If an object is dropped from the top of a building and it reaches the ground at t = 4s, then the height of the building is (ignoring air resistance)`(g=9.8ms^(-2))`

Options

A. 77.3 m
B. 78.4 m
C. 80.5 m
D. 79. 2 m

Answer

B

Explanation

`t = 4s" " h = ? `
`s = ut + (1)/(2) at^(2)`
u = 0 a = g s = h
`h = (1)/(2) g t^(2)`
`= (1)/(2) xx 9 / 8 xx 16`
= 78 . 4 m