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Friction Differences
Name: Veronica
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 2000-2001
Question:
is there a tested coefficient of friction between dry cardboard and plastic? What is the best way for a student to find coefficients of friction experimentally?
Replies:
Veronica:
Yes, cardboard and plastic both have a coeffecient of friction.
you will have to look those up in a Chemistry/Physics dictionary or
reference book. The equation that will determine a coefficient of friction
is Force due to friction = coefficient of friction X the normal force. So,
if you can measure the force needed to overcome friction, with a spring
scale, and you know the normal force (the weight) of an object, you can
calculate the coefficient of friction. Hope this helps.
Katie Page
Measuring a coefficient of friction, despite the apparent simplicity of the
concept, is quite difficult, fraught with experimental difficulties and
often resistant to interpretation.
The "classic" method is to measure the force that must be applied to a sled
to start it moving (static coeff. of friction) and keep it moving at a
constant speed in a horizontal direction. The method fails for a bunch of
reasons. Try the experiment again and the two surfaces aren't the same. They
may have "burnished" and become slippier,or may have "galled" and become
much less slippery.
Another arrangement is to place the sled on an variable inclined plane. The
angle at which the sled slips down the incline is called the 'slip angle' or
similar term. Much the same problem. There is an "overshoot" angle that can
exceed the "kinetic" angle.
Yet another configuration is to roll a ball bearing down a fixed angle
trough and measure how far the ball rolls after it hits the sample which is
horizontal at the bottom of the trough. This is called rolling coefficient
of friction. The physics of the mechanical process is difficult to interpret
theoretically, however.
To appreciate the complexity of the problem imagine the sleds to be feather
pillows, the ramps to be soft matresses, and the ball bearing to be a water
filled baloon. Given this imagry, try to figure out what a coefficient of
friction is!
Vince Calder
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Update: June 2012
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