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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 final velocity of the ball on reaching the ground .
Solution
Given, maximum height reached, s = 20 m
Acceleration due to gravity, g = 10 m/s2
Let v' be the final velocity of the ball on reaching the ground.
Considering the motion from the highest point to ground,
Velocity at highest point = 0 = Initial velocity for downward journey of the ball.
Distance travelled, s = 20m
Using the third equation of motion,
v2 - u2 = 2gs
or, v2 - 0 = 2 (10) (20) m/s
or, v2 = 400 m/s
or, v = 20 m/s
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