Also, choose an instantaneous axis of rotation at the contact point of the yo-yo with the surface. The only torque about this point is provided by the tension of the string. Basically the same as your line of action.
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Quist, Oren
Sent: Thursday, September 09, 2010 1:36 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] line of action
"line of action" is the way to solve this problem. It has always been one of my favorite problems and demonstrations.