Friction
And Forces in Two Dimensions
Review
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A force is a push or pull.
Newton’s 3 Laws of Motion:
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Objects resist changes in velocity unless acted
upon by a net force.
The magnitude of an object’s acceleration
depends on the net force and inversely on the
object’s mass.
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a = Fnet / m
For every force there is an equal and opposite
counter-force.
Friction

A force that opposes motion.
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It was once thought that friction was caused
by surface roughness.
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In this model, microscopic nooks and bumps on
the surface of an object lock and slip against
similar features on another surface.
It is now known that this model is incorrect.

Friction is caused by brief electromagnetic
interactions between charged particles in two
surfaces.
Friction

Two important types of friction:
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Static friction
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Kinetic friction
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Prevents motion from beginning
Prevents motion from continuing
Static friction is usu. stronger than kinetic
friction.

This is why, when you’re pushing a heavy object
across the floor, it’s harder to start it moving than
it is to keep it moving.
Friction

Ff =  FN
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Ff = Friction (N)
 = Coefficient of friction (no units)
FN = Normal force = mg cos
Coefficient of friction
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Unique for every pair of substances and set of
conditions.
Different CoF for static and kinetic friction.
Material 1
Material 2
Static
Aluminum
Aluminum
Aluminum
1.05-1.35
1.4
Coefficients
of
Friction
Mild Steel
0.61
0.47
Sliding
Cast Iron
Cast Iron
1.1
0.15
Copper
Cast Iron
1.05
0.29
Copper
Mild Steel
0.53
0.36
Copper
Steel (304 stainless)
0.23
0.21
Glass
Glass
0.9 - 1.0
0.4
Glass
Nickel
0.78
0.56
Leather
Oak (Parallel grain)
0.61
0.52
Nickel
Nickel
0.7-1.1
0.53
Oak
Oak (parallel grain)
0.62
0.48
Oak
Oak (cross grain)
0.54
0.32
Rubber
Asphalt (Dry)
0.5-0.8
Rubber
Asphalt (Wet)
0.25-0.75
Rubber
Concrete (Dry)
0.6-0.85
Rubber
Concrete (Wet)
0.45-0.75
Coefficient of Friction

CoF must be determined experimentally
for any pair of surfaces.

 = F f / FN
Sample Problem

The coefficient of static friction between
two blocks of wood is 0.500. What
applied force is necessary to start a 250.kg wooden box sliding along a horizontal
wooden surface?
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
Fa must be equal to or greater than Ff,static.
Ff =  FN
FN = Fw = mg
 FN = (250. kg)(9.81 m/s2)
 FN = 2450 N
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Ff = (0.500)(2450 N)
Ff = 1230 N
Sample Problem 2

A box with a mass of 40.0 kg requires an applied
force of 250. N to keep it moving at a constant
speed along a horizontal surface. What is the
CoF between the box and the floor?

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Ff =  FN
Ff =  mg
(250. N) =  (40.0 kg)(9.81 m/s2)
(250. N) =  (392 N)
 = 0.638
In lab today
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We will calculate the coefficient of friction
between a shoe and the surface of a ramp.
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You will volunteer the use of your shoes!
Place the shoes on a ramp.
Incline the ramp until the shoe begins to slide.
Measure the angle at which the shoe begins to slide.
Calculate the coefficient of friction using some
simplifying math.
Repeat the procedure but first wet the shoe’s sole and
the board.