Question

Whats the velocity of a ball falling with 100joules of kinetic energy and a mass of 2 kilograms?

Answer 1

Short answer: v = 10 m/s
Comment
This is just an application of KE = 1/2 m v^2
Givens
KE = 100 Joules
m = 2 kg
v = ???

Formula
KE = 1/2 m v^2

Substitution and Solve
100 = 1/2 * 2 * v^2
100 = v^2 Take the square root of both sides.
sqrt(100) = sqrt(v^2)
v = 10 m/s

Comment
There is no KE unless v > 0. 
You may have  PE when v = 0, but not KE.