**Goal:** Problem solving with rotational dynamics

**Source:** UMPERG-ctqpe140

A disk

on a horizontal surface sits against a curb. A string wound around the

disk is attached to a mass as shown. If R=5 cm and h=2 cm, the largest

m for which the disk will not move is

- Less than 2M
- 2M
- 3M
- 4M
- 5M
- Greater than 5M
- Cannot be determined.

## Commentary:

## Answer

(4) When m = 4M the torques about the contact point between the disk and

curb balance. Students find this problem very difficult although rather

simple. Many have the most difficulty with the simple geometry needed to

find the moment arms.