### Bicycle Wheel

A person standing on a turntable while holding a bicycle wheel is an
excellent laboratory to observe angular momentum conservation. A person is
initially stationary on the turntable, and the bicycle wheel is rotating
about a horizontal axis.

Initially, the angular momentum about a vertical axis is zero.

The person re-positions the wheel so its rotation axis is vertical. What
happens?

- The person remains stationary.
- The person rotates in the same direction as the wheel.
- The person rotates in the opposite direction as the wheel.

Since there is no external torque acting on the system, the total angular
momentum must remain zero. The person spins in the opposite direction as
wheel.

Re-positioning the wheel so its rotation axis is horizontal again should
stop the person's rotation. Flipping the wheel over so that its rotation axis
is again vertical but reversed will make the person spin in the opposite
direction.