4E Phy Prelim 2021 Answer

1 BLS4EPhyPrelim2021P1Answer 1. D 2. D 3. B 4. D 5. C6. B 7. C 8. B 9. C 10. A11. C 12. C 13. D 14. C 15. D16. B 17. C 18. B 19. D 20. A21. B 22. A 23. C 24. D 25. C26. D 27. B 28. A 29. C 30. A31. B 32. B 33. B 34. B 35. B36. C 37. B 38. A 39. B 40. C 1 DTimefromX–Yistimeforhalfaperiod.Timefor1period=75s/25=3.0sTimeforhalfaperiod=3.0s/2=1.5s 2 D 3 BUsingF=ma,5000–T=500×2T=5000–1000=4000N 4 DOrder of movement of thecar: stationary, increasingvelocity, constant velocityanddecreasingvelocityinthesamedirection(soAiswrongasforthelastpart,thecarmovesintheoppositedirection). 5 CSinceF2 >F1 ,theobjectwillmovetowardstherightandfrictionactstowardtheleft.Fortheobjecttoaccelerate,theresultantforcemustbemorethanzero.F2 –F1 –F>0OnlyoptionCispossible. 6 B 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑏𝑙𝑜𝑐𝑘 = 5 𝑐𝑚× 4 𝑐𝑚× 10 𝑐𝑚 ( )– 2 𝑐𝑚× 3 𝑐𝑚× 10 𝑐𝑚( )

2 = 200 𝑐𝑚 3 − 60 𝑐𝑚 3 = 140 𝑐𝑚 3 𝑚𝑎𝑠𝑠 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦×𝑣𝑜𝑙𝑢𝑚𝑒 = 15 𝑔/𝑐𝑚 3 ×140 𝑐𝑚 3 = 2100 𝑔 7 CThecenterofgravityisatposition5,thecenterofthebeam.Inorderforthebeamtobebalanced, thepivot shouldbebetweenposition5andposition7(whereW2 is).OnlypossibleanswerisC. 8 BTakingmomentaboutZ,takingclockwisetobepositive.Resultantmoment=20×3–10×2–5×2=30Nmclockwise 9 CInfig.9.1,pressureoftrappedair=75cmHg+5cmHg=80cmHgInFig.9.2,Pressureoftrappedair=75cmHg–5cmHg=70cmHgComparethetwosituations,theproductofpressureandvolumeshouldbethesame.Sincethetubeusedarethesame,thecrosssectionalareawillbethesame.P1 V1 =P2 V2 P1 L1 =P2 L2 80×8=70×L2 L2 =9.1cm(2s.f.) 10 APressureatXandYarethesamesincetheyareatthesamelevel.PX =PY hX pX g=hY pY ghY =hX pX ÷pY =8cm 11 CUsefulworkdone=Workdoneonthebox–workdoneagainstfriction=Fd–Rd 12 CPowerisdefinedastheworkdoneperunittime.P=WD÷t=F×d÷t=F×v (assumingthewatermoveatconstantspeed,v=d÷t)=mg×v (forcerequiredistheweightofthewaterpumpedout,W=mg)=150×10×20 (for1second)=30000W=30kW 13 DOptionA:Sincemoregasisaddedintothevessel,thusthenumberofparticlesperunitvolumeincreases. Thefrequencyof collisionwiththewall of thevesselincreasesthuspressureincreases.OptionB:Theintermoleculardistancedecreasesduetothenumberofparticlesperunitvolumeincreases.

3 OptionC: Sincemoregasisadded, massof themassincreasesthustheweightincreases.Option D: Average kinetic energy remains constant as it is proportional to thetemperatureofthegas.Sincethetemperatureiskeptconstant,theaveragekineticenergywillnotchange. 14 CCeramictileisabetterheat conductorthanthewoolencarpet,thusheattransferisslowerthroughthewoollencarpetascomparedtotheceramictile.Thusthewoollencarpetwillfeelwarmerthantheceramictile. 15 DSinceradiationistheonlymethodofheattransferthatisonlytotravelinvacuum,heatfromthesuncanonlybetransfertoearththroughradiation. 16 BThethermometer showserrorinmeasuringtemperature(constantlymeasured1 o Clower) but if you are measuring the difference in temperature, the error will beeliminated.Differencebetweenmeltingiceandboilingwater=99–(−1)=100 o C 17 CΘ=(Rθ –R0 )÷(R100 –R0 )×10020=(Rθ –2)÷(2.5–2)×100Rθ =2.1Ω 18 BEnergyrequiredtomelt8.0goficecube= 8 𝑔1000 𝑔 ×336 𝑘𝐽 / 𝑘𝑔 = 2688 𝐽T