Question

a boy throws a ball of mass 20g vertically upwards .It rises to max. height of 100m. At what height will K.E. be reduced to 70%

Answer 1

Potential power of the ball at 100 m =  kinetic energy at the bottom
The Kinetic energy is = mgH = (20/1000) x 10 x 100 = 20 J.
From the law of convertion we can say 30 % the initial kinetic energy is converted into potential energy at that point as kinetic energy diereses to 70 % of the initial value.

So, assume the point at height as H

than , $(30/100)*20=mgH$ (KE = 20)
         so, $H=30m$(ans)
          

Answer 2

30 meter ...........