This is one of the most important rules that scientists have found which describes natural phenomena. Unfortunately there is no non-circular proof of energy conservation -- in the end, all laws of physics that we know of are the result of observation, formation of hypotheses, making predictions, and testing them. Conservation of energy is one such law. If energy could be created or destroyed, all of our ideas of how the world works would have to be modified in some way (and weíd learn something very perplexing). But so far, energy seems not to be created or destroyed.
Energy can be converted from one form to another, though. Mechanical energy, such as the kinetic energy of motion, can be converted to heat energy, for example in the heating of a carís brakes when it slows down. Chemical energy in the gasoline of the car can be converted into both heat energy in the exhaust and heating the engine, and into mechanical energy to move the car. Potential energy, such as the gravitational potential energy stored in an object which is on a high shelf, can be converted into kinetic energy as the object falls down. Electrical energy can be converted to heat or mechanical energy or sound energy in a variety of useful ways around the house using common appliances.
It is often the conversion of one form of energy to another which is the most important application of this rule. Often predictions of the behavior of physical systems are very much more easily made when using the idea that the total amount of energy remains constant. And careful measurements of different kinds of energy before and after a transformation always show that the total always adds up to the same amount.
Historically, of course not all the forms of energy were known to begin with. Scientists had to keep inventing more forms to keep the law of energy conservation true. If that process had gotten too messy or complicated to make sense, we would have had to give up the law.
One very interesting feature of energy is that other forms can be converted into rest mass and back again (particle physicists do this every day in their accelerators). Einsteinís E=mc^2 gives the relationship between the rest mass of a particle (measured in standard mass units) and the amount of energy that corresponds to (measured in standard energy units). It even applies to other systems where particles are neither created nor destroyed. If a box contains some air at a temperature, and then is warmed up, it will become ever so slightly more massive because of the extra energy given to it. You can call that rest mass of the whole box or the mass equivalent of the kinetic energy of the particles in it- nature doesnít care what names you give it.
Tom J. (w Mike W.)
(republished on 07/25/06)