Re: Friction

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Thu, 29 Oct 1998, John DaCorte wrote:

> I was wondering if someone could help me with a demonstration I have had
> trouble with.  I place a block of wood on a tilted desk and tilt it to
> the angle that just makes the block move and secure the desk.  Then I put
> the block on another edge (with a different surface area) to show that the
> frictional force and the coefficient of friction do not change with
> surface area.  The problem is that the block always stays put on the side
> with the large surface area and slides on the side with the small surface
> area.  Any explanations?  Thanks in adavance for any help.

Here's mine:  Your experiment convincingly shows that the frictional force
*does* in general depend on the surface area and no expert testimony,
theory, or other armchair speculation to the contrary can change that fact
of nature.  The model of friction that we use in introductory physics is a
reasonable but not at all excellent approximation to reality. 

John
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