Re: Why is it "static friction?"

[Dan Crowe <dcrowe@SOTC.ORG>]

You need to know the inner and outer radii
and the radius of gyration of the spool.
If the radius of gyration is less than the
geometric mean of the inner and outer radii,
the frictional force is parallel to the tension.
If the radius of gyration is greater than the
geometric mean of the inner and outer radii,
the frictional force is antiparallel to the tension.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org


-----Original Message-----
From: Carl E. Mungan [mailto:mungan@USNA.EDU]
Sent: Friday, November 14, 2003 5:08 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Why is it "static friction?"


Ludwik wrote:

> static frictional
> force acts in the direction opposite to the direction of
> a causing force

Here's a favorite example of mine. Maybe it will amuse you. Consider
a cylindrically symmetric spool of thread. The thread is wound around
the inner diameter. The outer diameter makes contact with a rough
floor. The thread comes off the *top* of the spool and you pull on it
with a constant tension, so that the spool smoothly rolls in the
direction of the pulling (causing?) force without slipping.

Question: Since the spool rolls without slipping, which way does the
friction force point, forward in the pulling direction or "opposite
to the direction of the causing force"?

Hint: You might need more information to give a definitive answer.
What (minimum) additional information do you need?
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
       U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu       http://usna.edu/Users/physics/mungan/