    Question

# A ball thrown vertically upwards with a speed of 20m/s from the top of a tower reaches a certain height and then falls back to the earth in 6 s. Find the height of the tower. [Take acceleration due to gravity, g=10 m/s2]

A

100 m

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B

150 m

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C

160 m

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D

None of these

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Solution

## The correct option is C 160 m Let the height reached by the ball when thrown upwards be H and the height of the tower be h. Here, Initial velocity, u=20 m/s Final velocity after reaching height, H is v=0 acceleration, a=−g=−10 m/s2 [It is taken as negative because the direction of acceleration due to gravity is downwards, while the initial velocity is upwards] Using, v2=u2+2as 0=202−2gH H=2022g=20×2020=20 m Now, the ball will fall down from a height of 20 m from above the tower. Initial velocity, u′=0 time, t=6 s displacement, s=20+h acceleration, a=g=10 m/s2 Using s=ut+12at2, 20+h=0×6+12×10×6×6 ⇒20+h=180 ⇒h=180−20=160 m Thus, the height of the tower is 160 m  Suggest Corrections  5      Similar questions
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