(VJC) 2024 H2 Prelim P1 soln

2024 Preliminary Examination H2 Physics Paper 1 Suggested Solution 1 D 6 B 11 A 16 B 21 A 26 A 2 C 7 D 12 C 17 D 22 B 27 A 3 C 8 D 13 A 18 B 23 A 28 D 4 D 9 C 14 D 19 D 24 B 29 A 5 A 10 C 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 2. The velocity- time graph of a particle moving in a straight line is shown in Fig. 2: Fig. 2 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 11 = (2+10)(20) - (10 )(4) 22 = 120 - 20 = 100 m displacement 100Average velocity = = time 14 1 = 7.1 m s − Distance = Area under velocity-time graph 11 = (2+10)(20) + (10 )(4) 22 = 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.

3 3. 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. So this is the wrong statement. Option D: If object is moving downwards in the opposite direction of the resultant force, it will be decelerating.

4 4. The diagrams show a hollow metal cube, with a sealed opening, is suspended from a spring balance before and during immersion in water. A reduction in the balance reading occurs because of the immersion. Water is then allowed to enter the metal cube, through the opening, to fill up the empty space in the cube. Which of the following statements is correct? A Upthrust of the water on the cube increases, hence the balance reading will be further reduced. B The balance reading during immersion corresponds to the upthrust of the water on the cube. C The balance reading stays the same as the increase in weight of water in the cube is offset by the increase in upthrust of the water on the cube. D The balance reading increases due to the increase of gravitational pull on the cube. Ans: D Option A: Upthrust is equal to the weight of water displaced by the cube. There is no change to the volume of the cube, hence there is no change in the upthrust. Option B: Tension + Upthrust = Weight Tension = Weight – Upthrust The balance reading due to tension does not correspond to the upthrust of water on the cube. Option C: Upthrust does not change. As water fills up the empty space in the cube, the weight of the cube will increase. Hence, balance reading will not stay the same. Option D: Same reasoning as option C, upthrust does not change but weight increases, hence balance reading increases due to the gravitational pull (weight) on the cube.

5 5. Which row in the table gives the gravitational potential energy, the elastic potential energy and the kinetic energy of a bungee jumper during the first fall? Air resistance is negligible. Gravitational potential energy/ kJ Elastic potential energy/ kJ Kinetic energy/kJ A top middle bottom 120 60 0 0 10 120 0 50 0 B top middle bottom 120 60 0 0 30 60 0 30 60 C top middle bottom 120 60 0 0 30 120 0 60 0 D top middle bottom 120 60 0 0 60 120 0 0 0 Ans: A At the top: There is only GPE. Hence, EPE and KE equal to 0. At the bottom: All GPE is lost and converted to EPE as it is momentarily at rest, hence KE = 0. Option B is wrong. At the middle: GPE lost is converted to gain in EPE and KE. The bungee jumper is moving, so KE cannot be 0. Option D is wrong The total energy stays constant at 120 kJ. Option C is wrong. Only Option A gives the correct combinations of energy at the different positions.

6 6. An escalator is 60 m long and lifts passengers through a vertical height of 30 m, as shown. To drive the escalator against the forces of friction when there are no passengers requires a power of 2.0 kW. The escalator is used by passengers of average mass 60 kg and the power to overcome friction remains constant. How much power is required to drive the escalator when it is carrying 20 passengers and is travelling at 0.75 m s-1? A 4.4 kW B 6.4 kW C 8.8 kW D 10.8 kW Ans: B Power required to drive the escalator with 20 passengers = power to drive the escalator when there are no passengers + power to drive 20 passengers of an average mass of 60 kg each 3 ( sin ) 302.0 10 20(60)(9.81)( )(0.75)60 escalator escalator P Fv P mg v  =+ =+ =  + = 6414 = 6.4 kW 30 m 60 m

7 7. A turntable is rotating at a constant number of revolutions per second. What is the relationship between the angular velocity of a point on the turntable and the distance of the point from the centre of the turntable? A Angular velocity is directly proportional to the distance. B Angular velocity is inversely proportional to the distance. C Angular velocity is directly proportional to the distance squared. D Angular velocity is independent of distance. Ans: D Given number of revolutions per second, = constant angular velocity =2 is a constant, indep endent of distance. f f 8. A satellite of mass M is in circular orbit with radius r around the Earth with a period of T. A second satellite of mass 2 M moves around the Earth with radius 4 r. What is the period of this second satellite in terms of T? A 4 T B 2 T C 4 T D 8 T Ans: D 2 2 2 2 3 3 For the 1st satellite, 2() 2 (4 )For 2nd satellite, ' 2 8    = = = == E E E E GM M Mrwr GM M MrrT rT GM rTT GM

8 9. What is the approximate number of atoms in a cubic metre of an ideal monatomic gas at the temperature of 27 C and a pressure of 1 x 105 Pa? A 1 x 1022 B 6 x 1023 C 2 x 1025 D 3 x 1026 Ans: C 5 23 25 25 (1 10 )(1) (1.38 10 )(27 273.15) 2.41 10 2 10 pV NkT N N − =  =  + =  10. A fixed mass of monatomic ideal gas at a volume V at temperature T and pressure P has a root mean square speed c. After absorbing heat, its final volume is V and final pressure is 2P. What is the new root mean square speed of the ideal gas? A 0.5 c B 0.71 c C 1.4 c D 2