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Question

A ball is thrown vertically downward with a velocity of 20 m/s from the top of a tower. It hits the ground after some time with a velocity of 80 m/s. The height of the tower is :
(g=10 m/s2)

A
320 m
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B
300 m
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C
360 m
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D
340 m
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Solution

The correct option is B 300 m

Let the height of the tower is h.

The motion of the ball is constant accelerating motion with acceleration g=10m/s2.

Initial velocity of ball, u=20 m/s
Final velocity of ball, v=80 m/s

According to equation of motion for constant accelerating motion,
(Here, downward direction is negative and upward direction is positive)

v2u2=2(g)(h)

v2u2=2gh

(80)2(20)2=2(10)h

6400400=20h

6000=20h

h=300 m

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