A2L Item 094
- Description: Determine the friction force acting on a rolling disk pulled by a string wrapped about its circumference.
- Goal: Problem solving
- Source: UMPERG-ctqpe162
- Keywords: Rotational Dynamics, Rotational Motion, Mechanics, Forces, Torque
The question for students:
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:
- 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 for teachers:
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.