Q. 134.3( 25 Votes )
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 (g = 10 m/s2)
Answer :
Given,
Mass, m = 40 kg
Acceleration due to gravity, g = 10 m/s2
Height, h = 5 m
We know that,
Potential energy = m × g × h
= 40 × 10 × 5
= 2000 J
When the object is fall to its half way down, then its height above the ground = 
=2.5 m
Now, Potential energy = m × g × h
= 40 × 10 × 2.55 J
= 1000 J
Now, we know that according to the law of conservation of energy:
Total potential energy = Potential energy at half way down + Kinetic energy at half way down
2000 = 1000 + Kinetic energy at half way down
Kinetic energy at half way down = 2000 – 1000
= 1000 J
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