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Re: [Phys-l] Dynamics Question



Hi David

I wonder if you are making this harder than it seems to be and so have
wound up confusing yourself.

Based on what you have written, I would approach this by using

(1) F = m*a (where F, a are vectors. Even though not explicit, I
assume you know the constant force applied to the disk as the tension
in the thread);
(2) Torque = I*alpha (where Torque, alpha = angular acceleration are
vectors and Torque = r x F, where r is the vector from some arbitrary
origin to the point of application of the force);
(3) The moments of inertia I for a solid disk and a hoop are easy to
calculate or look up.

You know the net force on and the mass of the disk. So you know its
acceleration, which here is the acceleration of its CM.

You know the force is applied tangentially (since the thread is
wrapped around the disk), so you know there must be some torque with
respect to the center of the disk, which here coincides with its CM.

My apologies if this is too elementary and I have missed the point of
your question.

regards
-Krishna

On Jan 26, 2008 5:37 PM, David Abineri <dabineri@fuse.net> wrote:
I am stumped on the following and am looking for a good way to think
about the problem.

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?

How do the answers change if it is a uniform hoop?

Thanks for some direction on getting at these questions.

David Abineri



--
D. Abineri dabineri@fuse.net

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