Most physical "laws" are approximations. Like most standard "laws" of
physics, F=(mu)N works only sometimes. I teach my students that they must
not only learn the "law", they must learn the restrictions, assumptions, and
caveats as to when the law applies.
I'm not up on the details either, but I have read elsewhere that for an soft
elastic substance (such as car tires), the frictional force does indeed
depend on the surface area. Racing "slicks" maximize tire area on the road,
but are not so good on a wet pavement. For wet pavement, tire tread helps to
push water out from under the tire, avoiding hydroplaning. However, the
pattern of the tread is relatively insignificant. Does anyone remember the
radical "Aqua-tread?" I heard that it wasn't any better at
anti-hydroplaning than a standard tire, but that the deep central groove did
weaken the tire. If true, that explains why Aqua-treads went the way of the
8-track. It looked good in the store, though. Tread pattern is about sales
as much as it is about physics.
My students enjoy labs where the instructor doesn't know the answer. Your
students might enjoy an exercise where they test the extent to which the
frictional forces depend on surface area and speed by, for example, dragging
a "standard" wood block across strips of various surfaces, such as aluminum
foil, rubber shelf liner, a piece of tire tread, etc. etc.
Vickie
__________Original message follows _______
I did an archive search on this topic and only found somewhat vague answers,
so let me try to get the discussion going again.
We learn in my high school physics classes that the amount of area in
contact between two surfaces does not affect the amount of sliding friction
between them. Invariably, every year a student asks why are wider car tires
better at handling, etc. I respond, somewhat vaguely, that the tire is
rolling over the road with little or hopefully zero slippage, and therefore
it is a different phenomenon. One student this year, showed me a car
enthusiast listserv thread on the same topic. One posting stated that Ff =
mu * Fn only applies experimentally if mu < 1 and rubber versus road has a
mu > 1. Is that really the case? Does anyone have a high-school level
explanation for wider car tires that I can pass on to my students?
I copied and pasted the address for the car enthusiast listserv thread
below.
Greg Schade
Physics/Chemistry Teacher
Parkway West HS
St. Louis, MO
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