A boy stands on a cliff edge and throws a stone vertically upwards at time t = 0. The stone leaves his hand at 20 m s⁻¹. Take the acceleration of the ball as 9.81 m s⁻².

a. Show that the equation for the displacement of the ball is:

s = 20t − 4.9t² 

b. Calculate the height of the stone 2.0 s after release and 6.0 s after release.

c. Calculate the time taken for the stone return to the level of the boy’s hand. You may assume the boy’s hand does not move vertically after the ball is released.