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© Copyright 2000, Jim Loy
Let's invent a car that is run by a rapidly spinning
flywheel. Somehow we got it spinning (maybe I turned a crank for a few hours),
and now we want to see it climb a hill. Our test driver gets into the car, puts
on his helmet (and gloves), puts the car in gear, and drives part way up the
hill. Then the car stops, as the flywheel has stopped. Why did the flywheel
stop?
The car had energy (in its flywheel). This energy was used to raise the car to a certain height on the hill. Where did the energy go? Did we lose it? No, the car now has a different kind of energy (potential energy). So we didn't lose the energy. The energy merely changed form. Kinetic energy (energy of motion) in the form of a spinning flywheel became kinetic energy in the form of a moving car, and then became potential energy in the form of a car partway up a hill. We can now put the car in neutral and zoom down the hill, if we want to. So we can convert the potential energy back to the kinetic energy. And then, if we apply our brakes at the bottom of the hill, we can convert all that energy into heat energy (which is another form of kinetic energy).
All this is an example of the Conservation of Energy (also called the First Law of Thermodynamics):
Energy is conserved. It is neither created nor destroyed. It only changes forms.
Much of science (especially physics and chemistry) involves changing one form of energy into other forms.
Conservation of energy is a law of physics. In fact, it is one of the most basic, and well tested of all laws. It is called a law because it has never been violated. It always works. Every time that it seemed to be violated, it was shown (usually quite easily) that it was in fact not violated. See E=mc² (E=mc^2), where we find that mass is another form of energy.
Physicists sometimes say that the Law of Conservation of Energy says, "You can't win," or "You can't come out ahead," or "You can't get something for nothing." They then go on to say that the Second Law of Thermodynamics (the law concerning entropy, which I will write about later) says, "You can't even break even," or "You lose."