The ball has potential

Classical Mechanics Level 1

Jason is standing on the tenth floor of a building, which is 30 meters above the ground. He holds a tennis ball in his hand, which has mass 60 grams.

He is arguing with Percy, who is standing on the ground floor, regarding the potential energy of the ball.

Percy says that since the ball is 30 meters above the ground, its potential energy is given by U = m g h = 60 1000 × 10 × 30 = 18 Joules U = mgh = \frac{60}{1000} \times 10 \times 30 = 18\text{ Joules} .

Jason disagrees. He says that since the ball is at the same height as him, h h is equal to 0 0 , and therefore potential energy of the ball is zero.

Who is correct, Jason or Percy?

Neither of them are correct; the correct formula for potential energy is U = − G m 1 m 2 r U=-\frac{Gm_1m_2}{r} Jason is correct, since he is holding the tennis ball. Both are correct, since the reference point of potential energy, U = 0 U=0 , can be arbitrarily chosen. Percy is correct, since height is always measured from the ground.

Pranshu Gaba by Pranshu Gaba

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