**Goal:** Problem solving with rotational dynamics

**Source:** UMPERG-ctqpe135.5

A

disk, with radius 0.25 m and mass 4 kg, lies on a smooth

**horizontal** table. A string wound about the disk is pulled with a

force of 8N. What is the angular acceleration of the disk about its

center?

- 0
- 64 rad/s
^{2} - 8 rad/s
^{2} - 4 rad/s
^{2} - 12 rad/s
^{2} - None of the above.
- Cannot be determined

## Commentary:

## Answer

(6) The correct value of α

_{cm}is 16 rad/s^{2}. Students have the erroneousconcept of ‘conservation of force’. Many think that since the disk

moves, the full force cannot contribute to the torque about the center

of the disk.