Quick question: What is energy?

“Energy is what makes things go round.”

This is perhaps the easiest way to attempt an explanation of what energy, in fact, is. There is no harm in using this particular way to explain the term even though one may argue that the above sentence is only a definition of how we describe energy, not a definition of the term itself. If we do, we use this sentence within a certain framework, let us say in the classroom. There we put “energy” into context and give it deeper meaning.

With time at any rate. After all, we do need some patience.

If you know energy, you know physics. For physics really only deals with three questions:

1. Where is the energy?
2. Where did the energy come from?
3. How did the energy get there?

How, for example, is it possible that we can see Jupiter in the night sky? Jupiter is a planet which is a rather long way away from us. How do we answer the questions above?

1. The energy lies in the light which hits our eyes.
2. The energy came from the Sun which radiated the light.
3. The light hit Jupiter and got scattered towards us.

Where is the energy? Where did it come from? How did the energy get there? This is a so-called pitcher in American baseball. Pitchers are the ones who manage to throw objects, in this case a small ball, fastest. How and why humans can put enough energy into throwing a ball up to 170 km/h fast is a question many physicists find intriguing. Source: GNU Licensed (http://commons.wikimedia.org/wiki/File:Baseball_pitching_motion_2004.jpg)

Another example, one that is perhaps a little closer down to Earth, is shown in the picture series above. The physics of  baseball is more complex than you would imagine, even though questions 1 and 3 are easily answered. The energy lies in the movement of the ball, while the ball accelerated (and started rotating) because it was thrown. But where was the energy to begin with? Pitchers do not move before starting to “wind up” in the way they do. Does the energy lie in the body and waits to be released? That does sound a little strange. We may even think that we have misunderstood something here. So, what is energy?

I do not have an answer.

“Thanks for the help!” you might think now, and I would understand it if you did. We are teachers and we have a group of students in front of us who want to learn about energy. That is what it says in the curriculum, too, that they should be doing. The students should know the term and we should test whether they do. We cannot do what Newton did and think in different terms to start with. Or can we?

Again, I do not really have an answer, but let us examine more closely what happens when we follow Newton’s line of thought.

Newton looked at an object which was let go from an altitude. When it was let go from a greater height it was faster when it hit the ground. Its speed had to increase during flight. This is intiuitive enough for me to make the argument that the term “energy” is not really important in this particular case. We can look at Newton’s equation and get the students to calculate without knowing about energy.

Then we can introduce energy as an abstract and make its existence legitimate by providing not a definition but a reason: Science deals with things which are about as complex as it gets. Therefore, when scientists tackle a new problem they try to find out what does not change first and take it from there. For example, when using a laser to measure the speed of an object, we do this on the basis of the speed of light not changing significantly as the light moves through air.

Whatever stays constant, is important in physics and, for that matter, in nature.

Now, speaking of nature, we cannot expect her to necessarily follow our lines of thought [1].And that means that whatever does not change is not necessarily a phenomenon or an expression which is easily understood by us.

Energy simply is one such phenomenon or expression. We use it because it is not supposed to change, no matter what does change within the physical system we look at; and here is the deal: The moment we have used it in one case, we will recognize it again and again in others like falling apples, chemical reactions, deformed cars after an accident, snooker, planetary motion, quantum physics – or baseball.

Will our students accept such an approach? Or can they be tested about energy like this? I do not know, but based on my own experience I am rather sure about one thing: They will have a better understanding of what energy is at the end.

What do you think?

[1] This can be an important conclusion in itself.

Alexander is a physicist, teacher and science communicator who is currently working at the Norwegian Centre for Space-related Education at Andøya Space Center in Norway. Even though, in his case, work and play do overlap, the content on this webpage is entirely private. You can follow Alexander on Twitter, Facebook and Google +