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Demonstration: What force stops a climber on a rope from falling? • The more two objects are pressed together,
the greater the friction.
• This is the force that saves a falling climber.
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First Questions
• In what direction is friction?
• What would walking be like, without friction?
• What is friction?
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What affects friction?
• Materials?
• Smoothness?
• Surface area?
• Amount two objects are pressed together?
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What makes friction?
• Molecules of one object attract molecules of the _ _ _ _ _ object.
• At its core, friction is the attraction between _ _ _ _ _ _ons (negatively charged) and
_ _ _ _ons (positively charged).
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What makes friction?
• Molecules of one object attract molecules of the other object.
• At its core, friction is the attraction between _ _ _ _ _ _ons (negatively charged) and
_ _ _ _ons (positively charged).
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What makes friction?
• Molecules of one object attract molecules of the other object.
• At its core, friction is the attraction between electrons (negatively charged) and protons (positively charged).
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Quantifying Friction
• Consider a bag of groceries on a table
• You pull the bag with 0.1 N. What does it do?
• Draw all the forces on the bag.
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You pull toward the right. Draw all the forces.
Groceries
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Pull
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PullFriction
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PullFriction
Table
Weight
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More about the grocery bag.
• If you double your force, the bag remains stationary . How much is the friction force now? ____ Newtons.
• In fact, even if you pull with 8 N, the bag does not budge.
• Only if you pull with more than 8 N does the bag move.
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Make a chart of your pulling force (P) & the friction force (f).
P (Newtons) f (Newtons)
0.0
0.1
0.2
4.0
8.0
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Make a chart of your pulling force (P) & the friction force (f).
P (Newtons) f (Newtons)
0.0
0.1
0.2
4.0
8.0
0.0
0.1
0.2
4.0
8.0
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Graph friction vs. Pull
f
P
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Graph friction vs. Pull
f
P
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Aside: Quantifying Materialsand Friction
• Each pair of materials can have a different amount of friction between them
• Think of a pair of materials that has an unusually large amount of friction.
• The greek letter ‘’ (“myoo”) is the coefficient that describes the amount of friction between two materials.
• The greater , the more the friction.
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Friction and materials
static kinetic
Concrete
and tire 1.0 0.8
Metal on
oiled metal 0.15 0.06
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force _ _ _ _ _ _ _ _ _!
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.
• But during sliding, f has one value, the “kinetic” value. f = kN.
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.
• But during sliding, f has one value, the “kinetic” value. f = kN.
• During sliding, it takes _ _ _ _ force to pull the bag than it did to get it to start to slide.
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.
• But during sliding, f has one value, the “kinetic” value. f = kN.
• During sliding, it takes less force to pull the bag than it did to get it to start to slide.
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When P > 8 N, the bag moves.
• As the bag begins to slide, the friction force decreases!
• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.
• But during sliding, f has one value, the “kinetic” value. f = kN.
• During sliding, it takes less force to pull the bag than it did to get it to start to slide. Try to include this in your graph. (Hmm…)
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Pull or Push (Newtons)f (N)Motion f=kN No Motionf = Pf max = sN
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“I thought weight (or mass) ought to fit into the amount of
friction, but it’s not on the graph?
So, does it matter?” …Yes, more weight leads to a greater
Normal, which leads to more friction.
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Example: Pull a crate with a force with 80 N. What happens?• The mass of the crate is 10 kg
• The s = 0.9
• The k = 0.7
• Draw all the forces that act on the crate.
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You pull toward the right. Draw all the forces.
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The forces
PNfW = Mg
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How much is the friction?
• If it is moving, f=kN
• If it is not moving, the most the friction could be is fmax = sN
• Either way, we need to know the value of the Normal force.
• How can we get that value?
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How to get the Normal
• It is not accelerating vertically, so Fy = ?
• N - Mg = 0
• N = Mg = (10)(9.8) = 98 Newtons
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How much is the friction?
• If it is moving, f=kN = (0.7)(98) = 68.6 N• If it is not moving, the most the friction
could be is fmax = sN = (0.9)(98)=88 N• If you pull with 80 N, how much friction is
there?• So, what happens?• What would happen if instead you pulled
with 90 N?
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What happens if youpull with 90 N?
• a = F / M
• a = / 10
• a = Newtons / 10 kg
• a = m/s2
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Last Example: Moving a crate.
• You are taller than the crate. So when you push it, you end up pushing sideways, and down. Draw the forces.
• When you pull it (maybe by a handle, maybe with a rope) you end up pulling sideways and up. Draw the forces.
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Pushing the crate (one)
Push23 degreeshorizontal line
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Pushing the crate (two)
P
N
f
W
23 deg
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Pulling the crate (one)
Pull23 degrees
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Pulling the crate (two)
PN
f
W
23 deg
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Which case will have the greater acceleration?
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The Crate, with numbers:
• The crate has a mass of 100 kg (weight of 980 Newtons).
• The Push or Pull will be 23˚ off the horizontal, with a magnitude of 180 N.
• The coefficient of kinetic friction between the crate and the floor is 0.1
• What is the acceleration of the crate in both cases?
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Push (Sideways and Down)
ax = Fx / M (Newton’s Second Law)
ax = [ Px - f ] ÷ M
Uh Oh, What is the value of f ?
f = kN
But now we need the value of the Normal.
How do we get N? …
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Push (Sideways and Down)
ay = 0, so Fy = 0
N - Psin - Mg = 0
N = Psin + Mg
N = 180sin(23˚) + 980
N = 70 N + 980 N
N = 1050 Newtons
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Push (Sideways and Down)
ax = Fx / M
ax = [ Px - f ] / M
ax = [ Pcos - kN ] ÷ M
ax = [ 180cos(23˚) - 1050) ] ÷ 100
ax = [166 - 105 ] ÷ 100
ax = 0.61 m/s2 {Done with the Push}
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Pull (Sideways and Up)
ax = Fx / M
ax = [ Px - f ] / M
To get the friction, we need the normal force.
How will the normal compare to the normal for the Push?
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Get the Normal, for the Pull
Fy = 0
N + Psin - Mg = 0
N = -Psin + Mg
N = -180sin(23˚) + 980
N = -70 N + 980 N
N = 910 Newtons
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Pull (Sideways and Up)
ax = Fx / M
ax = [ Px - f ] / M
ax = [ Pcos - kN ] ÷ M
ax = [ 180cos(23˚) - 910) ] ÷ 100
ax = [166 - 91] ÷ 100
ax = 0.75 m/s2 {Done with the Pull}
Compare the Pull with the Push.
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The acceleration is greater if you don’t press the objects together.