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Question

On a certain planet, a stone weighing 2 kg is thrown vertically downwards with a velocity of 5 m/s from a height of 30 m. It took 4 s to reach the ground. Calculate the work done by gravity.

A
75 J
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B
60 J
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C
60 J
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D
75 J
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Solution

The correct option is D 75 J
It is given that: S=30 m,u=5 m/s
t=4 sec

By kinematic eqn, as it is dropped the acceleration due to gravity (g)

S=ut+12gt2, we get the value of g as,

30=5×4+12×g×16
g=108=1.25 m/s2

work done by gravity Wg=mg.s
=2×1.25×30
Wg=75 J

(Here the work done is positive as the force and the displacement are in the same direction)

hence option D is the correct answer

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