Question

The position vector of two particles of mass 4 kg and 2 kg are , respectively, r1 = 3t i^ + t j^ +2t2 k^ and r2 = 3 i^ +(t2-1)j^ +4t k^ where t is in seconds and the position in metres. Determine the position vector of the centre of mass of the system , the velocity of the cm and the net force acting on the system