A car of mass 1200kg pulls a trailer of mass of 400kg along a straight horizontal road. The car and trailer are connected by a tow-rope modelled as a light inextensible rod. The engine of the car provides a constant driving force of 3200N. The horizontal resistances of the car and the trailer are proportional to their respective masses. Given that the acceleration of the car and trailer is 0.4ms⁻², find the resistance to motion on the trailer