I think it was Feynman that called energy "the currency of the
universe." We use that as a very rough definition. "Whatever you want
to do, it will cost you. If you want to speed something up, slow it
down, change its temperature, stretch it... it will cost you energy.
The energy it costs you doesn't disappear. It just isn't yours anymore.
The energy now belongs to the object you just finished playing with.
The total amount of energy (currency, dollars & cents) stays constant."
We also talk about how one might lose a penny or two in the cracks of
the couch. That doesn't mean the money no longer exists. It just means
we can't use it. We could pay someone (or figure out what the going
rate is) to go looking through the couch cushions, then we could use
those few pennies again.
Paul Lulai . . . To wonder is to begin to understand
Physics Instructor
Saint Anthony Village Senior High School, ISD 282
3303 33rd Avenue N.E.
Saint Anthony Village, MN 55418