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Re: TORQUE



David Abineri wrote:
We are discussing a pully with a string going over it and the string not
slipping against the pulley with two different masses attached to each end
of the string, one on each side of the pulley.

I drew the free body diagram of the pulley showing a supporting force at
the center and a Tension downward on the left and also one downward on the
right of the pulley.

The student's comment was "aren't all parts of the string that are in
contact with the pulley also supplying a torque on the pulley?" In other
words, he is not comfortable with the notion of just the two tensions
supplying torque but feels there are other torques that need to be included
in the net torque acting on the pulley.

I think this is a problem where it is crucial to be very aware of what "the
system" is. Your diagram shows the external forces on a "system" made up
of the pulley plus a piece of string draped over the top. The student's
diagram is for the "system" made up of the pulley alone. They are both
correct, but yours is easier to analyze.

--
--James McLean
jmclean@chem.ucsd.edu
post doc
UC San Diego, Chemistry