4 A car of mass 1500 kg is pulling a trailer of mass 300 kg along a straight horizontal road at a constant speed of \(20 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). The system of the car and trailer is modelled as two particles, connected by a light rigid horizontal rod. The power of the car's engine is 6000 W . There are constant resistances to motion of \(R \mathrm {~N}\) on the car and 80 N on the trailer.

1. Find the value of \(R\).
   The power of the car's engine is increased to 12500 W . The resistance forces do not change.
2. Find the acceleration of the car and trailer and the tension in the rod at an instant when the speed of the car is \(25 \mathrm {~m} \mathrm {~s} ^ { - 1 }\).