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Dry or unlubricated surfaces. Three laws
govern the relationship between the frictional force f and
the load or weight L of the sliding object for unlubricated
or dry surfaces:
(a) “For low pressures (normal force per
unit area) the friction force is directly proportional to
the normal load between the two surfaces. As the pressure
increases, the friction does not rise proportionally; but
when the pressure become abnormally high, the friction
increases at a rapid rate until seizing takes place.”
(b) The value of f/L is defined as the
coefficient of friction µ. “The friction both in its
total amount and its coefficient is independent of the area
of contact, so long as the normal force remains the same.
This is true for moderate pressures only. For high
pressures, this law is modified in the same way as the first
case.”
(c) “At very low velocities, the friction
force is independent of the velocity of rubbing. As the
velocities increase, the friction decreases.”
The third law (c) implies that the force
required to set a body in motion is the same as the force
required to keep it in motion, but this is not true. Once a
body is in motion, the force required to maintain motion is
less than the force required to initiate motion and there is
some dependency on velocity. These facts reveal two
categories of friction: static and kinetic. Static friction
is the force required to initiate motion (Fs).
Kinetic or dynamic friction is the force required to
maintain motion (Fk).
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