The unhelpful sounding physics definition: Energy is the capacity of a physical system to perform work
We know energy: light, heat, electricity, kinetic (motional) energy, potential (the energy something gets when we lift it up) energy etc.
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The initial definition is actually more helpful than it might first appear
In general we are more interested in what energy transitions can do for us than the energy itself
For example electricity is a form of energy - but we are interested in what it can do for us - we can use it to generate heat, run washing machines, pumps etc.
Energy commodities that are themselves not energy contain chemical energy that may be released when we burn them:
Gas can be burnt for heat, and to generate electricity
Coal can be burnt for heat, and to generate electricity
Oil can be burned for heat, and in automobiles and trucks for locomotion
Energy = force x distance
Energy = power x time (or power = energy per unit time)
Physicists measure energy in:
Ergs (cgs system, one dyne force acting over one centimetre)
Joules (SI unit, one Newton force acting over one metre)
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