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Re: rolling

Surely both comments are correct: the tangential force (of the wrapped
string) moves at twice the velocity thus working with twice the power, but
you still need the forward friction force of the table to account for the
acceleration in sigma F = ma. It's interesting that an accelerating force
can be doing no work, which is, I imagine, what the remark about pseudowork
is about.


At 18:41 04/02/04 -0600, Brian Whatcott wrote:
I see that I posted a confused note at lunch time in this same thread.

I shall need to try not to compound the error.

If a constant force is applied tangentially to a rolling cylinder in the
direction in which the cylinder is rolling by means of a horizontally
unrolling line wrapped round the cylinder,
won't that line move twice as fast as a line which applies a force of
of the same magnitude in the same direction (say by a wire halter and axle)
at the height of the center of the cylinder?
If the distance of application is doubled in unit time for unit force
that indicates more power?....

Hmmm....experimentally: a line wrapped around a bike's front wheel tire
top pulls faster than a line pulling at the axle with the same force....

Brian W

At 07:21 AM 2/4/2004, Skip,you reponded:
>Agreed, but it is not more force, and yet the acceleration increases.
>-----Original Message-----
>=46rom: Brian Whatcott [mailto:betwys1@SBCGLOBAL.NET]
>Sent: Wednesday, February 04, 2004 06:52
>Subject: Re: rolling
>If you put more work in, (or at a faster rate), you get more
> kinetic energy in the barrel.
>The force applied at the tangent of the barrel moves faster,
>further, than the same force applied through an axial tether.
>Brian W
>At 04:39 PM 2/3/2004, you wrote:
> >Speaking of rolling.
> >Some students and I were working on the problem wherein a rope
> > is wound around a barrel to pull it forward.
> > The rope pulls the top of the barrel as it unwinds.
> >When you calculate the acceleration of the barrel, it is
> >greater than would it would be if the rope simply pulled at
> > the center of mass. This must imply that the friction force
> > on the barrel pushes it forward, a surprising result to us
> > at least.
> >Anything wrong with our analysis?
> >skip

Brian Whatcott Altus OK Eureka!

Mark Sylvester
Duino Trieste Italy