# A2L Item 094

Goal: Problem solving

Source: UMPERG-ctqpe162

A uniform disk with R=0.2m rolls without slipping on a horizontal
surface. The string is pulled in the horizontal direction with force
15N. The disk’s moment of inertia is 0.4 kg-m2. The friction
force on the disk is:

1. 0
2. 15N, to the right
3. 10N, to the left
4. 5N, to the right
5. 5N, to the left
6. None of the above
7. Cannot be determined

### Commentary:

(4) This problem can be done without the arithmetic complication of
finding the mass from the center-of-mass moment of inertia. This is an
excellent problem for stressing multiple solution methods. This is a
situation where two equations are needed. They can be either the linear
dynamical relation and a rotational dynamical relation, or just two
rotational relationships about different points. Some students may
answer (7) because they are unfamiliar with the expression for moment of
inertia about the CM or because they do not know the Parallel Axis
theorem.

# A2L Item 095

Goal: Problem Solving

Source: UMPERG-ctqpe164

A uniform disk with mass M and radius R sits at rest on an incline
30° to the horizontal. String is wound around disk and attached to
top of incline as shown. The string is parallel to incline. The
tension in the string is :

1. Mg
2. Mg/2
3. 2Mg/5
4. Mg/4
5. None of the above
6. Cannot be determined