2024 VJC H1 Prelim P1 soln

2024 Preliminary Examination H1 Physics Paper 1 Suggested Solution 1 D 6 B 11 D 16 D 21 B 26 A 2 B 7 C 12 A 17 D 22 A 27 B 3 C 8 D 13 B 18 D 23 B 28 A 4 C 9 B 14 D 19 A 24 A 29 A 5 D 10 D 15 A 20 B 25 B 30 C 1. Which one of the following estimates is unrealistic? A The potential energy of a man at the top of Bukit Timah Hill is 100 kJ. B The volume of air in a classroom is 150 m3. C The mass of an apple is 100 g D The average temperature of fire is 300 K. Ans: D A: Height of Bukit Timah Hill = 160 m, potential energy of man = mgh = (60)(9.81)(160) = 94 kJ B: Volume of air in classroom = volume of classroom = 7 x 8 x 3 = 147 m3 C: 100 g for an apple is quite reasonable right… D: 300 K is 27 oC, so is unrealistic for the temperature for fire. 2. The rate of energy transfer, H through a conducting slab is found experimentally to be given by TH kA l  where the proportionality constant k is the thermal conductivity of the material, A is the cross-sectional area of the slab, l is the length of the slab and ΔT is the temperature difference at opposite faces of the slab. Which of the following are the base units of k? A kg m2 s-2 K-1 B kg m s-3 K-1 C kg m s-2 K-1 D unitless Ans: B -1 3 3 3 1 2 2 (J s )(m) (kg m s )units of k = kg m s Km K m K        Hlk A T

2 3. Two projectiles are fired from a gun as shown in Fig. 3. Fig. 3 The first projectile leaves the gun at a velocity of 140 m s−1 at an angle to the horizontal of 75°. The second projectile is fired from the gun at the same position with the same speed but at an angle of 15°. How much time later should the second projectile be fired so that both projectiles land at the same point on the ground at the same time? Ignore the effects of air resistance. A 10 s B 15 s C 20 s D 30 s Ans: C Vertically, from , sinTime to reach max. height, 2 sinTime of flight, 2 2(140)sin 751st projectile, time of flight = 27.6 s9.81 2(140)sin152nd projectile, time of flight = 7.399.81         y y yv u a t ut g ut g s Time difference = 27.6 - 7.39 = 20.2 s

3 4. The velocity- time graph of a particle moving in a straight line is shown in Fig. 4 Fig. 4 What are the average speed and velocity of the particle between time t = 0 and t = 14 s? Average speed / m s-1 Average velocity / m s-1 A 7.1 7.1 B 7.1 10 C 10 7.1 D 10 10 Ans: C Displacement = Area under velocity-time graph 1 1 = (2+10)(20) - (10 )(4) 2 2 = 120 - 20 = 100 m displacement 100Average velocity = = time 14 1 = 7.1 m s  Distance = Area under velocity-time graph 1 1 = (2+10)(20) + (10 )(4) 2 2 = 120 + 20 = 140 m displacement 140Average velocity = = time 14 1 = 10 m s  t/ s v/ m s-1 Above time axis: positive displacement Below time axis: negative displacement Distance is a scalar and has no direction. So areas above and below time axis are considered as positive.

4 5. An aeroplane, flying in a straight line at a constant height of 500 m with a speed of 200 m s-1, drops an object. The object takes a time t to reach the ground and travels a horizontal distance d in doing so. Ignoring air resistance, which one of the following gives the values of t and d? t /s d /km A 25 10 B 25 5 C 10 5 D 10 2 Ans: D 2 2 1Vertically, 2 1500 2 500 2 10 s9.81      y u ys u t a t gt t Horizontally, = = 200 (200)(10) 2 km xd u t t 6. A ball falls vertically and bounces on a moving conveyor belt. What is the direction of the resultant force due to the conveyor belt acting on the ball? A B C D Ans: B When the ball bounces on the conveyor belt, there is an upward force on the ball by the conveyor belt to cause it to rebound upwards and there is a forward force in the same direction of motion of the conveyor belt to cause it to move forward. The resultant of these two forces is diagonally upwards towards the right. Direction of motion of conveyor belt

5 7. The diagram shows two opposite vertical forces of magnitude 1.2 N and 2.1 N acting on an object. Which of the following statements is wrong? A The magnitude of the resultant force is 0.9 N. B The object is accelerating and moving up. C The object is decelerating and moving up. D The object is decelerating and moving down. Ans: C Option A: The magnitude of the resultant force is 2.1 – 1.2 = 0.9 N Option B: The resultant force is 0.9 N upwards since the force exerted on it upwards is larger than the force exerted on it downwards. If object is moving upwards in the same direction of the resultant force, it will be accelerating. Option C: If object is moving upwards in the same direction of the resultant force, it will be accelerating and not decelerating. Option D: If object is moving downwards in the opposite direction of the resultant force, it will be decelerating.

6 8. Two trolleys are placed together on a horizontal runway with a compressed spring between them. When they are released, the 2.0 kg trolley moves to the left at 2.0 m s–1. How much energy was stored in the spring? A 4.0 J B 6.0 J C 8.0 J D 12 J Ans: D Find final speed v of the 1.0 kg trolley: Taking  as +ve and using conservation of momentum, Total initial momentum = total final momentum 0 = 2.0(-2.0) + 1.0(v) v = 4.0 m s-1 Energy stored in spring is converted to gain in KE of trolleys. Energy stored in spring = 2 21 1(2.0)(2.0) (1.0)(4.0) 122 2   J 9. A steel spring was stretched to a length of 58 cm when a 30 N weight was hung from it. The same spring was stretched to a length of 72 cm when a 51 N weight was hung from it. What was the original length of the spring? A 15 cm B 38 cm C 44 cm D 61 cm Ans: B Using F = kx, where x is stretched length minus original length L, 30 (58 ) ---- (1) 51 (72 ) ----(2) k L k L     Solving the two equations simultaneously, L = 38 cm 2.0 kg 1.0 kg

7 10. Two blocks, one made of wood and the other of iron, are arranged at rest on the ground. Which of the following statements is correct? A The force exerted by the ground on the iron block in (1) is greater than the force exerted by the ground on the wooden block in (2) because the iron block, being denser than the wooden block, exerts more force on the ground. B The force exerted by the wooden block on the iron block in (1) is equal to that exerted by the iron block on the wooden block in (2). C The force exerted by the iron block on the wooden block in (1) is bigger than that exerted by the wooden block on the iron block in (2). D The force exerted by the wooden block on the iron block in (1) is smaller than the force exerted by the ground on the wooden block in (2). Ans: D Option A: This is wrong because the normal reaction force that the ground exerts on the iron in (1) and wood in (2) is the same, since the normal reaction force = total weight of the two blocks, which are the same in both situations. Option B: The force that the wooden block exerts on the iron block in (1) is equal to the weight of the wooden block and the force that the iron block exerts on the wooden block in (2) is equal to the weight of the iron block. Hence, the