bedok south 4E5N Prelim 2020 Sc(Phy) Marking Scheme

Bedok South Secondary SchoolSec 4Exp/ 5NA Science (Physics) Prelim Exam 2020 Markingscheme Paper 2 Section ANo. Answers Marks1 Markers’ comments:● Most candidates are able to give an appropriate scale.● A number constructed parallelogram without indicatingthe direction of forces.● Some drew the resultant force incorrectly and manywere unable to state thedirection. 2a 1

Correct time period and line drawn (B1) for each line. Markers’ comments:● This question is well attempted.2b Distance travelled = m 12 9 + 17( ) 2.5( )= 32.5 Average speed = 32.5 / 17 = 1.91 m/s[Allowerror carry forwardfrom the diagram, as longas student displayunderstanding in how to calculate the total distancebased on the graph drawn.] Markers’ comments:● Generally well attempted.● Weaker ones still continue to assume that distance= speed x time. B1B1 2c Acceleration = = m/s 2 0−2.55 − 0.50 Deceleration = 0.50 m/s 2 Markers’ comments:● Some left answer as m/s 2 which shows thatcandidates were unable to− 0.50 the differenc between acceleration & deceleration. B1B1 3a 10 m/s 2 or 9.8 m/s 2 or 9.81 m/s 2 Markers’ comments:● Candidates were unable to distinguish between velocity& acceleration as manystated 0. B1 3b(i) 2000 – 800 = 1200 N B13b(ii) 1200 / 80 [C1]= 15 m/s 2 [A1] C1A14a [C1]𝑃 = 𝐹𝐴 = 14 000 𝑁500 𝑐𝑚 2 = 28 N/cm 2 [A1] C1A1 4b Plarger = Psmaller [B1] 𝐹𝐿𝑎𝑟𝑔𝑒𝑟 𝐴𝐿𝑎𝑟𝑔𝑒𝑟 = 𝐹𝑆𝑚𝑎𝑙𝑙𝑒𝑟 𝐴𝑆𝑚𝑎𝑙𝑙𝑒𝑟 = 1 𝐹𝐿𝑎𝑟𝑔𝑒𝑟 𝐹𝑆𝑚𝑎𝑙𝑙𝑒𝑟 = 𝐴𝐿𝑎𝑟𝑔𝑒𝑟 𝐴𝑆𝑚𝑎𝑙𝑙𝑒𝑟 1440 𝑐𝑚 2 500 𝑐𝑚 2 >Therefore Flarger > Fsmaller . [B1] B1 B1 2

OR Flarger = 28 N/cm 2 × 1440 cm 2 = 40 230 N [B1]Since Fsmaller = 14 000 N, Flarger > Fsmaller [B1] Markers’ comments:● Generally well attempted and for those who can recall the Pressure formula. B1B1 5a Elastic Potential Energy B1 5b(i) KE = ½ x m x v 2 1900 = ½ x 45 x v 2 [C1]v = 9.19 m/s [A1] C1A15b(ii) Loss in KE = Gain in GPE1900 = m x g x h1900 = 45 x 10 x h [C1]h = 4.22 m [A1] Markers’ comments:● Some candidates wrote KE = GPE which is not acceptableconceptually. Youare supposed to write loss in KE = Gain in GPE. C1A1 6a(i) Foam-filled wall: Ittraps airwhich is a poor conductorof heat.Itreduces heat loss through conduction Markers’ comments:● There is a need to identify air is trapped in thefoam filled walls as a significantnumber of candidates just stated foam is a poor conductorof heat. B1B1 6a(ii) Shiny outer casing: Ashiny surface is a poor emitterof heat.Itreduces heat loss by radiation. Markers’ comments:● Some candidates stated that shiny surface is a poorabsorber of heat which isnot applicable in this situation.● Some phrases their answer without clarity e.g. Shinyouter casing is a pooremitter of heat. This can be better with just “Shinysurface is a poor emitter ofheat.” B1B1 6b During evaporation,the more energetic particlesatthe surface of the liquidbreakthe forces of attraction andleft the liquid.Leaving behind particles withlower kinetic energyand thus,lowering thetemperatureof the liquid. Markers’ comments:● This question is