Question

At time t = 0, a ball is dropped from a height is 180 m, towards a floor. On each collision, the ball losses a tenth of its speed. The figure below gives the speed-time graph of the ball, taking g = 10 ms^-1

Then corresponding to the point D on the plot, t, is

(1) 10.8 s

(2) 11.4 s

(3) 16.8 s

(4) 18.0 s

Answer 1

(3) 16.8 s

Let t_A be the time taken to fall through height h = 180 m, after the ball is dropped.

Let t_BD be the time taken to bounce up from the floor, reach the maximum height and then come down to hit the floor again.

Hence the total time taken is

t_OD = t_OA + t_BD = 16.8 s