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Re: A problem

At 8:06 PM +0530 8/1/01, D.V.N.Sarma, you wrote about A problem:


Here is a question about which I request list members comments.

A uniform cube with a edge 'a' rests on a horizontal plane whose
friction coefficient equals 'k'. The cube is set in motion with an
initial velocity, travels some distance over the plane and comes to
rest. Explain the disappearance of the angular momentum of the
cube relative to an axis lying in the plane at right angles to the
cube's direction of motion.


INTERESTING!!

others write:

the frictional force supplies a torque which changes the angular momentum
Wolfgang

but Wolgang's force exerts zero torque around the specified axis!!!!!

Force of normal reactioon due to the surface would not act through the
center of the bottom surface. It would act closer towards the front edge.

Surendranath. B.


This would indeed create a net 'stopping' torque since it would then
exceed the opposing torqe due to gravity.

Can we easily see HOW MUCH closer to the the front edge the normal
force is? _*I*_ can't!

But I DO remember an ancient discussion of the work-energy theorem
from H&R that said we could consider that the stopping force of
friction could be considered to be acting AT THE CENTER OF MASS of
the cube (or any extended object being considered.) It works for for
energy considerations and prolly for ang. mom. also?

(I really think that CM isn't stressed as much as it could/should be
in intro courses.)

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