Question

A stone is dropped from the peak of hill . It covers a distance of 30m in the last second of its motion . Find the height of the peak.

Answer 1

Let the height of the peak be h m and the stone dropped from the peak takes t sec to reach the ground. It also covers 30m in the last sec i.e. t th sec.

Taking acceleration due to gravity g=9.8m/s2and initial velocity u=0 we can write applying equation of motion under gravity. In

h=0×t+12×g×t2.......[1]

30=0+12×g(2t−1).....[2]

From equation [2] we have

30=12×9.8(2t−1)

⇒2t−1=304.9

⇒2t=304.9+1

⇒t≈3.56s

Inserting the value of t=3.56s in [1]

we get

h=12×g×t2=12×9.8×3.562≈62.1m