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Lubricants and Friction
Name: Allison B.
Status: Student
Age: 11
Location: N/A
Country: N/A
Date: April 2002
Question:
I was doing an experiment on the coefficient of friction
using an inclined plane for my school project. Part of my project was to
look at the effects of lubricants. When I used wood-on-wood and added
either water or oil as a lubricant, the coefficient increased. When I had
a small piece of wood I could raise the plane to a vertical. Is this due
to surface tension, viscosity, or something else?
Replies:
The reason, I believe, is that the fluid makes an airtight seal
between the block of wood and the inclined plane. Especially if the
block is pressed tightly against the plane atmospheric pressure will
hold it firmly in place since if the block is raised slightly from the
plane, a very good vacuum is formed between the plane and the block.
There are also molecular forces between the wood molecules and the
water molecules, but I suspect the vacuum force is the dominant
reason.
This is different from using Johansson Gage blocks, which are machined
so perfectly that they can be "wrung" together so tightly that the
molecular forces hold them together with a large force; the two blocks
almost become one.
Unfortunately I cannot think of a simple decisive set of experiments to
pin down the phenomena. I did note that if you wet one side of an
index card (heavy paper) and press it against the side of a desk or
other smooth surface, it feels glued on. It can be removed easily,
however, by "peeling" it off. This is at least consistent with the
effect being due to vacuum between the paper and the desk.
Best, Dick Plano...
The fluid is soaking into the wood. This in effect INCREASES the adhesion
between the surfaces both by surface tension, viscosity as you suspect --
and also by "softening" the surface of the wood, making it stick better. You
will need to use a smooth metal block and a smooth metal inclined plane, so
that the block can "ride up on" the lubricating fluid.
Vince Calder
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Update: June 2012
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