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

AI Thread Summary
In the film One Eight Seven, the bicycle's rotation causes the chair to spin due to the conservation of angular momentum. When the bike wheel is spun horizontally, it acquires angular momentum, which necessitates an equal and opposite reaction from the chair to maintain the system's total angular momentum at zero. The chair does not rotate when the bike is vertical because the angular momentum is not effectively transferred in that position. The force causing the chair to rotate comes from the interaction of the spinning wheel with the chair, creating a torque that results in the chair's movement. This demonstration illustrates fundamental physics principles, particularly the conservation of angular momentum.
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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.
 
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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)
 
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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