Friction Tests
In this experiment, a thin layer of silicon was
attached to the surface of a long track to simulate
contact with human skin (medical literature shows
silicon to have a friction coefficient similar to
the human palm). Small square patches, two inches
per side, were cut from a conditioned leather ball
and a new synthetic ball and affixed to the flat
bottom surface of a hollow carrier, in order to make
good contact with the silicon surface. A mass was
placed inside the carrier to approximate the mass of
the ball. A line was attached to the carrier and run
parallel to the surface of contact and over a
near-frictionless wheel. To measure the coefficient
of static friction, hanging weights were added to
the line in 10g increments until the carrier began
to slide across the silicon surface (as shown in the
diagram below and left). This experiment was first
performed with the ball’s surface dry. Then to
simulate human sweat, Visine was added to each 4
square inch slice of the ball’s surface, one drop at
a time.
Initial friction tests show a much higher
coefficient of friction for the synthetic ball when
dry. The coefficient of friction between the surface
of the synthetic ball and a silicon surface is about
3.2 for our experimental setup. The friction
coefficient is 1.7 for the leather ball, using the
same procedure. Friction tests with Visine applied
to the ball’s surface show that the coefficient of
friction increases for the leather ball. After
repeated application of drops, the coefficient
increased gradually by at least 30% for the leather
ball, thereby making it more "gripable". After
quasi-saturation, adding drops reduced the
coefficient by 20%, relative to a dry ball. However,
for the synthetic ball, the coefficient of friction
reduces immediately by 55% with the first drop of
liquid. A larger reduction is seen with repeated
application of liquid. In conclusion, the wet
synthetic ball is significantly more slippery
compared to wet leather balls.
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