A2L Item 096
- Description: Determine the friction force acting on a disk held fixed on an incline by a string wrapped around the disk and attached to the top of the incline.
- Goal: Problem solving
- Source: UMPERG-ctqpe166
- Keywords: Forces, Friction, Mechanics, Problem Solving, Rotational Dynamics
The question for students:
A uniform disk with mass M and radius R sits at rest on an incline 30° to the horizontal. A string is wound around disk and attached to top of incline as shown. The string is parallel to incline. The friction force acting at the contact point is:
- Mg/2, down the incline
- Mg/2, up the incline
- Mg/4, up the incline
- Mg/0.86, down the incline
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(3) Balancing torques about the center of the disk determines that the friction force points up and is equal to the tension in the string. (The other forces, gravity and normal pass through the point and contribute no torques.) Balancing torques about the contact point determines the tension readily.