Energy is the ability to do work.
If we apply a force to an object, it accelerates. We know this from Newton's Second Law, F = m a.
We do work on an object when we exert a force F on it and move it through a displacement s,
It has an acceleration a = F / m
and it has a final speed vf which we can find from
vf2 = vi2 + 2 a ( sf - si)
vf2 = vi2 + 2 a s
vf2 = vi2 + 2 ( F / m ) s
vf2 = vi2 + 2 ( F s ) / m
vf2 = vi2 + 2 W / m
W = (1/2) m vf2 - (1/2) m vi2
W = [ (1/2) m v2 ]
We define this quantity as the Kinetic Energy, KE,
KE = (1/2) m v2 W = KE
When we do work on an object, we change its Kinetic Energy. Because an object moves it has energy or it can do work on something else. We call this Kinetic Energy.
Power Potential Energy Return to ToC, Work and Energy (c) 2002, Doug Davis; all rights reserved