# Homework Help: 2011 F=ma #13

1. Jan 31, 2012

### usamo42j

1. The problem statement, all variables and given/known data
The apparatus in the diagram consists of a solid cylinder of radius 1 cm attached at the center to two disks of radius 2 cm. It is placed on a surface where it can roll, but will not slip. A thread is wound around the central cylinder. When the thread is pulled at the angle θ = 90◦ to the horizontal (directly up), the apparatus rolls to the right. Which below is the largest value of θ for which it will not roll to the right when pulling on the thread?

The figure can be found at www.aapt.org/physicsteam/2012/upload/WebAssign-exam1-2011-1-4.pdf (Problem 13)

2. Relevant equations
τ=Fr sinθ
I'm not sure about others; maybe the ones relating angular/translational kinematics

3. The attempt at a solution

I'm confused. Isn't the only torque caused by the force on the thread, so wouldn't the discs always roll to the right?...

2. Feb 1, 2012

### tiny-tim

hi usamo42!
what about the torque of the friction force?