Work and Energy #2

Question

Using Conservation of Energy, if a rock is thrown up in the air and reaches a height of 12 m before it begins to fall, how fast was it thrown?

Answer 1

Using energy considerations, remember that when a ball falls from height h, the velocity at the ground is

SQRT(2gh) where g = acceleration due to gravity.

So, the velocity as it leaves the throwers hand is the same as if he dropped it from 12 meters. In other words, under the force of gravity, matter accelerates and decelerates the same!

Therefore, the initial velocity would be SQRT(2*9.8*12) = 15.34 m/sec