2021 RI Prelims H2 Phy Paper 2 Soln

© Raffles Institution [Turn over 2021 Raffles Institution Preliminary Examinations – H2 Physics Paper 2 – Suggested Solutions 1 (a) (i) When bodies in a system interact, the total momentum of the system remains constant, provided no net external force acts on it. B1 B1 It is important to include the word “net”. There can be external forces acting on the system. As long as the “net” external forces is zero, conservation of momentum can be applied. Note that conservation of momentum is not only confined to collision problems. Hence no credit is given for simply st ating “Total momentum before and after collision is constant”. Also, there is no need to confine the discussion to just “two bodies”. (ii) Relative speed of approach = relative speed of separation u  (u) = vB  vA 2u = vB  vA …………. (1) Conservation of momentum mA u  mB u = mA vA + mB vB …………. (2) mA(1) + (2): (2 mA u ) + (mA u  mB u ) = (mA vB  mA vA ) + (mA vA + mB vB) 3 mA u  mB u = mA vB + mB vB 3 AB B AB mmvu mm   B1 B1 B1 Some candidates worked backwards from the result and quote some expressions. No credit is given if there are no bas is and no explanation given for these expressions. Candidates who tried to use conservation of kinetic energy will end up with very messy expressions and most likely do not obtain the result needed. For elastic collisions, using the relative speed relationship is neater. The sign convention is given in the question (taking right to be positive). Candidates using the wrong convention may still end up with the required answer, but no credit is given for such case, as candidates failed to follow the instructions given. (b) (i) 1 3 3(0.62) (0.059) (4.4)0.62 0.059 11.7 m s AB AB mmvutennis mm      B1 Some candidates confused the collision between the basketball and tennis ball with that of the collision with the ground. (ii) The impact force is much greater than the weight of the tennis ball / basketball. (Hence, gravitational forc e, which is an external force, is neglected when applying conservation of momentum.) B1 The scenario in part (b) (which is a vertical motion) is an extension to the result in part (a) (which is a horizontal motion). Hence the force of gravity (which is an external force) may made the application of conservation of momentum invalid. The

2 © Raffles Institution reason why it may still be applied is that the impact force is much larger than the gravitational force (weight) of the objects. This is due to the short time duration of impact between the two objects. Repeating what the question has already pr ovided will not earn the credit, e.g. stating that the “speed of the basketball before and after impact with the ground is the same” will not earn any credit. Simply stating that there are no external fo rces is also not correct, as gravitational