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Why did the bike rotate in the physics experiment in the film 187?

  1. Nov 23, 2009 #1
    In the beginning scene of the film One Eight Seven, Samuel Jackson's character has one of his students hold a bicycle while sitting in a chair, and pump the pedals until the wheel got up to around 180 rpm.

    When he told the student to tilt the bike to the right, the chair started rotating.

    Why? I can't think of any legitimate physics reason why this would happen.
  2. jcsd
  3. Nov 23, 2009 #2
    It is due to conservation of angular momentum.
    If he sits down on the chair with the wheel not spinning, the total angular momentum of the system is zero.
    If he then starts spinning the wheel when it is horizontal, he gives the wheel angular momentum, and so the chair must spin in the opposite direction to balance the angular momentum and keep the total angular momentum of the system at zero.

    (you can't create a net angular momentum out of nothing, just like you can't create a net momentum out of nothing)
  4. Nov 23, 2009 #3
    I understand that it happens due to conservation of angular momentum, but I have two questions about it:

    1. why doesn't the chair rotate when the bike is held vertical and its wheel is spun. Doesn't the wheel have angular momentum in this state as well?

    2. aside from the conservation principle at work, what actual force is causing the chair to rotate. where is it coming from? what is the force explanation for this demonstration of conservation of angular momentum?
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