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Re: Friction



Hi,
Just a few quick comments on the block.

The presences of finger oils make a difference, and one usually picks up
the block by the edges. Try a light sanding of all sides of the block you plan
to use. This will clean the surfaces and help insure they are of a similar
polish. If you glue your sand paper on a "know flat" surface
( plate glass works well) then with only a little care, sanding should also
"flatten" the surfaces nicely. Also check that the surface of the tilting
board is uniform.

The rule of thumb about friction only applies to "clean hard
surfaces". There was an nice article on friction maybe 18 months ago in
Scientific American on friction at the microscopic level. At that level the
frictional force was proportional to actual contact area, but this contact area
was proportional to the normal force

Thanks
Roger

*****************************

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