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You've probably heard the term "energy" for most of your lives. When you were a toddler your parents might have complained, "he/she has so much energy I just can't keep up with him/her." The term has lots of everyday meanings.

But in your science classes you certainly will have heard the term in its more technical sense -- these molecules have a particular binding energy; ATP is the energy currency of the cell, and so on.  But what is energy really?  For a physicist, in the end everything is energy -- because we have learned that mass is a form of energy.  But that doesn't help very much.  Here are a few key ideas to get us started.

• At beginning, the best starting point for building up the concept of energy is motion. The place where historically the idea of energy started and the place where we will start is with the sense that energy is something we will associate with moving objects -- a quantification of the idea of motion.
• At the end, energy becomes the universal currency of physics -- it's stuff that can, in principle, be converted into motion or that has come from motion.  

The way this works is that we follow a process that has turned out to be immensely useful in organizing our thinking about physics and indeed about much of science.

1. We look at our usual "simplest possible cases" and decide how we might quantify the concept of motion.
2. We find two ways of doing this -- momentum and energy.  With one of them, energy, we find there is a conservation law that allows us to introduce concepts that look like transformations of the energy of motion into other forms.
3. Every time we find a situation that looks like the sum of the energies we have defined is not conserved, we try to introduce a new kind of energy in order to keep things conserved.

This process sounds circular. What good is it if every time our law fails we introduce a term to save it? Isn't this sloppy science?

Well, no. We might decide this was a useless process if we had to introduce a new kind of energy for essentially every new experiment or phenomenon we looked at. But we start with the energy of motion (kinetic energy) and quickly add the energy of relative position (potential energy). We soon discover that something being hotter can be considered a kind of energy (thermal energy) and corresponds to an increase of an internal hidden motion of an object's atoms and molecules. We then discover that there is energy stored in the structures of atomic bonding to form molecules (chemical energy) and this, in the context of quantum mechanics, can be interpreted as kinetic and potential energies distributed probabilistically. We know that light can carry energy, and finally, Einstein suggests that mass can be considered a form of energy. And that's where it now stands -- sort of.

So over a period of 400 years we have invented 6 kinds of energy.* And they have served us to describe millions of experiments and situations. This is very much a part of our modeling of the world and science's ways of knowing. We create ways of thinking about phenomena that are appropriate for those phenomena (choosing a channel on cat television) and then try to stitch our understandings together (building coherence). With energy, we have developed one of the most powerful tools in the scientific arsenal -- and it all begins with thinking about motion.

Follow-ons:

• Kinetic energy and the work-energy theorem
• Energy of place -- potential energy
• The conservation of mechanical energy
• Energy losses -- thermal energy
• An essential energy storage for life -- chemical energy
• Going beyond -- nuclear energy, mass energy, dark energy, and ...?

* As of this writing, the scientific community has learned that the galaxies of our universe seem to be moving away from each other at increasing speeds -- accelerating rather than slowing down as would be expected from the fact that they attract each other gravitationally. Some source is providing huge new energies of motion to objects of galactic scale. If, as we expect, there is still a conserved total energy, this implies that there is a new kind of energy that is being transformed into kinetic energies of galaxies. We don't know what this is, but for now we are calling it "dark energy". Stay tuned!

Joe Redish 7/29/11