**Goal:** Hone the concept of angular velocity and link to rotational inertia and angular momentum

**Source:** UMPERG-ctqpe122

A child is standing at the rim of a

rotating disk holding a rock. The disk rotates without friction. The

rock is dropped at the instant shown. As a result of dropping the rock,

what happens to the angular velocity of the child and disk?

- The angular velocity increases.
- The angular velocity stays the same.
- The angular velocity decreases.
- cannot be determined

## Commentary:

Answer

(2) is the correct response if the rock is simply dropped. However, some

students selecting this choice may think that the angular velocity is

maintained by some external agency. Students selecting answer (1) are

likely misapplying conservation of angular momentum.

Background

This question is useful for revealing whether students understand the

concept of moment of inertia and its relationship to angular momentum.

Many students reason that after the rock is dropped, the moment of

inertia is smaller and the angular velocity must increase to conserve

angular momentum.

RevealingQ

When dropped, does the rock have angular velocity? Just before being

dropped does the rock have angular momentum? Just after being dropped

does the rock have angular momentum?

Suggestions

It is possible to do a traditional demonstration of the role of moment

of inertia in angular momentum and then just drop the weights when arms

are extended.