**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-m^{2}. The friction

force on the disk is:

- 0
- 15N, to the right
- 10N, to the left
- 5N, to the right
- 5N, to the left
- None of the above
- Cannot be determined

## Commentary:

## Answer

(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.