Energy

Energy is defined as the capacity to do work. This is why work and energy carry the same dimensional formula, $[ML^2T^{-2}]$, and the same SI unit, the joule (J) -- doing work on a system is precisely how energy flows into or out of it.

Energy shows up in nature in many forms -- mechanical, electrical, thermal, nuclear, and so on -- and many machines exist specifically to convert energy from one form into another. This unit restricts attention to mechanical energy, which is further split into two types:

• Kinetic energy, the energy a body possesses because it is moving, and
• Potential energy, the energy a body possesses because of its position or configuration relative to its surroundings. …