Friction

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Friction Inclines Problem-Solving Approaches
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4.8 Applications Involving Friction, Inclines
The origin of friction is electric forces between
atoms of materials in contact.
We will be concerned with two kinds of frictional
forces forces between stationary objects and
forces between moving objects.
Frictional forces result from surfaces in
contact. Frictional forces oppose the sliding of
surfaces in contact. Static friction forces point
opposite to the direction of motion an object
would move over a surface if no friction were
present. Figure out which way an object would
move without friction present. Then Fstatic is
aimed the opposite direction.
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If the object is sliding down the plane, the
frictional force is kinetic and directed up the
plane.
v
If the object is stationary, then the component
of its weight down the plane wants to pull it
down. The frictional force is static and is
directed up the plane.
If some object is pushing (or pulling) the object
up the plane, the frictional force is kinetic
and is directed down the plane.
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If some force is just about to push (or pull) the
object up the plane, the frictional force is
static and is directed down the plane.
Of course, friction acts between when a surface
is horizontal or vertical, as well as tilted.
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Experimentally, the magnitude of frictional
forces is observed to be proportional to the
normal force between objects, and the direction
is opposite to the motion, or the impending
motion.
We model the magnitudes of frictional forces
using these two official starting equations
Usually, µs gt µk. What does this have to do with
antilock brakes?
Here are a couple of toys to play
with http//physics.bu.edu/duffy/java/SecondLaw2
.html http//physics.bu.edu/duffy/java/SecondLaw
.html
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Static friction forces start out zero and
increase up to a maximum.
If I try to pull a lead brick across a rough
surface using very little pull, the friction
force will be small, but sufficient to overcome
my pull.
The friction force will increase as I increase
the pull.
Eventually, when my pull exceeds the maximum
static friction force, the brick starts
accelerating.
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In the case of both static and kinetic friction
forces, the friction force opposes motion of one
surface across the other, so it is directed
parallel to the surfaces.
The friction force is not necessarily directed
opposite to an applied force, as in the diagram
to the right.
Here are some notes from another class. They
represent a different way of presenting the above
material. I am going to paste them into this
lecture, so you are about to get a 2nd
perspective on what I just covered.
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Where would we be without friction? slip-sliding
away Friction is not a separate kind of
forceit is a result of electromagnetic forces
between microscopic objects. Friction forces
oppose actual motion (kinetic friction) or motion
that would take place if no friction were present
(static friction).
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Friction is a force, so it is a vector. Friction
forces occur at contact points between surfaces
and are always directed parallel to the (local)
surface. Kinetic friction forces on a surface
point in the direction parallel to the surface
and opposite the velocity. Static friction force
direction figure out which way an object would
move without friction. Then is aimed in the
opposite direction.
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Work friction examples (a) what is the force of
friction on a block at rest on an inclined plane
(b) what is the maximum angle for block to remain
at rest on an inclined plane.
4.9 Problem SolvingA General Approach
This is what weve been doing all along!
There is no new material in this section.