1. The work-energy principle states that the change in the total mechanical energy (kinetic and potential) of a particle is equal to the net work done on that particle.

A child’s toy of mass 250 g is placed at rest on the surface of a swimming pool and it immediately begins to sink vertically.  The pool is 1.2 metres deep and immediately before the toy hits the bottom of the pool it is moving with speed 0.4 m s^-1. 

i) Find the loss in gravitational potential energy of the toy between as it moves to the bottom of the swimming pool.

ii) Find the gain in kinetic energy of the toy between as it moves to the bottom of the swimming pool.

iii) Use the work-energy principle to find the work done against the resistive force of the water on the toy. 

iv) Assuming that the resistive force is constant, find its magnitude.