Re: [Phys-l] Dynamics Question

[John Denker <jsd@av8n.com>]

On 01/26/2008 06:37 PM, David Abineri wrote:

> A uniform disk of mass M and radius R has a massless, thin thread 
> wrapped around the rim and this thread is pulled by a constant force in 
> the plane of the disk.  This force is the net force acting on the disk 
> (ie in deep space away from any other masses) .  What is the linear 
> acceleration of the CM of the disk and what is the angular acceleration 
> about the CM caused by this force? 

The key conceptual point is this:
   Force is not motion.

If you know the force, you know the force ... and the issue
of whether the object is moving and/or accelerating and/or
rotating or not has got nothing to do with it.

Some students have a hard time becoming comfortable with this
conceptual point.  I don't claim to understand 100% why, but
one hypothesis is that students have a tendency to empathize
with the object, and therefore to use (or misuse) an accelerating 
reference frame.

Some problems are easier in an accelerated reference frame and
some are simpler in the lab frame.  This problem is a poster
child for the sort of problems that are easier in the lab frame.