Question

A ball bounces to 80% of its original height.What is the fraction of its mechanical energy lost in each bounce ?

Answer 1

Let a ball fall from a height h, 
thenKinetic energy of ball at the time striking the ground= Potential energy of ball at height h

so, K.E=mgh

Kinetic energy of ball due to rebounce will be :

K.Er =P.E of ball at height h1= mgh1

Loss of kinetic energy due to re-bounce=K.E -K.Er

=mgh -mgh1
=mg(h-h1)
=mg(h-80h/100)
=mg(20h/100)
= 0.2 x mgh

Fractional loss in K.E in each re-bounce will be :
=K.E-K.Er/K.E
=0.2 x mgh/mgh
=0.2
=0.2 x100% 
=20%

Fraction of its Mechanical energy lost in each rebound is 20%