• Potential (Chemical, Elastic, Gravitational, Nuclear)
• Heat
• Kinetic
• Electrical
• Electromagnetic
• etc.

In each case it is reasonable to expect that ‘energy’ could mean something completely different. However, in essence the definition of energy for each of these wildly different bits of physics can be boiled down to:

Energy is the capacity to perform mechanical work

‘Work’ is again a bit tricky to understand (see e.g. http://en.wikipedia.org/wiki/Mechanical_work), but in a simple case Mechanical work may be defined as (in one dimension) a force multiplied by a distance.

The units of force are kg m / s^2 (mass x acceleration). The units of work (and therefore energy) are Force x distance (mass x length^2/time^2).

So physically “the energy of an object represents the force it can exert multiplied by the distance over which it can apply this force” (Force x Distance), or equivalently “energy is the amount of work an object can perform”

Not being a physicsy type, can you tell me what the unit of energy kg m^2 s^-2 actually means? Its easy to see what units of speed mean (how far in how long!) for example, but what is that unit telling me about energy?

Cheers,

Pete