Friction

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Friction

• Pages 23-26 in text

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What is Friction

• Friction is a force
• A frictional force arises when two substances
  contact each other.
• The molecules of each surface interact according
  to Newtons Laws of Motion.
• Friction always opposes motion, i.e., it is
  opposite to the direction of velocity.

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Types of Friction
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Contact Force

• Force that occurs between objects that are in
  contact with each other.
• Contact forces can be resolved into components
  that are perpendicular and parallel to the
  surfaces in contact.
• The perpendicular component is called the normal
  force.
• The parallel component is called friction.

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Contact Force in Running
During the push off phase in running, the normal
force acts upward on the runner, while the
friction force acts forward on the runner. The
frictional force is the only force capable of
moving the runner horizontally down the track.
The normal force can only accelerate the runner
upwards.
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Friction and the Normal Force

• The maximum frictional force is proportional to
  the normal contact force.
• An increase in the normal force results in an
  increase in the maximum friction.
• This is because the molecules on the two surfaces
  are pushed together more, thus increasing their
  interactions.

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Increased Weight, Increased Normal Force,
Increased Friction
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Friction and Surface Area

• Friction is not affected by the size of the
  surface area in contact.
• If the normal force remains constant, but the
  contacting surface area is increased, then the
  normal force is spread out over more molecules,
  thus the force on each molecule is reduced.
• Amontons (1699)
• What about race car tires?

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Calculating Friction

• Ff_max ?FN

• Ff_max is the maximum force of friction
• ? (Mu) is the coefficient of friction
• FN is the normal force
• Friction can range in value from -Ff_max to
  Ff_max

• ? depends on the types of surfaces that are
  interacting. It would be low for rubber on ice,
  but high for rubber on asphalt. It also depends
  on whether the surfaces are moving relative to
  each other ( ?static or ?dynamic )

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Friction Example
A 5 kg block of wood rests on a ceramic counter.
If the coefficient of static friction between the
block and the counter is 0.4, what horizontal
force is necessary to move the block.
?Fy may FN mg may 0 FN mg
Normal force FN mg 5 x 9.81 49 N
?Fx max Fh Ff max 0 Fh Ff
Fh Friction force ?FN 0.4 x 49 19.6 N
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Horse Pulling Cart
According to Newtons 3rd Law, these forces are
equal and opposite. So, if the horse pulls
forward on the cart with the same force as the
cart pulls back on the horse, how will the horse
ever move the cart?
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Solution
Friction acts on the horses feet but very little
acts on the wheels of the cart. Drawing a free
body diagram reveals the answer. The horse and
cart are one system so the forces in between them
are internal and cannot produce a change in
motion of the system.
FF
FF
FN
FN
Force of friction on the wheel which opposes the
motion of the horse-cart system
Friction force resulting from the horse pulling
back on the ground
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Tug of WarFat Bastard vs. Phil Pfister
Fat Bastard Pull Force 3000 N Mass
210 kg Height 1.8 m
Pfister Pull Force 3000 N Mass
120 kg Height 1.8 m
Both competitors are wearing the same footwear
which has a coefficient of friction of 1.5 with
the rubber floor they are competing on. If both
men employ the same technique, who wins?
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Two Free Body Diagrams
Fat Bastard
Pfister
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Fat Bastard Wins

• Both competitors have a force of 3000 N pulling
  on them from the rope.
• Fat Bastards extra mass gives him a potential
  friction force (3090 N) which is greater than the
  force of the rope, so he doesnt move.
• Pfisters maximum friction force (1170 N) is less
  than the force of rope, so he is pulled toward
  Fat Bastard.

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Would it be better to pull up or down on the rope?

• Suppose competitor A was taller than competitor
  B.
• A would be pulling on an upward angle, while B
  would be pulling on a downward angle.
• Who has the advantage?

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Pulling Up On The Rope
?Fy may FN Fy1 mg may 0 FN mg Fy1
Friction force Fx1 ?FN
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Pulling Down On The Rope
?Fy may FN Fy1 mg may 0 FN mg Fy1
Friction force Fx1 ?FN
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Midterm Example Question
y
5 kg
Fx1
40?
x
A 5 kg box is being pushed up a 40? incline with
an acceleration of -2 m/s/s. If the coefficient
of dynamic friction between the incline and box
is 0.2, then what is the value of Fx1? Remember
that friction always opposes the direction of
motion.