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Hey so I've just learned about angular velocity and momentum and how torque changes it.

Looking at a wheel spinning around an axis, with one end being held up by a rope, what causes the wheel to rotate downwards over time, and eventually fall?

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I didn't saw it rotating backwards, the only thing which i saw was that the wheel stops after some time, that can be easily attributed to the friction the ball bearing offers.... – Vineet Menon Sep 25 '11 at 12:19

Friction. The wheel is held up due to the effects of a torque on the angular momentum of the wheel; when the wheel no longer has angular momentum (because it stops spinning due to friction) precession no longer occurs. Wikipedia has more information on precession, but I think this MIT video gives a better intuition.

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