newton's third law of motion: motion of connected particles 2?

Question

A car of mass 1200 kg, towing a carvan of mass 800 kg, is travelling along a motorway at a constant speed of 20 m/s. There are resistance force on the car and the carvan ,of magnitude 100 N and 400 N respectively.Calculate the magnitude of the force on the carvan from the towbar,and the driving force on the car.
The car brakes suddenly, and begins to decelerate at the rate of 1.5 m/s^2. Calculate the force on the car from the towbar. What effect will the driver notice?

Answer 1

When at constant speed, the net force on the carvan must be zero -- no acceleration. So the tension in the towbar is 400N. (The carvan's airdrag is 400N in the negative direction [if forward is positive].) The car must be applying 500N to the road.

When braking, the carvan is under negative acceleration of -1.5 m/s^2. It's mass is 800 kg, so the net force applied to the carvan is
F = m*a = -800 kg*1.5 m/s^2 = -1200N

Until the vehicles slow down a bit, the carvan's air drag is still negative 400N. So the towbar is applying the remaining 800N force to the carvan, in the negative direction.
By Newton's 3rd, the car feels an equal and opposite force pushing it along. The driver will feel that the car does not slow as easily as it would without the carvan.