Re: Why is it "static friction?"

["Carl E. Mungan" <mungan@USNA.EDU>]

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/