In the question number 67, the time taken by the ball to reach the ground is

Options

A. 2 s
B. 3 s
C. 5 s
D. 7 s

Answer

C

Explanation

Let `t_1` be the time taken by the ball to reach the highest point.
here, v = 0, u = `20 m s^(-1), a = - g = - 10 m s^(-2), t = t_1`
As `v = u + at`
`therefore 0 = 20 + (-10)t_1` or `t_1 = 2 s`
Taking vertical downward motion of the ball from the highest point to ground.
Here, u = 0, `a = +g = 10 m s^(-2)`, S = 20 m + 25 m = 45 m, `t = t_2`
As `S = ut + 1/2 at^(2)` `therefore` 45 = `0 + 1/2(10)t^(2)_2`
`t^(2)_2 = 45 xx 2/10 = 90/10 = 9` or `t_2 = 3 s`
Total time taken by the ball to reach the ground = `t_1 + t_2 = 2 s + 3 s = 5 s`