A0.50 kilogram ball is held at a height of 20 meters. what is the kinetic energy of the ball when it reaches halfway after being released?

While the ball is held 20 meters above the floor,

                                   Potential energy = (mass) (gravity) x (height) =

                                                        (0.5 kg) x (9.8 m/s²) x (20 m) =  98 joules.

When it's dropped and reaches 1/2 of its initial height, it has half as much
potential energy.  The rest has turned into kinetic energy  =  49 joules.



Want to know what else you can figure out ?  Watch this . . . 

                                                 Kinetic energy = (1/2) x (mass) x (speed)²

                                                          49 joules = (1/2) x (0.5 kg) x (speed)²

Divide each side by (0.25 kg):       49 joules / 0.25 kg = speed²

Take the square root of each side:        7/0.5 = speed = 14 meters per second

That's the speed of the ball after falling 10 meters ! 
How cool is that !