Friction: Always Rubbing
Me the Wrong Way
Physics: Principles with Applications, 6th
edition
Giancoli
© 2005 Pearson Prentice Hall
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Friction
On a microscopic scale, most surfaces are rough.
The exact details are not yet known, but the force
can be modeled in a simple way.
Friction is a force that always
opposes motion.
There are many different types of
friction, the two we are concerned
with are:
 Kinetic or sliding friction –
Friction between two surfaces that
are moving relative to each other.
 Static Friction –
Friction between two surfaces that
are at rest relative to each other.
We can model kinetic or sliding friction with
an equation which relates kinetic friction to
the normal force:
f k  k FN
Where  k is the coefficient of kinetic friction,
and is different for every pair of surfaces.
We can model static friction with an inequality:
f s   s FN
Where  s is the coefficient of static friction, and
is different for every pair of surfaces and is
generally larger than the coefficient of kinetic
friction.
Another way to think about kinetic friction is that
it is the force that must be overcome to keep an
object sliding with a constant velocity.
Static friction can be thought of as the force that
must be overcome to start an object sliding. It
has a maximum value (hence the inequality) and
once this value is reached the object will begin
to slide.
The static frictional force increases as the applied
force increases, until it reaches its maximum.
Then the object starts to move, and the kinetic
frictional force takes over.
Inclined Planes or Ramps
An object sliding down an incline has three forces acting on
it: the normal force, gravity, and the frictional force.
• The normal force is always perpendicular to the surface.
• The friction force is parallel to it.
• The gravitational force points down.
To analyze a
problem
involving
inclined plane
you will want
to use
components
perpendicular
to the surface
of the plane
and parallel to
the plane.


If the object is sliding,
then we use kinetic
friction. If the object is
at rest, the forces are
the same except that we
use the static, and the
sum of the forces is
zero.
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