2021 JPJC Prelim H2 Physics P3 Solutions

[Turn over 2021/JPJC/Prelim/9749/03 Answers to 2021 JC2 Preliminary Examination Paper 3 (H2 Physics) No. Solution Remarks 1(a)(i) Upthrust is the net upward force acting on a body submerged in a fluid due to the difference in pressure at the top and bottom of the immersed portion of the body. [1] for correct answer 1(a)(ii) Upthrust sea 23 4 5 4 (7.5 10 )(1.03 10 )(9.81)5 606.3 606 N U V g         [1] for correct equation and numerical substitution [1] for correct answer 1(a)(iii) 606.3 (8.0)(9.81) 527.8 530 N (shown) U T mg T U mg      [1] for correct method and numerical substitution 1(b)(i) [1] for correct T, U and D 1(b)(ii) Resolving forces horizontally, sin35 --- (1)TD  Resolving forces vertically, cos35 --- (2) cos35 T W U UWT   Substitute (2) into (1), [1] for correct equations by resolution of forces sea bed W T D direction of current 35° sea surface U

2 2021/JPJC/Prelim/9749/03 No. Solution Remarks  sea 32 ( )tan35 tan35 (1.03 10 )(7.5 10 )(9.81) (8.0)(9.81) tan35 475.7 476 N D U W Vg mg          [1] for correct numerical substitution [1] for correct answer 2(a)(i) Work done per unit mass in bringing a small test mass from infinity to that point. [1] for correct answer 2(a)(ii) Potential at infinity is taken to be zero. Due to the attractive nature of the gravitational force, work done by an external agent to bring any mass from infinity to that point is always negative. Hence the potential at any point must always be negative. [1] [1] 2(b) [1] for correct shape [1] for at least 3 correct plots (i.e. r, 2r and 4r) 2(c)(i) Gravitational force provide the centripetal force for the spacecraft to move in circular orbit. 2 2 11 24 31 (6.67 10 )(6.0 10 ) 4(6400000) 3953 4.0 10 m s (shown) GMm mv RR GMv R        [1] for statement [1] for correct equation and numerical substitution

3 2021/JPJC/Prelim/9749/03 [Turn over No. Solution Remarks 2(c)(ii) 1. 11 24 11 24 66 10 (6.67 10 )(6.0 10 )(8600) (6.67 10 )(6.0 10 )(860 0) 3(6.4 10 ) 4(6.4 10 ) 4.48 10 J fi fi U U U GMm GMm RR                                       [1] for correct equation and numerical substitution [1] for correct answer 2(c)(ii) 2. 11 24 6 31 22 22 10 (6.67 10 )(6.0 10 ) 3(6.4 10 ) 4565 4.57 10 m s 11KE 22 1 (8600)(4565 3953 )2 2.24 10 J f fi GMv R mv mv              Since total energy = kinetic energy + gravitational potential energy and there is more loss in GPE than gain in KE, the total energy decrease. [1] for correct quantitative method [1] for correct explanation and conclusion 3(a) The first law of thermodynamics states that the increase in internal energy of a system is the sum of the heat s