A body falls freely from rest. It covers as much distance in the last second of its motion as covered in the first three seconds. The body has fallen for a time of :

Options

A. 3s
B. 5s
C. 7s
D. 9s

Answer

B

Explanation

`S_(nth) = u +(g)/(2) (2n-1)`
` = 0 +(10)/2 ( 2n-1)`
` = 5 (2n-1)`
` S_3 =0 +(10)/(2) xx (3)^2 = 45`
Given `: S_(nth ) = S_3`
` therefore 5( 2n-1) = 45 `
which gives n=5.