Re: Friction

[Doug Craigen <dcc@cyberspc.mb.ca>]

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.

Two possibilities spring to mind.  Either the coefficient of friction
isn't constant between these two surfaces, or else the two sides of the
block you choose don't have the same frictional properties.  I would
want to be sure it wasn't the latter before giving too much credence to
the former.  I haven't seen  your block of wood, but differences in the
grain on different faces may be the "problem".  How about gluing
something of constistant surface onto the two faces of the wood (e.g.
felt)?  In fact, why not have felt of two different sizes glued onto
opposite faces of a flat piece of wood?

A related demonstration:
Set the table angle to be just below that where the block starts to
slide.  Now push horizontally on the block with your finger.  The result
gives a simple show that the static coefficient of friction is larger
and also shows that it doesn't break down into components (has anyone
else come across students who think that they should use the static
coefficient of friction in the direction perpendicular to the motion of
a sliding object?).

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Doug Craigen          "Technology with purpose"
                       http://www.dctech.com