(i) When an object is thrown at a certain angle, it rises to a certain height and then comes back while moving in a parabolic path. Let h be the maximum height to which it rises.
As it rises, its kinetic energy decreases but its potential energy increases.
Potential energy gained by it in rising to the highest point = work done against gravity.
= - mg x h = -mgh
As the body falls down, it gains kinetic energy = work done by the force due to gravity.
mg x h = mgh
Net work done by the force due to gravity
= - mgh + mgh = 0
(ii) Here, m = 20 kg,
u = 5 m s^-1, v = 2 m s^-1
According to work energy principle work done by a force = change in kinetic energy.
= 1/2 mv^2 - 1/2 mu^2
= 1/2 x 20 x (2)^2 - 1/2 x 20 x (5)^2
= 40 - 250
= - 210 J