assumption 2020 4E5N SciPhy Prelim Ans

Sec 4 Express / 5 Normal Academic Preliminary Examination2020Science (Physics) 5076Answer Keys and Marking Scheme PAPER 1 – 20 M1 2 3 4 5 6 7 8 9 10 B A D D C A C C D C 11 12 13 14 15 16 17 18 19 20 C D A D C A D C B C PAPER 2 SECTION A – 45 M1a diameter of Earth 10 Mmdiameter of an atom 0.1 nmdiameter of human hair 0.1 mm distance between Bukit Panjangand Bugis by car 15 km B2 – all correct, B1 - 2 to 3 correct, B0 – 0 to 1correct B2 1b Taking avernier callipers /micrometer screw gauge,measure the thickness of200 pieces of papers.Then divide by 100 to get theaverage thicknessof1 piece of paper. B2 – all 3 points, B1 – 2 points, B0 – no marks ifjust mention instrumentwithout description of steps B2 2a B1 – correct plotted pointsB1 – smoothness of the graph B2

2b Constant acceleration for the first 6 seconds.Constant velocity from 6 to 12 seconds.Decreasing deceleration from 12 to 22 seconds. B2 – all 3 points, B1 – 2 points, B0 – 0 to 1 point (ecf from 2a) B2 2ci (ecf from 2b) M1 A1 2cii (ecf from 2c) B1 3a For a body in equilibrium, thesum of anticlockwisemoments must beequal to the sum of clockwise momentsabout thesamepivot. *note: must include “same pivot” to award B1 B1 3b M1 A1 3c Move the counterweight toincreasetheperpendiculardistanceto thepivot,So that for thesame moment, F can be reduced. Or Reduce theweightof the pole,So that the total clockwisemomentdecreases, learningto a smaller F. Accept other reasonable answers. B1B1 4a Energy cannot be created or destroyed.However it can be converted from one form to another.Total energy must remain the same. B2 – all 3 points, B1 – 2 points, B0 – 0 to 1 point B2

4bi 1. M1A1 4bi 2. M1 A1 4bii The kinetic energy at Cis less than the gravitationalpotential energyatA because some of the gravitational potential energyisconverted to heat(thermal energy) and/or sound energydue to friction.(or loses energy tosurrounding as heat and/or sound energy) B1 B1 5a Air/plastic/foam is a bad conductor of heat, so heatloss from the water isreduced. Conduction cannot happen through vacuum. B1 B1 5b The plastic cap prevents convection from setting upabove the water. Convection cannot happen in a vacuum. B1 B1 5c The silvered surfaces reflectradiantheat back intothe water, reducing heatloss. Or The silvered surfaces arepoor emitter of radiation,reducing heat loss. B1 6ai 1. 1.25 s B16ai 2. 0.8 Hz (ecf from 6ai 1) B1 6aii The ball will movedownwardto the trough and moveback up again. B16bi Ultraviolet radiation B16bii Might get sunburnt.Might have skin cancer.Ionise the human cells. Accept other reasonable answers. B1 7a The car door isnegatively-chargedas the positively-charged droplets areattractedtothe door. B1B1

7bi An electric field is a region where an electric chargewill experience a force. B17bii B0 if lines touch or cross each other. B1 8a Current is the rate of flow of charges. B18b 0 A because current flows through S1 as it is a lowerresistance wire. OR 0 A because when S1 is closed, short circuit o