Friction
Pages 23-26 in text
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Dr. Sasho MacKenzie - HK 376
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What is Friction
• Friction is a force
• A frictional force can exist when two
substances contact each other.
• The molecules of each surface interact
according to Newton’s Laws of Motion.
• Friction always opposes motion, i.e., it is
opposite to the direction of velocity.
• If there is no motion, then friction opposes
the sum of all the other forces which are
parallel to the surfaces in contact.
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Types of Friction
Dry Friction
Fluid Friction
Occurs between the non-lubricated
surfaces of solid objects
Occurs with fluids,or
lubricated surfaces
Static Friction
When dry friction acts between
two surfaces that are not
moving relative to each other
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Dynamic Friction
>
When dry friction acts
between two surfaces that are
moving relative to each other
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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
Friction Force
Normal Force
Resultant
runner’s
force on
push
runner
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 friction 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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 Weight means  Normal Force,
and therefore,  Maximum Friction
5 kg
10 kg
Surfaces are more compressed together and there
are more interactions between molecules
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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 is not always = FF_max
Assume:
FF_max = 50 N
50
Fapplied
25
10
N 10.2 kg FFriction
50
0N
50
40
10
25
0N
N
Static Friction
Dynamic Friction
Ffriction
(magnitude)
What is the acceleration?
What is the normal force?
Calculate S?
Calculate D?
25
10
0
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10
25
50
Fapplied
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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.
Fh
5 kg
Fh
Free body
diagram
mg
Ff
FN
Normal force = FN = mg = 5 x 9.81 = 49 N
Fh= Friction force = FN = 0.4 x 49 = 19.6 N
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Fy = may
FN – mg = may = 0
FN = mg
Fx = max
Fh – Ff = max = 0
Fh = Ff
Dr. Sasho MacKenzie - HK 376
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Horse Pulling Cart
According to Newton’s 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 horse’s 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.
mg
N
Friction force
resulting from the
horse pulling back
on the ground
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N
Force of friction on
the wheel which
opposes the motion of
the horse-cart system
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Tug of War
Fat 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
3000 N
3000 N
2060 N
2060 N
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Ff = FN
= 1.5 x 2060
= 3090 N
1180 N
Ff = FN
= 1.5 x 1180
= 1770 N
1180 N
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Fat Bastard Wins
• Both competitors have a force of 3000
N pulling on them from the rope.
• Fat Bastard’s extra mass gives him a
potential friction force (3090 N) which
is greater than the force of the rope, so
he doesn’t move.
• Pfister’s 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
rope
FN = mg + Fy1
Fx2
Friction force = Fx1 = FN
mg
Fy1
This component
increases N
FN
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Bigger N, means larger
friction force
Fx1
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Pulling Down On The Rope
rope
This component
decreases N
Fy = may
FN + Fy1 – mg = may = 0
FN = mg – Fy1
Fy1
Fx2
Friction force = Fx1 = FN
mg
Smaller N, means less
friction force
FN
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Fx1
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Midterm Example Question
y
Fx1
x
40
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.
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