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How would you explain conservation of angular momentum to a six year old?

  1. Jul 8, 2007 #1
    The other day, a staff member at a science museum I've started volunteering at told me that when spinning on a lazy-susan type device, the main reason the spinning slows down when I extend my leg is because of "air friction."

    Being tactful, I didn't try to explain to her why that doesn't make sense. But it made me think, and I realized that I don't think I could really explain why the spinning slows down (and why it speeds back up when I pull my leg back in) to a six year old, which is the main age group of the museum.

    How would you explain it to a six year old?
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  3. Jul 8, 2007 #2


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    I seriously wouldn't know. I have a six-year old boy, but I wouldn't know how to explain that to him either. Ok, you could invent something like: when you have something turning and you make them bigger, then they turn slower, and when you make them smaller, they turn faster or something, but you'll be confronted soon with counter examples (like a bike and a train, which one goes faster etc...).
    At 6 years, normal kids are just getting acquainted with the natural number system, and some intuition for operations (mostly addition). Very difficult to explain what's a conserved vector quantity!
  4. Jul 8, 2007 #3


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    I used to have six year olds.
    Frankly, I don't think you are going to be able to do much better than just saying conservation of energy.
    When asked what that is then It is one of the rules of how the world works.

    Now if you could just explain it to adults :rolleyes:
  5. Jul 9, 2007 #4


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    You could try to explain the concept of linear momentum first. That an object at speed tends to remain at that speed, noting that in the real world friction evetually slows it down.

    If the child can grasp this concept then explain than an object tends to remain at it's current speed when traveling in a circle, and if the radius is decreased, the object continues to maintain it's speed, but since the radius is smaller, the rate of rotation is increased.

    For an example of this, get a small piece of pvc pipe, and tie a string to a small but relatively dense object. Thread the loose end of the string through the pipe, spin the object around in a circle, then pull on the string to decrease the radius.
  6. Jul 9, 2007 #5
    Sit them on a swivel chair, then tell them to stick their legs out and swing them from left or right. When they swing left, the chair will move clockwise, and when right, anti-clockwise. Actually, a six year old might be too small fo this effect to be very noticeable. If you spun the chair around and asked them to extend and retract their limbs to slow down and speed up the rotation it might work.
  7. Jul 9, 2007 #6


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    Yeah we should probably conquer this issue before we try to explain things to 6-year-olds! By what I read, I think the tour guide that the OP mentioned actually believed friction was cause, not angular momentum.:rolleyes:
  8. Jul 9, 2007 #7
    Theres also a change in energy right? You can show that with the pvc pipe example. How it takes energy to pull the object into tighter circles, but its easier (releases energy) to let the ball farther out.
  9. Jul 9, 2007 #8
    I would use the ice skater example. You can show a video of an iceskater to the child and point out to them that every time as the skater goes into a circular jump they hold out their arms as far as can be while they spin into the jump. Then as soon as they leap into the air they pull in their arms which causes them to spin faster. Then just as they land they stick their arms out which slows down their spinning so that they can regain their balance easier. The explanation could be that the spinning ice skater can increase or decrease how fast she is spinning by pulling her arms in or extending them out during the spin, and that the reason why this works is because of conservation of angular momentum.

    Of course, if the 6th grader asks what those "bumps" are on the ice skaters chest that's when you just nervously say, "yes, momentum is conserved for those too" and proceed to buy them a Nintendo game or something...
    Last edited: Jul 9, 2007
  10. Jul 9, 2007 #9
    We have a spinning device at the museum that is very good at illustrating that when you extend your leg (or butt, or arm, etc.) your spinning slows down; when you pull it back in, you speed up. The tour guide was trying to explain it in terms of "surface area" and "air friction."

    All of these explanations are great, and are helping me. Yes, the problem of explaining to the children's parents is what comes first... I wonder if I can even explain it to myself!
  11. Jul 10, 2007 #10
    When I discuss physics with nonphysics people, I always refer to the centrifugal force as "the merry-go-round effect" The farther out on the merry go round you are the "harder" the merry go round is trying to push you off, which means it gets "tired" and cannot spin as fast. I know, pretty sloppy.....but this is a 6 year old we are talking about. However, if you stop and think about how centrifugal force is tied to the entire energy of the system this is actually more or less correct. I have a 6 year old as well, so I'll check tonight if he buys this arguemunt.
    Last edited: Jul 10, 2007
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