If a ball is held in the air 10meters above the ground and has a gravitational potential energy of 20 j, how much kinetic energy will it have just before its hits the ground

20 J

Explanation:

The law of conservation of energy states that (if we neglect air resistance) the mechanical energy of an object in free fall is conserved:

where

E is the mechanical energy, which is the sum of

U = potential energy

K = kinetic energy

When the ball is held 10 meters above the ground, its gravitational potential enegy is U = 20 J, while its kinetic energy is K = 0 (because the ball is at rest). Therefore, its mechanical energy is

E = U + K = 20 J + 0 = 20 J

Just before hitting the ground, its potential energy is zero (because its height is now zero), and since the mechanical energy must be conserved, we still have that E = 20 J. So, we can find the kinetic energy just before hitting the ground:

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