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The “where did the energy come from?” game

April 14, 2009

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.

13 Comments leave one →
  1. mllamoreux permalink
    April 16, 2009 6:59 pm

    Great game. I will use it.


  2. Jill Alvarado permalink
    April 22, 2009 3:27 pm

    I have used this game with high school students to teach about energy transfer and the types of energy (kinetic, chemical, etc.). We then make the point that all our energy comes from the sun with one exception – geothermal energy! Things like volcanoes and hydrothermal vents (and life & phenomena associated with them) all derive their energy from the earth itself. This then makes a point that if the sun went out, there would still be life on the planet. And from there, the game can lead to great questions and discussions about physics, “life finds a way”, conservation, solar and geothermal power, ecosystems, etc…. that is, if you can hold their attention that long. :o)

  3. carefreeabandon permalink
    April 23, 2009 2:18 pm

    When I tried this game (and I tried it long before I read it on ur blog) the answer was always the Sun. (I wonder if I’m right)
    Also, is your blog private? That might explain why I can’t subscribe to it. But it is on my blog roll (no sarcasm here. I like Physics…I think)

    • gravityandlevity permalink*
      April 23, 2009 2:24 pm

      As far as I know, this blog should be completely public. I’m not sure why you’re having trouble subscribing… I am unfortunately very new to the world of blogging.

  4. carefreeabandon permalink
    April 25, 2009 4:30 pm

    Oh, okay, never mind. I’ll try and figure it out.

  5. jacobus permalink
    May 2, 2009 12:47 am

    Oh yes, this is the Prime Mover. Aristotle’s metaphysics strikes again!

  6. Barbara permalink
    July 20, 2009 3:30 pm

    I like the game, but I’m having trouble connecting some things back to the sun. For example, what about Nuclear power plants? The energy comes from the breaking of uranium, which is mined from the Earth…which I assume has been there since the Earth formed…

    Also, what about hydroelectric plants? The energy really comes from the gravitational potential energy of the water, which I guess comes from the shape of the Earth…how does that relate back to the sun?

    Maybe by Earth Science skills aren’t good enough to fill in the gaps…

    • gravityandlevity permalink*
      July 20, 2009 3:44 pm

      Hi Barbara,

      This is a pretty hard game, isn’t it? Every question is ultimately difficult and has an answer that is full of interesting physics.

      I’ll answer the easy one first: you’re right that hydroelectric plants run off the gravitational potential energy of water. But the water got that potential energy by being evaporated from some lower altitude and then rained down on the mountains (or wherever the river has formed from). So ultimately their gravitational potential energy came from the sun heating a lake or ocean.

      As for uranium… you got me on that one. Uranium has an intrinsic energy due to the instability of its nucleus (the nucleus is in some kind of intrinsically tense, high-energy state). Heavy elements like uranium can only be formed by the collapse and subsequent explosion of a dying star. The remnants of this exploded star formed a gas cloud called a nebula, from which high density chunks eventually emerged that became the planets. The fact that we have uranium on Earth is a sign of our planet’s violent birth. The energy stored in a uranium nucleus does not come from our current sun, but from a giant, now-deceased star that gave birth to the Sun and its planets.

      I am not a cosmologist, so if anyone out there would like to add to/correct this explanation, please feel free.

      • August 21, 2009 1:10 pm

        Nope, sounds about right to me. (I’m not an astronomer, but I play one on the Web: is my other Web site.) The only thing I’d add is that a supernova nebula is relatively short lived, so that the uranium (and all the other remnants of the supernova) may be fairly well mixed into the interstellar medium before a solar system is formed. On the other hand, the shock wave of expanding material from a supernova explosion could itself cause a solar system to be formed, by compressing the interstellar medium enough for a nucleus (which will become a star or group of stars) to form. I’m not aware that there’s a good way to figure out how exactly we got our uranium, though supernova core collapse is one sure thing about that.

      • Stephen permalink
        August 9, 2012 4:01 pm

        To my knowledge there is no such thing as intrinsic engergy due to instability. A wobbly stack of blocks does not have intrinsic energy due to its instability. It has potential energy due to its height above the table and ultimately above the surface of the earth. Its instability makes it easy to transfer the potential energy to kinetic energy with a slight push. The same can be applied to Uranium. Just like blocks can be arranged in various stable and unstable configurations so to atomic constituents can be arranged. Uranium happens to be an unstable configuration of protons and neutrons mainly due to its size. When given a slight push (colliding a proton with the Uranium nucleus) the “potential” energy stored in the mass of the Uranium nucleus is released as electromagnetic radiation and kinetic energy of various small particles. So the energy produced by a nuclear power plant comes from the mass of the Uranium atom. As for the rest of the explanation (how does Uranium get its mass?), your reasoning seems plausible to me.


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