3.3. Energy

Obviously a course on relativity will involve energy! We’ll keep it simple, although there’s actually nothing simple about energy.

My high school coverage of energy taught me that it comes in various forms like: thermal energy, radiant energy, chemical energy, nuclear energy, electrical energy, motion energy, sound energy, elastic energy and gravitational energy.

Nope. We’ll see that Einstein made this all much simpler, while making it subtle. Let’s get simple and remember that “classical” forms of energy could be boiled down to just

• energy of motion—kinetic energy

• energy that’s just waiting to become energy of motion—potential energy

The easiest way to think about potential energy is holding a book above the floor. It has “gravitational potential energy” since were you to let go, it would fall—it would gain kinetic energy as it sped to the ground. Pull on the bow with an arrow in play and the bow string would have “elastic potential energy” which it’s eager to convert into kinetic energy as it goes back to its original shape. It transfers that motion to the arrow which gains kinetic energy also.

A really important idea is one of conservation. Not the kind that you might think of in nature, although that’s important too. No, “conservation laws” in physics are a big deal, although they can be expressed in a simple way:

What goes in, comes out.

You might have heard that energy can neither be created nor destroyed. Add up all of the energies in some system at some time, let nature do her thing—warm up, cool down, undergo a chemical reaction, slow down, speed up, drop from a height—and then add up all of the energies in the system at some later time and you’ll get the same number.

I’ll make this a little more sophisticated, but the basic rule of energy conservation is just that: whatever goes in comes out.