A ball dropped from 9th stair of multi storied building reaches the ground in 3 sec. In the first second of its free fall, it passes through n stairs then n equal to

Options

A. 1
B. 2
C. 3
D. 4

Answer

a

Explanation

Let height of one storey be h.
Total height =9h
A ball is dropped from height 9h in t=3
`therefore 9h=0+(1)/(2)g(t)^(2)[u=0]`
`9h=5xx3xx3`
h=5m
Let, in the first second it crosses the n storiesof total height nh
`therefore nh=0+(1)/(2)g(1)^(2)`
`nh=5`
n=1 storey