In the first video Walter Lewin shows a bicicle
wheel hanging from a rope
which is attached to one side of the axis
of the wheel.
When the wheel does not rotate
it will hang in a horizontal position.
However, when the wheel does rotate
it will keep its initial
vertical position while hanging from the rope.
The wheel does so because in order to move to the horizontal position
it has to change the direction
of the angular momentum vector for which it would need a torque.
There is a torque present in this case,
but not in the right direction for the wheel to move
to the horizontal position.
That we will discuss on the next page.
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