# A2L Item 177

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?

1. 0
6. None of the above.
7. Cannot be determined

### Commentary:

(6) The correct value of αcm is 16 rad/s2. Students have the erroneous
concept 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.

# A2L Item 087

Goal: Hone angular kinematic quantities and distinguish them from linear kinematic quantities.

Source: UMPERG

A mass moves in a circle with uniformly increasing anglular velocity.

As the angular velocity ω increases, the linear acceleration of
the mass has…

1. constant magnitude and direction.
2. constant magnitude, changing direction.
3. changing magnitude, constant direction.
4. both magnitude and direction changing.