2022 RSS Phy Prelims P2 Ans

Riverside Secondary School Prelim Examination 2022 Science for Sec 4E Solutions and Marking Scheme Deduct HALF mark per question or per part of a question for incorrect significant figures given. Deduction is capped at maximum 2 marks per section. Section A ( 50 marks) Qn Answer Marks Remarks 1a  Measure the time taken for 20 complete movements using a stopwatch. Let the time taken be t1.  Repeat the timing of 20 complete movements using the stopwatch. Let the time taken be t2.  Calculate the average time taken, <t>, for 20 complete movements. <t> =  Calculate the average time taken for one complete movement. Average time = [1] [1] [1] Can accept 10 movements 1b the distance moved in the direction of the force along the track [1] 2a 0 to 15 s – curve with increasing gradient to 120 m 15 s to 25 s – straight line from 120 m to 320 m [1] [1]

2b Kinetic energy = = 3.6 x 107 J [1] [1] 2ci Work done is the product of the force and the distance moved in the direction of the force. [1] 2cii Distance = 560 - 320 = 240 m [1] 2ciii Acceleration = = - 0.83 m/s2 Force while train decelerates = 1.8 x 105 x 0.83 = 1.5 x 105 N (2 sf) OR 3.6 x 107 = F x 240 F = .5 x 105 N (2 sf) [1] [1] Accept 1.49 x 105 N (3 sf) 3ai Weight is the force of gravity acting on the object. [1] 3aii Mass = 2.6 x 103 x 0.15 x 2.0 x 10-2 = 7.8 kg Weight = 7.8 x 10 = 78 N [1] [1] 3b Resultant force = (1.3 x 105 – 1.0 x 105) x 0.15 = 4500 N Direction of force is outwards [1] [1] [1] 3c Pressure = 12 x ρ x 10 9.6 x 104 = 12 x ρ x 10 ρ = 800 kg/m3 [1] [1] 4ai Moment = 25 x 0.72 = 18 Nm [1] 4aii Work done = 25 x = 28. 3 J (3 sf) [1] [1]

Or Work done = 25 x = 28. 1 J 4b The moment of force P about hinge is greater than the moment of force F about the same hinge. Since the perpendicular distance of the line of action of P to hinge is greater than F, hence the door rotates. [1] [1] 5a The initial temperature of the oil is higher. The rate of temperature decrease will be higher / the oil loses energy faster when hotter. [1] [1] 5b The attractive forces between molecules draw the molecules closer. Hence, the potential energy of the molecules decreases. The potential energy lost is the thermal energy lost to the freezer. Hence, the temperature of the oil remained constant from t = 3600 s to t = 10 800 s. [1] [1] [1] 5ci Q = mlf = 45 x 10-3 x 5.7 x 104 = 2600 J (2 sf) [1] [1] 5cii Rate = = 0.36 J/s [1] ecf allowed 5d The specific heat capacity of oil in the liquid state is less than oil in solid state. The temperature decreases of oil in liquid state decreases more quickly. Or The rate of decrease of temperature of oil in solid state is slower than that for liquid state because the specific [1] [1]

heat capacity for oil in solid state is higher than that for oil in liquid state. 6a Electrical energy to light energy and thermal energy [1] 6b Current decreases Voltage decreases Power decreases [1] No half mark 6ci Total current = 0.50 + 2.0 = 2.50 A Total R = = 4.8 Ω [1] [1] 6cii The cross-sectional area of the filament with greater resistance is smaller / smaller radius / smaller diameter [1] 7a At least one arrow on the electric field lines 1 M Field lines equally spaced out 1 M At least 4 field lines drawn [2] 7bi The negative charges on the grains move towards the positively charged metal plate. Unlike charges attract. [1] 7bii The top layer of grains, being positively charged, are attracted to the negatively charged plate. [1] 7c Energy = 2000 x 4.0 x 10-4 = 0.80 J (2 sf) [1] [1]

8a There is a flow of electrons in the wire as they are attracted to the positive terminal K from the negative terminal J. Direction of convention current is from K to J. [1] [1] 8b Direction of F is downwards. Using Fleming’s left hand rule. When the first finger points in the direction of the magnetic field and the second finger points in the direction of the current, the thumb will point in the direction of the force. [1] [1] 9a n = = = 1.5 OR n = = = 1.5 [1] [1] 9b 1.5 = sin r = r = 22⁰ [1] [1] 9ci Rays 1, 2 and 3 spend 10.0 μs, 10.3 μs and 10.8 μs in the cable respectively. As the pulse that enters the cable lasts 0.1 μs, each pulse that leaves the cable also lasts 0.1 μs. [1] 9cii [2] 1 M for same brightness 1M for drawing the pulses of light at the correct time 9ciii time between pulses = 0.9 μs [1] Fig. 9.4

number of pulses per second = = 1.1 x 106 pulses [1] 9civ Over long distances, there is an increase in time (or distance) for the pulses that leave the cable. Hence, there is an overlap of pulses if use to transmit data at high rate. [1] 10ai & 10aii Label x-axis and y-axis 1 M 2 complete cycles with constant amplitude and period 1 M (graph can start from trough instead of peak) P can be the peak or the trough 1M [2] + [1] 10aiii Amplitude/ e.m.f. increases Frequency of the current increases or period decreases [2] 10b There will be an alternating current in the primary coil. A changing magnetic field is created in the core which links with the secondary coil / A magnetic field cuts the secondary coil continuously A e.m.f. is induced in the secondary coil [1] [1] [1] 10c For a given power, the current in the transmission cable will be low when the voltage is high. Hence, the thermal energy loss in cables will be minimized. (P = I2R) [1] [1] P

EITHER 11ai weight of horse in mid-section of horse and labelled force upwards on at least two legs and labelled force downwards on saddle area labelled F [3] 11aii There is an equal and opposite force that the horse exerted upwards on the rider. [1] [1] 11bi [1] No half mark 11bii The centre of gravity of the fence will be lower and the line of action of the weight of the fence will fall outside the base at a bigger angle of tilt. [1] 11c 3000 = 520 x 10 x h h = 0.58 m maximum height above the ground = 1.4 + 0.58 = 2.0 m (2 sf) [1] [1] [1]

OR ai 6.0 cm [1] aii Drawing of 2 rays 2M Label of image I and dotted image drawn 1M Minus 1 M for missing arrow / solid image [3] aiii height = 1.8 cm [1] Accept 1.7 cm to 1.9 cm aiv Magnification = = 0.6 [1] ecf allowed av A real image can be projected on the screen Light can pass through a real image Virtual images are upright. [2] Any 2 b The speed of light becomes slower in glass. The wavefronts travel a shorter distance in the same time or left-hand side of wavefront slows down first before the rest of the other wavefronts. Hence, it changes direction. [1] [1]