2020 RI Prelims H2 Phy Paper 3 Soln

© Raffles Institution 9749/03   2020 Raffles Institution Preliminary Examinations – H2 Physics Paper 3 – Suggested Solutions Section A 1 (a) (i) The direction of motion after collision will be along the same line of motion before collision. OR The direction of velocities after collision will be along the line joining their centres of gravity. (ii) Their relative speed of separation is equal to their relative speed of approach. (b) Take direction to the right as positive. By conservation of linear momentum,     12 1 2 12 12 22 4.0 2 2.0 2 0 2 ----- (1) mu m u mv mv m m mv mv mv mv     Since collision is elastic,    12 2 1 21 21 4.0 2.0 6.0 ----- (2) uu v v vv vv        Subst. (2) into (1),   11 1 1 1 02 6 . 0 01 2 3 12 4.0 m s3 mv m v mv v       Velocity of A is –4.0 m s–1 i.e. body A moves to the left with speed 4.0 m s–1. From (2),  1 2 6.0 4.0 2.0 m sv   Velocity of B is 2.0 m s–1 i.e. Body B moves to the right with speed 2.0 m s–1. (c) In the collision in Fig. 1.3, the styrofoam increases the duration of impact as it is compressible. Thus, for the same change in momentum, the impact forces experienced by the balls are smaller compared to the collision in Fig. 1.2. Hence the external gravitational forces (weight) on the balls are comparable in magnitude to the impact forces and they contribute to the change in the momentum of the system such that the momentum of the system is no longer conserved. Comments (a) (i) Did not accept “parallel” to initial line of motion, “horizontal axis” etc. (ii) Did not accept “kinetic energy is conserved”, “12 21uu v v ” as question asks to “describe subsequent motion” (i.e. qualitativ e description of speed/ velocity is required). (b) Some are still unaware that 11 2 2 11 2 2mu mu mv mv  and 12 21uu v v are vector equations, so they forget to substitute numbers with negative signs for the velocities which are in the opposite direction. Note that u1 and u2 in the question are speeds.

2    © Raffles Institution 9749/03 (c) Poorly done. This part reveals common misconception about the principle of C.O.M. Many students reasoned that since the speeds after collision have reduced (as styrofoam has absorbed some energy), the total momentum is not conserved. This is far from the truth. Think of this example: suppose the col lision in (b) results in a completely inelastic collision, then (4.0) 2 ( 2.0) 30mm m v v   , final v < u1 or u2 but total momentum is still conserved. Basically, students need to understand one has to use a vector sum approach when adding individual momenta. Some students mixed up C.O.M. with cons ervation of kinetic energy and erroneously concluded that the collision being inelastic (loss of k.e.), violates C.O.M. Another common issue is students did not recognize the force exerted by