Re: Friction

[David Dockstader <DRDOCK00@UKCC.UKY.EDU>]

On Sun, 14 Jul 1996 15:46:34 -0400  said:
> Say more about this.  The only semi-pertinent references I found were from
> the CRC Handbook of Chemistry and Physics, 1985-1986...
>     ...You also have me wondering about anti-lock brakes!
I have no specific references to give you. You might get some insight from
reading Car & Driver or looking through books in your engineering library.
I've done both, but no specific reference sticks in my mind.  My comments are
based mostly on emperical evidence.

1.Your values for the coeficent of friction are correct.  Skid pad tests,
acceleration data for 4 wheel drive high performance cars, and braking data
all show max accelerations around 1g so mu must be around 1.  (Two wheel drive
max acc is around 0.5g.

2.  Max acc is always achieved with some slippage of the tires.  Therefore,
slipping friction must be greater than static friction.  Drag racers do not
spin their tires because they are out of control.  They do it because it gives
the max acc.  Likewise a race car in a turn is deliberately set up to slide.
Sliding is faster.  To stop the slide you have to slow down!

3.  More misconceptions about friction:  It is independent of surface area:
Not true for tires.  Why do you think they put wide tires on performance cars?
It is independent of speed:  Only within limitations, which for some materials
are pretty restrictive.  When we have lubricated bearings we still talk about
friction, but now it is really viscosity.  In my books viscous forces are
certainly speed sensitive.  Mu is a constant: only over a limited range
of speeds and forces.

Leigh makes some good points.  The term "friction" is a catch all term which
covers a multitude of sins, so neat generalizations are impossible.  If we
confine our generalizations to "frictional forces" and then tell the students
that it is necessary to figure out what those forces are on a case by case
basis I think we'd give them a more honest picture.  If we have students slide
a few blocks and then tell then imply that this is a good model to use for
all friction, we've done the student a real disservice.  Leigh is right that
F=ma is in an entirely different league than mu=F/N and students really
should be taught in such a way that they understand this difference.

I'm still looking for stuff that will show greater forces for kinetic friction,
variation with area, and variation with speed.  I want to teach the students
to believe what they see, and not what they read.  I've not had success with
these demonstrations in the past, but I suspect it is because the effects are
small and my measurements were not that accurate.  Has anybody experimented
with motion detectors and force probes connected to a computer or graphing
calculator to look for these effects?  My guess is that with their accuracy
it should be possible to show that at least for some materials our simple
linear models are not a very good approximation.