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Question
An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
Solution
Potential energy of the object = m × g × h
= 40 kg × 10 ms-2 × 5 m
= 40 × 10 × 5 kg m-2m
= 2000 J
Let v be the velocity of the object at half the height, i.e., after travelling 2.5 m.
Then v2 - u2 = 2as
v2 - 02 = 2 × 10 ms-2 × 2.5 m
So, v2 = 50 m2 s-2
Then,
Kinetic energy `1/2 mv^2`
= `1/2 xx 40 kg xx 50 m^2 s^-2`
= 1000 J ...(∵ 1 J = 1 kg m2 s-2)
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