A stone is dropped from a certain height which can reach the ground in `5 s`. It is stopped after `3 s` of its fall and then it is again released. The total time taken by the stone to reach the ground will be .

Options

A. (a) `2 s`
B. (b) 3 s`
C. ( ` 4s`
D. (d)` none of these`.

Answer

C

Explanation

Height from where the stoneis dropped = 1/2 gt^2`
` = 1/2 xx 9.8 8 xx 5%2 = 4.9 xx 9 m`
Vertoca, dpwmward dostamce covered by stone in `3` seconds ` 1/2 xx 9.0 xx 3^2 = 4.9 xx 9 m`
Velocity attained `= 9.8 xx 3 ms^(-1)`
Remaining distance ` 4.9 xx 25-4.9 xx 9`
`= 4.9 xx 16 m`
If ` t_1` is the tiem taken to cover the remaining distance when stone is dropped again (u=0)`
` 4.9 xx 16 1/2 xx 9.8 t^2`
` t= sqrty16 = 4 s`.