Re: friction (STATIC)

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Referring to my questions (see bolow) "Promod R. Pratap" wrote:

> I was always under the impression that the equation:
> F_st =mu_st * N only applied when the object is about to slide.

It is true that mu_st in our tables is for objects "about to slide".
But for an object at rest, as we push it stronger and stronger
(before sliding), the net force must be zero. Thus F_st increases
too. N=m*g (on a horizontal floor) does not change. So we say
that mu_st must be changing. N remains constant and changes
in mu_st are due to something else than N. The mu_st must be
proportional to the pushing force.

I was speculating that some work is done and some heat is
produced when a resisting object (before the coefficient of
friction drops down to mu_k) when an object is pushed. I
suppose Q can be measured in a very sensitive calorimeter.
Small displacements of the C.M. can also be measured. In
a model, however, we ignore small W and Q. There is
nothing profound in this observation.
Ludwik Kowalski

> > 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