The kinetic energy of a body, in purely Newtonian mechanics, is Ec = (m × v²) / 2. Since energy is measured in J, mass in kg, and speed in m/s, it follows nicel

The kinetic energy of a body, in purely Newtonian mechanics, is Ec = (m × v²) / 2. Since energy is measured in J, mass in kg, and speed in m/s, it follows nicely that J = kg × m² / s². Or, we can also take the potential energy of a body at some height h above the Earth: Ep = m × h × g, where g is the gravitational acceleration, ~9.8m/s², and we get the same units. It's really not in any way surprising - this is basically the definition of the Joule. The link between units of kinetic energy and units of thermal energy is actually more surprising. The surprising thing about E=mc² is that it gives a definition of energy for a completely stationary body outside any external field. One way to look at it is actually that this is simply the kinetic energy of the body, and that all "stationary" bodies are in fact moving with speed c on the t coordinate in 4D space-time ("a body which is not moving in space at all is moving with speed c towards the future"). [Note that of course speeds are all relative to some reference frame.]