Re: Friction

[palmer@sfu.ca (Leigh Palmer)]

> > Everyone seems anxious to teach that static friction is greater than sliding
> > friction and demonstrate it with objects on ramps etc.  Yet, for most of us
> > car tires are the most improtant application of friction we experience, and
> > they work the other way around.  Anybody have any good demonstrations to show
> > a case where static friction is less, to simulate tires?  Quite honestly, I
> > think this is another case like the half a lens, and tides, where well
> > intentioned teachers who are out of touch with the real world mislead  their
> > students.  In this case wrong ideas about friction could lead to an accident.
>
> Howdy again,
>
> So, are you saying that a car stops faster if it's skidding than if the
> tires are rolling (i.e., there is no RELATIVE motion between the point(s)
> of contact of the tire and the surface: therefore STATIC friction)?  I
> wonder why they made  anti-lock brakes?

You're telling me that antilock brakes keep the tires in rolling contact
(that is, not slipping) as I understand it. That should be easy to test.
If that were the case, then on a sufficiently clean piece of pavement no
skid marks would be left in a panic stop with antilock brakes. As I have
no equipment with which to test that (we have one car, a 1989 Dodge
Caravan), I can't find out. Will someone else do the test? It will
surprise me a bit if no skid mark is left. I had always been led to
believe that antilock brakes worked on a slightly skidding tire. Will
someone tell us how they do work? It won't settle this question, but Dave
has already given the correct answer to it, so that doesn't matter.

I think what ABS does is to pump the brakes fast, but I don't really know.
That would produce a bunch of miniskids, I think, and steering should not be
impaired. The idea of working around the skid point is to maintain control,
I think; it does not make the car stop faster than it could with a
practiced driver at the wheel. What it does is prevent loss of control
by an unpracticed driver. The business of stopping faster is one I
didn't think as important. What happens when an ABS equipped car does
a panic stop on snow, by the way? There is little doubt in my mind that
the degree of traction is greater for a rolling tire than for a skidding
tire in that case. It is my guess that a panic stop on snow isn't any
better idea with ABS than without it.

I'm naive about ABS; I want to know. If you are teaching about friction
coefficients of various flavors then you will probably get these same
questions in class. Can you answer them? I can't. Dave's right; this is
the most important (but also the most obvious) example of friction in
the mind of the student. If we use cars in problems involving traction we
should know the answers to such questions.

One of my last semester's students just asked me a question (by email):

>   I was doing some backyard observing after around 11:30pm on Saturday,
> July 13th when at around 1:10 am I noticed an object brighten suddenly
> about 15 degrees due east from the star Deneb.  The object went from being
> invisible to the naked eye to suddenly brightening in intensity and even
> rivaling the star Deneb.  The star-like object brightened for about five
> seconds and then disappeared and about one minute later it again went
> from being invisible to intensifying to the brightness of Deneb but this
> time it sustained this brightness for only a few seconds...

He went on to ask me what he had seen, and I gave him an excellent answer,
if only my best guess (a Molniya trajectory satellite).

I'm very pleased that my students still ask me questions, even during the
summer. I believe they do that because I have taken an interest in real
world questions, and I have not given them glib answers in the past. You
can't "figure out" how ABS brakes work from first principles, but surely
it is not beyond the capability of a physics teacher to understand how
they work and to pass that information on.

Leigh