Two balls of mass 3kg and 7 kg respectively move towards one another with speeds 5ms^-1 and 2ms^-1 respectively on a smooth table. If they collide and join, what velocity do they move off with?

m = 3kg ; 7kg u = 5ms^-1 ; -2ms^-1 (Taking the direction to the right as positive). momentum before = m1u1 + m2u2 = (3x5) + (7x-2) = 1 momentum after = (m1+m2)v2 = (3+7)v = 10v By the principle of conservation of momentum; momentum before must equal momentum after therefore 1 = 10v. v = 0.1ms^-1. It is positive therefore it moves off towards the right.