2024 Serangoon SS Sci(Phy) Prelim Paper 2 ANS

2024 SSS 4E Sci(Phy) Prelim Paper 2 Section A & B Marking Scheme Section A 1 (a) When a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot. [1] (b) About the pivot, sum of clockwise moments = sum of anticlockwise moments (10 × 0.8) + (15 × 4.8) = (𝐹 × 3.2) --- [1] 3.2𝐹 = 8 + 72 = 80 𝐹 = 25 N --- [1] Only 1m to be awarded if perpendicular distance value(s) is/are incorrect 0m to be awarded if student mixed up ACW vs CW moments [2] [Total: 3] 2 (a) 𝐺𝑃𝐸 = 𝑚𝑔ℎ = (450)(10)(30) --- [1] 𝐺𝑃𝐸 = 135000 J --- [1] [2] (b) Gain in KE = loss in GPE (20 m) 𝐺𝑃𝐸 = 𝑚𝑔ℎ = (450)(10)(20) --- [1] 𝐾𝐸 = 90000 J --- [1] [2] (c) 𝐾𝐸 = 1 2 𝑚𝑣2 𝑣 = √2𝐾𝐸 𝑚 = √2(90000) 450 --- [1] 𝑣 = 20 m/s --- [1] [Total: 6]

3 (a) The wave travels parallel to the direction of the medium’s particle vibration [1] (b) 𝑓 = 1 𝑇 = 1 0.2×4 = 1 0.8 --- [1] 𝑓 = 1.25 Hz --- [1] [2] (c) [1] (d) 𝑣 = 𝑓𝜆 = 1.25 × (8.0 × 10−6) --- [1] 𝑣 = 10 × 10−6 = 0.000010 m/s --- [1] ECF allowed [2] [Total: 6] 4 (a) 𝑛 = sin 𝑖 sin 𝑟 1.50 = sin 𝑖 sin 25 𝑖 = sin−1(1.50)(sin 25) --- [1] 𝑖 = 39.340 = 39° (2 s.f.) OR 39.3° (3 s.f.) --- [1] [2] (b) 1m for correctly drawn emerging light ray parallel to light ray incident on side WX -1m for missing arrow head [1] [Total: 3]

5 (a) 𝑀 = 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒 𝑠𝑖𝑧𝑒 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡 = 5 15 --- [1] 𝑀 = 1 3 = 0.33333 = 0.33 (2 s.f.) OR 0.333 (3 s.f.) --- [1] [2] (b) Camera / eye [1] (c) The image will be magnified and the distance between the image and the lens will increase (image will be further behind the lens) [2] [Total: 5] 6 (a) (i) Effective resistance of whole circuit = ( 1 5 + 1 15) −1 + (5) --- [1] = 8.75 Ω (3 s.f.) --- [1] [2] (ii) 𝑉 = 𝐼𝑅 𝐼 = 𝑉 𝑅 = 12.0 8.75 --- [1] 𝐼 = 1.3714 = 1.37 A (3 s.f.) --- [1] [2] (b) Effective resistance of whole circuit = ( 1 5 + 1 5) −1 + (5) --- [1] = 7.50 Ω (3 s.f.) --- [1] [2] [Total: 6]

7 (a) [1] manganese arsenide dropped away from magnet touching both metal contacts (close circuit) [1] (b) When the surrounding temperature is 45 °C or higher, manganese arsenide becomes non-magnetic and is no longer attracted by the magnet. [1] the weight of the slab of manganese arsenide and the tension in the springs pull it down to touch both metal contacts. [1] This closes the circuit which causes the electric bell to ring. [2] [Total: 3] 8 (a) [1] when the switch is closed, the solenoid will become a temporary magnet / electromagnet. The iron rods will be magnetized by induction such that the same ends will have the same polarity (pole). [1] Since like poles repel, the iron rods