A lorry of mass m, and travelling initially at speed v along a horizontal road, is brought to rest by an average braking force F in time t. Ignoring any other resistive forces, what distance is travelled by the lorry during this time?

Answer:

23) C

We first need to find the work done by the braking force. The work done would be equal to the energy of the lorry. Since the lorry was moving, its energy would be kinetic energy.

So work done by braking force = KE of lorry
Work = ½(mv²)
We know that work done = force x distance
Force x distance = ½(mv²)
Distance = mv² ÷ 2F

Hence, C is correct.