A very large number of balls are thrown verticlaly upwards in quick successions in such a way that the next ball is thrown when the previous one is at the maximum height. If the maximum height is 5m, the number of balls thrown per minute is (take `g=10m//s^(2)`)

Options

A. 80
B. 120
C. 40
D. 60

Answer

D

Explanation

Let the time interval of throwing two successive balls be t sec. so after t seconds the velocity of each ball will be zero and it will reach a vertical height of 5m.
`thereforev=u-g timplies u=g t`. (where `v=0`)
where u is the upward velocity of projection
Also `s=ut-(1)/(2)g t^(2) or 5=g t*t-(1)/(2)g t^(2)`
or, `t=sqrt((10)/(g))=sqrt((10)/(10))=1` sec.