Re: Friction

[dtmarx@siu.edu (David T. Marx)]

On November 1, 1996 Dwight Souder wrote:
>        Greetings everyone.  The other day, I was talking to our school's
> tutor.  We were talking about what I was going over in Physics when he saw
> something in my notes.  I copied a quote from the Modern Physics by Trinklein
> (Holt publishing) on p. 81 in the section dealing with Friction.  The quote
> states that, "Friction is practically independent of the area of contact.  The
> force needed to slide a block along a table is almost the same whether the
> block lies on its side or on its end.  When the block is on its end, the
> increased pressure causes the actual area of contact to be the same as when
> the block is on its side."  To me, this seems very logical and I've
> tested/demonstrated this many times.  One of the teachers asked a question
> that really made me think.  He wanted to know if the above statement is true,
> then why do race car/drag racers use wide tires?  If the above statement is
> correct, then wouldn't be possible to use narrower tires and get the same
> effect?  Also, apparently race car drivers use "slicks" (tires with no tread)
> to try to optimize the "grip" with the road.  Does it really matter if there
> is tread on a tire on dry road?
>        If you have any information that you can give me, I'd greatly
> appreciate it!  I'm not very knowledgeable of race/drag cars, so the above
> information may be slightly incorrect dealing with the tires.
>
> Dwight
> dsauder@nwosun.discovery.k12.oh.us

Just the drive the point home about what is meant by "contact area..."
        A distinction must be made in this demonstration between apparent
(or nominal) area and the so called 'real contact area."  In the 1950s,
Bowden and Tabor conducted a now famous study of slightly polished steel
flats in contact under various applied loads.  They measured the real area
of contact via electrical conductance.   I don't have the results in front
of me, but I'll do my best to recall the details.  The "loads" varied from
5 g to 500 kg; and the number of contacts varied from 3 to 35,
respectively.  The average contact radius (assuming the contact is
circular) is on the order of 10 micrometers.  The ratio of real contact
area to the nominal contact area is on the order of 1/1000.