The Chief Examiner also reported that this question was poorly attempted by majority of

the candidates who attempted it. Candidates were reported not to apply the distance

formula correctly and their responses indicated poor knowledge of conservation of linear

momentum.

In part(a), candidates were expected to respond as follows:

Distance covered by first body = S1 = u1t + ½ a1t2 = 8t + 10t2

Distance covered by second body = S2 = u2t + ½a2t2 = 22t + 9t2

Since the distance between them is 15 m, it implied that S1 – S2 = 15m i.e. 10t2 + 8t - which gave t2 – 14t = 15. Solving this quadratic equation gave t = 15 seconds.

In part (b), candidates’ expected response was:

Total momentum before impact = 25 x 8 + 15(-3) = 155 gms-1

Total momentum after impact = 25 x 5 + 15v = (125 + 15v) gms-1

Since momentum is conserved, then 155 = 125 + 15v. Solving this equation gave

v = 2 ms-1. The change in momentum of the 15 g ball was given by m(v-u) = 15 = 15 x 5 = 75 gms-1.