# Angular momentum conservation

• hitemup

## Homework Statement

A person stands on a platform, initially at rest, that can rotate freely without friction. The moment of inertia of the person plus the platform is IP. The person holds a spinning bicycle wheel with its axis horizontal. The wheel has a moment of inertia Iw and angular velocity WW. What will be the angular velocity WP of the platform if the person moves the axis of the wheel so that it points vertically upward?

## Homework Equations

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L_initial = L_final

## The Attempt at a Solution

I didn't understand why the angular momentum is conserved only in vertical axis. There is no external force, so what has happened to angular momentum in the horizontal? Wıth regard to what should I choose an axis so that I can apply the conservation?

Last edited:
There is no external force, so what has happened to angular momentum in the horizontal?
External forces do come into play to prevent the platform from rotating horizontally. The platform is constrained to freely rotate only about a vertical axis.

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