A ball dropped from the top of a tower covers a distance `7x` in the last second of its journey, where `x` is the distance covered in the first second. How much time does it take to reach to ground?.

Options

A. 3 s
B. 4 s
C. 5 s
D. 6 s

Answer

B

Explanation

In first second distance travelled,
`x=(1)/(2)xx g xx t^(2)=5m`
(for `g=10 ms^(-2))` in last second `7x=35 m`
Now, `S_(t)th=(u+at-(1)/(2)a)`
`rArr 35=0+10xx t-(1)/(2)xx10`
`therefore t=4s`