Question

A ball is dropped from the top of a cliff. By the time it reaches the ground, all the energy in its gravitational potential energy store has been transferred into its kinetic energy store. If the ball is travelling at 20 m/s when it hits the ground, what height was it dropped from? (Assume that the gravitational field strength is 10 N/kg.)
The answer is 20 meters, how do I get that?

Answer 1

U = mgh, Ek = 1/2*m*v^2

U = Ek (conservation of mechanical energy)

⇒ mgh = 1/2*m*v^2

∵g = 10, v = 20

⇒ 10h = 1/2*400

⇒ h = 20 (m)