Question

A stone thrown vertically upwards takes 4 s to return to the thrower. Calculate

1. initial velocity of the stone
2. maximum height attained by stone. (Take g = 10 ms⁻²)

Answer 1

A stone thrown vertically upwards takes 4s to return to the thrower,
⇒ Time is taken by the stone to reach maximum height = t = `4/2`
t = 2s
Initial velocity of stone = u =?
Maximum height attained by stone = h =?
Acceleration = a = −g = −10ms⁻²

(i) v = u + at
Velocity of stone at heighest point = v = 0
0 = u + (−10) 2
u = 10 × 2 = 20 ms⁻¹ = 20 ms⁻¹

(ii) v² − u² = 2aS
(0)² − (20)² = 2 (−10) h
−20h = −400
h = `400/20` = 20 m