Definition of Energy

What is energy? The ability to do work.

Energy (scalar) comes in several basic forms: Kinetic, Potential; these comprise Mechanical Energy, as well as Thermal, Electromagnetic, Chemical, Sound, etc.

1. Kinetic Energy (K)   K=½mv2. The energy of an object due to its motion.

2. Potential Energy (U): The energy of an object due to its position or due to its geometrical configuration.

Examples: Gravitational potential energy  U=mgh.
Energy stored in a spring   U=½kx2.

3. All other energy forms (O): Thermal, Electromagnetic, Chemical, Sound, etc.

The MKS energy unit is the joule (J).   1 J = 1 N m = 1 kg m2/s2