ASRJC 2022 Prelim P3 Guide

1 9749/03/ASRJC/2022PRELIM [Turn Over Anderson Serangoon Junior College 2022 JC2 H2 Physics Prelim Mark Scheme Paper 3 1a The rate of change of momentum of a body is proportional to the resultant force acting on the body and takes place in the direction of the resultant force. B1 B1 1bi The acceleration of object X is dependent on the resultant of the two forces, weight and tension, acting on object X. Since the weight and tension are in opposite directions, the resultant force must be less than its weight and hence, has an acceleration less than g. B1 1bii Let T be the tension in the rope. Considering the FBD of object X, by N2L, ↓+: mXg – T = mXa T = mXg – mXa ………. (1) Considering the FBD of object Y, by N2L, ↑+: T – mYg = mYa T = mYg + mYa ………. (2) Equating (1) and (2), we have mXg – mXa = mYg + mYa g (mX – mY) = a (mX + mY) Hence, XY XY mmag mm −= + M1 M1 A1 1biii Since the motion is linear with uniform acceleration, using s = ut + ½ at2 ↓+: 21 6.0 3.05.0 0 [ (9.81)]2 6.0 3.0 t−=+ + t = 1.749 s = 1.7 s C1 A1 2a The object is in equilibrium and hence resultant force is zero. Before lowering into water, sF mg= After lowering into water ' ' ' ( ' ) s ss ss F Up mg F Up F Up F F k x x += += = − = − Hence, upthrust = decrease in spring force = k∆x [Note: cannot just be substituting numbers, but must indicate it is representing ∆x] = 42 (0.0045) = 0.189 N = 0.19 N (to 2 s.f.) M1 A1 2b Upthrust = Vρg V(1000)(9.81) = 0.19 V = 1.94 x 10—5 m3 M1 A1

2 9749/03/ASRJC/2022PRELIM 2c By Newton’s 3rd Law, force of water on mass = - force of mass on water. Since force of water on mass is the upthrust, magnitude of force of mass on water is therefore same magnitude as upthrust. Hence magnitude of force of mass on water = 0.19 N M1 A1 2d When fully inside water, 1 () sF V g mg ke V g mg e mg V gk    += += =− So, first part of graph while inside water is a horizontal flat line since e is independent of s. While it is moving out of water, 1 () sF V g mg ke V g mg e mg LA gk    += += =− L is the submerged height of the mass and thus varies linearly with s. So, the second part of the graph is a linear line with increasing e. When the mass is fully out of water, the spring force is thus equal to weight and there is no upthrust, so the graph is a horizontal flat line independent of s. B1 B1 B1 3ai The gravitational potential at a point is the work done per unit mass in bringing a small test mass from infinity to that point. B1 B1 3aii Gravitational potential is always negative because the gravitational potential is taken to be zero at infinity B1 e s