An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

Given :         

 Mass of the object,        M=40kg

 Height up to which it is raised,      h=5 m

Potential energy,       P.E = Mgh

P.E = 40×10×5=2000J

Let its kinetic energy be E_k​ when it is half way down i.e  h′=2h​

According to conservation of energy,           

 K.E_i​+P.E_i​=K.E_f​+P.E_f​

0+2000 = E_k​+ Mg (2h)​

2000 = E_k​+ 40×10×25​

∴  E_k​=1000J

Hence the kinetic energy is 1000 J.