An elevator without a ceiling is ascending with a constant speed of 10 m//s. A boy on the elevator shoots a ball directly upward, from a height of 2.0 m above the elevator floor. At this time the elevator floor is 28 m above the ground. The initial speed of the ball with respect to the elevator is 20 m//s. (Take `g=9.8m//s^2`)
(a) What maximum height above the ground does the ball reach?
(b) How long does the ball take to return to the elevator floor?

Answer

A, B, D

Explanation

`Absolute velocity of ball =30 m//s`

(a) Maximum height of ball from ground
`=28+2+(30)^2/(2xx9.8)=76m`
(b) Ball with return to the elevator floor when,
`s_1=s_2+2`
or `10t=(30t-4.9t^2)+2`
Solving, we get `t=4.2s`