Kinematics (OCR A Level Physics)

Use the equations for momentum and kinetic energy to derive an expression for the kinetic energy E_k of a particle in terms of its momentum p and mass m.

Fig. 20.1 shows an electric motor used to lift and lower a load.

Fig. 20.1

At time t = 0 the load is on the ground with displacement s = 0.

Fig. 20.2 shows the variation of the displacement s of the load with time t.

Fig. 20.2

Fig. 20.3

On Fig. 20.3, sketch a graph to show the variation of the velocity v of the load with time t.

You do not need to insert a scale on the v axis.

Describe how the kinetic energy and the gravitational potential energy of the load varies from t = 0 to t = 2.0 s.

During the downward journey of the load, the string breaks at t = 4.0 s. It then falls vertically toward the ground. The mass of the load is 120 g.

Air resistance is negligible.

Calculate the velocity V of the load just before it hits the ground.

The load hits the ground and comes to rest in a time interval of 25 ms.

Calculate the average force F exerted by the ground on the load.