Energy of motion is usually called "kinetic energy."
A simple example of the transformation of kinetic energy into potential energy, and vice versa, is afforded by the pendulum.
When passing through its position of equilibrium, since gravity can do no more work upon it without changing its fixed point of support, all the energy of oscillation is kinetic. At intermediate positions the energy is partly kinetic and partly potential.
Available kinetic energy is possessed by a system of two or more bodies in virtue of the relative motion of its parts.
If a body whose mass is m grammes be moving with a velocity of v centimetres per second relative to the earth, the available kinetic energy possessed by the system is Zmv 2 ergs if m be small relative to the earth.