of graph:

• Straight line passing through the origin [1]
• Positive gradient (upward to the right) [1]

Sketch notes: The graph of P against 1/V should show a straight line with positive slope starting from (0,0), demonstrating direct proportionality between pressure and reciprocal of volume.

17 (a) Answer: [1]

Water is replenished in the reservoir by rainfall/river flow naturally, so the energy source is continuously renewed / the water cycle is sustained by solar energy.

17 (b) Answer: [2]

Working: $E_p = mgh = 500 \times 10 \times 150 = 750\,000 \text{ J} = 750 \text{ kJ per second}$

Or: 750 000 W or 750 kW of gravitational power

• Correct formula [1], correct substitution and answer with unit [1]

17 (c) Answer: [3]

Working: $\text{Electrical power output} = \frac{85}{100} \times 750\,000 = 637\,500 \text{ W}$

Or: $= 0.85 \times 750 = 637.5 \text{ kW}$

Or approximately 638 kW or 6.4 × 10⁵ W

• Correct efficiency application (output = 0.85 × input) [1]
• Correct substitution [1]
• Correct answer with unit, rounded appropriately [1]

18 (a) Answer: [2]

Phenomenon: Doppler effect [1]

Explanation: As the car approaches the radar source, the reflected waves are compressed, resulting in a higher detected frequency (blue shift). The relative motion between source and observer causes the frequency shift. [1]

18 (b) Answer: [2]

Working: $\text{Speed} = 42 \text{ km/h} = \frac{42 \times 1000}{3600} = 11.67 \text{ m/s}$

$\text{Distance} = 11.67 \times 2.5 = 29.2 \text{ m} \approx 29 \text{ m}$

Or using km/h directly: $\text{Distance} = 42 \times \frac{2.5}{60 \times 60} = 0.0292 \text{ km} = 29.2 \text{ m}$

• Conversion to consistent units [1], correct calculation and answer [1]

19 (a) Answer: [3]

• Independent variable: Type of insulating material / wrapping material [1]
• Dependent variable: Temperature of water (at given times) / rate of cooling / temperature drop [1]
• Controlled variable: Any one from: initial water temperature; volume/mass of water; thickness of wrapping material; size/shape of beaker; environmental temperature; time intervals [1]

19 (b) Answer: [2]

Marking points:

• Single final temperature only tells you which beaker lost most heat overall [1]
• Time-based data reveals rate of cooling / allows comparison of cooling curves / shows if insulation affects initial rate differently from long-term performance / identifies when most heat is lost [1]

20 (a) Answer: [2]

Marking points:

• Metals expand when heated, but brass expands more than invar (different expansion coefficients) [1]
• Since brass is on top and expands more, the strip bends downward (toward the invar side) / unequal expansion causes bending [1]

20 (b) Answer: [2]

Marking points:

• When heated sufficiently, the free end bends down enough to touch the electrical contacts below [1]
• This completes the circuit containing the bell/alarm, allowing current to flow and sound the alarm [1]

20 (c) Answer: [1]

Any one from:

• Use a metal that expands more easily than brass (for top layer)
• Reduce the gap between free end and contacts
• Increase the length of the strip
• Use thinner metal layers
• Use a metal with higher expansion coefficient for the top layer

TOTAL MARKS: 60