a ball is dropped from a tall building with height 30.0m. What is the speed of t

Question

a ball is dropped from a tall building with height 30.0m. What is the speed of the ball at the height of 20.0m? What part of energy is in kinetic energy form at height of 20.0m? At what height does the ball have equal amount of gravitational potential and kinetic energy?

I found the velocity to be 14.0m/s

I found the kinetic energy at h=20m to be 33%

I posted this question before, and the person who answered it said that the ball has equal amounts of gravitational potential energy

and kinetic energy at h=15m; however, he did not explain how he came about that answer.

Can someone help me understand why kinetic and gravitational potential energy are the same at 15m?

Explanation / Answer

By energy conservation

KE1 + PE1 = KE2 + PE2

KE1 = 0 as initially v = 0

PE1 = mgH, where H = 30 m

PE2 = mgh, where h is the height where PE and KE will be equal

KE2 = 0.5*m*v^2

and you already know that at height h

v = (2gh)^0.5

KE2 = 0.5*m*2*g*h = mgh

Now in first equation

0 + mgH = mgh + mgh

h = H/2

Comment below if you still have any doubts.

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