Re: Friction

[Ken Fox <kfox@stega.smoky.org>]

On Thu, 29 Oct 1998, John DaCorte wrote:

> Hi,
>
> 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.
>
> John DaCorte 

Friction deoms are a real problem. To do what you are trying to do
requires the coefficient of friction be the same on the two surfaces.
There is the problem.
The grain of the wood is different on the two edges and this causes the
difficulty. It is okay to discuss that the great change in area does not
give a proportional change in friction coefficient.

One thing to try is to cover the block with paper so the two surfaces may
be more the same. I have had students do this with two styrofoam cups
slightliy loaded with marbles. One cup has had half its bottom removed.
The results they give shows good agreement with theory.  

Hope this makes sense.

Ken Fox
( who spends as much summer time as possible on that beautiful island that
John
gets to teach on. I am jealous! ;-) )
Smoky Hill HS
Aurora CO