Re: Advice needed on simple physics problem

dkjk_at_bigpond.net.au
Date: 02/14/05 

Date: 13 Feb 2005 20:59:04 -0800

Steven Gray wrote:
> dkjk@bigpond.net.au (James Stokes) wrote in
> news:a3ea3c76.0502130411.3bfe41d6@posting.google.com:
>
> >> Hi group,
> >>
> >> I need some advice on the following hypothetical problem. In an
effort
> >> to lift a cylinder of radius r over a step of height r, a man
wraps an
> >> inextensible rope around the cylinder and pulls the rope with a
> >> constant horizontal force T. Assuming that the cylinder rolls
without
> >> slipping around the edge of the tread, the work done by the force
T is
> >> W = T*r*pi/2. The change in gravitational potential energy is mgr,
so
> >> the applied force is T = 2m*g/pi I would like to know if this
solution
> >> is realistic.
>
> >The method looks good, but I believe that you've miscalculated the
work
> >done as W=T*r*pi/2. Unless I've gotten the picture wrong, the
amount of
> >rope pulled under tension is not r*pi/2, but r+r*pi/2.

I'm pretty sure the length is r*pi/2. I can't see how you obtained
r+r*pi/2.

> On second thought, the method is flawed as well. The tension
required to
> do the task is not constant- it's greater at the start and less at
the end.
> If sufficient force is applied to get the cylinder moving and that
same
> force continues to be applied, then the cylinder will have kinetic
energy
> when it gets to the top of the step.

I agree. Any thoughts on a correct solution?
 
> --
> Steve Gray
> sgray2@cfl.rr.com

Thanks in advance.

James

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