The “where did the energy come from?” game

If you read the last post, you know how I feel about energy.  The concept of energy, and how it flows from one place to another, is the most important single idea that we have in physics.

There is a nice little game you can play to get you thinking about the ebb and flow of energies in the universe.  I call it the “where did the energy come from?” game.  It works like this: think of any object that has energy.  Then ask “where did it get that energy?”.  Once you figure that out, ask the question again “where did that energy come from?”.  Repeat ad nauseum, tracking the energy as far back as you can manage.  Can you find out where it originated?  It’s kind of like the game where a little kid keeps asking his parents “why?”, but now you’re an adult (with nerdy tendencies), and it’s a lot more fun.

Of course, I am not the inventor of this game.  It has been around a long time, and probably plenty of you reading this post have played it before.  But it’s a remarkable game for a few reasons.  1) It can be played and enjoyed with only a very basic understanding of physics, or it can be played and enjoyed with a great deal of physics training (as you get better at physics, you get better at filling in the details of how exactly the energy was stored and how it was transferred).  2) It is an entirely non-mathematical way of reviewing some of the most important ideas in physics.  3) It takes surprising and unpredictable turns along the way.  4) It has a surprising conclusion.

I’ll give you a few examples here, and then say a few words about the “great conclusion”.  I remember it was surprising to me when I first played this game, although it may not surprise the biology majors out there.  When I was a TA and introduced this game to my students, several of them seemed genuinely moved by it, which was probably the greatest moment of my TA-hood.

Example 1: A baseball is hurtling toward home plate.  It has kinetic energy.

• The baseball gained energy from the pitcher’s muscles, which had stored chemical energy in the form of ATP.
• The chemical energy in his muscles came from chemical energy in the food he ate (let’s assume he ate a steak for lunch before the game).
• The chemical energy in the steak came from chemical energy in the food that the cow ate: grass.
• Chemical energy in the grass comes from photosynthesis.  Photosynthesis is the process of making chemical energy out of electromagnetic energy from the sun.

Example 2: A plastic grocery bag has been blown high into the sky by the wind.  It has gravitational potential energy.

• The grocery bag was pushed up into the sky by collisions with air molecules (wind), so its gravitational potential energy was formerly kinetic energy of air molecules.
• Wind comes from pressure gradients: air flows from high-pressure to low-pressure areas.  Higher air pressure is a result of higher air temperature (PV = NRT).  Higher air temperature is a direct result of heating from the sun.

Example 3: My car is driving on the freeway.  It has kinetic energy.

• My car got its kinetic energy from the rotational kinetic energy of the engine’s crankshaft.
• The crankshaft got its energy from being pushed by the engine’s pistons (their linear kinetic energy became rotational kinetic energy of the crankshaft).
• The pistons got their kinetic energy from combustion: a series of controlled gasoline-induced explosions that turned stored chemical energy into kinetic energy.
• Gasoline is made of ancient organic matter: long-dead plants and animal whose bodies have decomposed and compressed, turning them into a thick, black slurry.  The chemical energy in gasoline is still the same chemical energy that existed in the living bodies of those ancient plants and animals.
• Those ancient plants and animals got their chemical energy either from the sun, or from the food they ate (which inevitably got it from the sun as well).

These are just a few examples.  But try it, and you’ll almost certainly come to the same conclusion.  Just about everything on the Earth owes its energy to sunlight (except for maybe in places like this).  Of course, if you’re really good, you can answer the question “where did the sun get its energy?”, and eventually you’ll get back to the Big Bang.  I, personally, am not confident enough in my cosmology to go that far.