Question

A ball is thrownvertically upward with a velocityof 20m/s from the top of a building if height 25m from the ground. How high will the ball rise??

Answer 1

(a)

H = u²/(2g)

H = 20²/(2×10)

H = 20 m

The ball will reach 20 m high from the point of projection.

It reaches 20 + 20.5 = 40.5m high from the ground.

(b)

t1 = 2u/g

t1 = 2×20/10

t1 = 4 seconds

If v is the velocity with which it hits the ground then,

v = √(u² + 2aS)

v = √(20² + 2×10×25)

v = 30 m/s

v = u + at

t2 = (v - u)/g

t2 = (30 - 20)/10

t2 = 1 second

Time taken for it to hit the ground = t1 + t2

= 4 + 1

= 5 seconds

Note: I took g = 10m/s².