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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.
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Solution
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
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