7. A car of mass 1200 kg tows a trailer of mass 800 kg along a straight level road by means of a rigid towbar. The resistances to the motion of the car and the trailer are proportional to their masses.
Given that the car experiences a resistance to motion of 300 N ,
- find the resistance to motion which the trailer experiences.
Given that the engine of the car exerts a driving force of 3 kN ,
- find the acceleration of the system,
- show that the tension in the towbar is 1200 N .
When the system has reached a speed of \(24 \mathrm {~m} \mathrm {~s} ^ { - 1 }\), it continues at constant speed until an electrical fault causes the engine of the car to switch off. The brakes are used to apply a constant retarding force until the system comes to rest.
Given that the retarding force of the brakes has magnitude 1 kN and assuming that the original resistances to motion of the car and the trailer remain constant,
- calculate the distance that the system travels during the braking period,
- find the magnitude and direction of the force exerted by the towbar on the car.
- Comment on the assumption that the original resistances to motion of the car and the trailer remain constant throughout the motion.