Re: PHYS-L Digest - 17 Oct 2000 to 18 Oct 2000 (#2000-374)

["Promod R. Pratap" <pratapp@UNCG.EDU>]

I was always under the impression that the equation: F_st =mu_st * N only
applied when the object is about to slide.  Otherwise, F_st is independent of
N.  Take the case of a light box and a heavy box sitting on the floor.  Now
apply the same small force to each box; this force is smaller than mu_st*N
for either box (with mu_st) being the constant evaluated just before the
boxes slide).  In that case, the F_st must be the same for both boxes, and is
therefore independent of N.  Of course, one can say that mu_st increases
differently for the two boxes, but that would mean the mu_st is a function of
N.  In the latter case, mu_st must be proportional to 1/N when the box is
stationary.

Any comments?

Promod Pratap
On Wed, 18 Oct 2000 02:01:23 -0700 Automatic digest processor
<LISTSERV@lists.nau.edu> wrote:

> ----------------------------------------------------------------------
>
> Date:    Tue, 17 Oct 2000 23:11:35 -0400
> From:    Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>
> Subject: Re: Friction (STATIC)
>
> brian whatcott wrote:
>
> > It is indeed helpful to come at length upon a physical system...
>
> Brian's message made me think about a peculiarity of static friction.
> Is it associated with heating? Is it associated with the macroscopic
> work done by the applied force?
>
> What happens when we push a box before it starts sliding? The
> applied force increases continuously and so does the static force
> of friction. If F_st =mu_st * N then mu_st must increase gradually.
> What is behind the increase mu_st from zero to the maximum
> quoted in tables? Perhaps the center of mass does move a little,
> heat is produced and work is done, like in kinetic friction. It is
> like stretching and breaking tiny elastic springs, I suppose.
> Ludwik Kowalski
>
> ------------------------------
>
> End of PHYS-L Digest - 17 Oct 2000 to 18 Oct 2000 (#2000-374)
> *************************************************************

----------------------
Promod R. Pratap
University of North Carolina at Greensboro
Department of Physics and Astronomy
PO Box 26170
Greensboro, NC 27402-6170
Phone: (336)334-3214 (office), (336)334-4279 (lab)
FAX: (336)334-5865