A car of mass 1200 kg is towing a trailer of mass 400 kg along a straight horizontal road using a tow rope, as shown in Figure 2.

The rope is horizontal and parallel to the direction of motion of the car.

• The resistance to motion of the car is modelled as a constant force of magnitude $2R$ newtons

• The resistance to motion of the trailer is modelled as a constant force of magnitude $R$ newtons

• The rope is modelled as being light and inextensible

• The acceleration of the car is modelled as $a$ ms^–2

The driving force of the engine of the car is 7400 N and the tension in the tow rope is 2400 N.

Using the model, find the value of $a$.

1.2. In a refined model, the rope is modelled as having mass and the acceleration of the car is found to be ${a}_{1}$ ms^–2.

State how the value of ${a}_{1}$ compares with the value of $a$.

1.3. State one limitation of the model used for the resistance to motion of the car.