A ball is dropped from certain height above the ground. After 5s, the ball passes through a plane of glass held horizontally and instantaneously loses `20%` of its velocity. If the ball takes 2 more seconds to reach the ground the height of the glass above the ground is

Explanation

In 5s velocity gained v = gt = 50 m/s. Velocity after passing through the glass pane
` = (80)/(100) xx 50 = 40m//s`

Height of the glass pane above the ground is
`h = ut + (1)/(2)gt^(2) = 40 xx 2 + (1)/(2) xx 10 xx (2)^(2) = 100 m`