Question

The position vector of two particles of mass 4.0 kg and 2.0 kg are, respectively, kjir ˆ2ˆˆ3 2 1 t tt + += r and ( ) k jir ˆ 4ˆ1ˆ3 2 2 t t + −+= r 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.