The maximum height is reached is `5s` by a stone thrown vertically upwards and moving under the equation `10s=10ut-49t^(2)`, where `s` is in metre and `t` is in second. The value of `u` is

Options

A. `4.9ms^(-1)`
B. `49ms^(-1)`
C. `98ms^(-1)`
D. None of these

Answer

B

Explanation

Given equation is `10s=10ut-49t^(2)` or
`s=ut-4.9t^(2)`
`implies(ds)/(dt)=u-9.8t=v`
When stone reaches the maximum height, then `v=0`
`impliesu-9.8t=0rarru=9.8t`
But time `t=5s`
So, the value of `u=9.8xx5=49.0ms^(-1)`
Hence initial velocity `=49ms^(-1)`