Question

A ball is thrown vertically upwards. It goes to a height 20 m and then returns to the ground. Taking acceleration due to gravity g to be 10 ms^-2, find: the initial velocity of the ball.

Answer 1

Given, maximum height reached, s = 20 m

Acceleration due to gravity, g = 10 m/s²

Let 'u' be the initial velocity.

At the highest point, velocity = 0

Using the third equation of motion,

v² - u² = 2gs

or, 0 - u² = 2 (10) (20) m/s

or, u² =  -(400) m/s          ...[Negative sign indicates that the motion is against gravity]

or, u = 20 m/s