Friction
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Friction
friction: The resistance to motion that occurs
when the surface of two media or
materials are in contact.
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Friction
friction: The resistance to motion that occurs
when the surface of two media or
materials are in contact.
examples: solid on solid
fluid on solid
fluid on fluid
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Friction
Types of friction:
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Friction
Types of friction:
static friction; frictional force that
occurs between two surfaces that
are not in motion relative to each
other.
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Friction
Types of friction:
1. static friction; frictional force that occurs
between two surfaces that are not in motion
relative to each other.
 Force is of sufficient strength to
prevent relative motion to each other.
 The bonds (Vander Whals Forces)
between the two surfaces are
sufficiently strong to prevent relative
motion by opposing forces
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Applications Involving Friction
On a microscopic scale, most surfaces are rough. The
exact details are not yet known, but the force can be
modeled in a simple way.
For kinetic – sliding – friction, we write:
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Friction
Types of friction:
2. kinetic friction (sliding friction):
occurs when surface of one
substance is moving (sliding) over the
surface of a second substance.
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Friction
Types of friction:
2. kinetic friction (sliding friction): occurs when
surface of one substance is moving (sliding)
over the surface of a second substance.
 Sufficient to retard but not prevent
the motion
 The bonds are sheared by a force
opposing the friction
 will eventually stop the motion if the
friction is not overcome by a
sufficient counter force
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Friction
Types of friction:
3. rolling friction; when an object is
rolling over another surface without
slipping without slipping
 The surfaces in contact form
static friction bonds
 The static bonds are broken by
a prying mechanism rather than
a shearing mechanism
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Friction
Types of friction:
3. rolling friction; This is a difficult type
of friction to analyze.
 We will not cover rolling friction in
depth
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Friction
• Measuring Friction;
 Frictional force = normal force ( coefficient of friction)
 Ff = F N μ
 FN = normal friction
 μ = coefficient of friction (NO UNITS)
 Two types of coefficients
1) static (μs)
2) kinetic (sliding) (μk)
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Friction
• Static Friction:
– We observe that the frictional force is
proportional to the normal force. For static
friction:
Ff ≤ FN (μs)
– The static friction may not always be at maximum
value. The strength of the frictional force varies
with circumstance.
– It will break when the maximum static frictional
force is exceeded.
Ff MAX = FN (μs)
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Friction
• Kinetic Friction:
– When the static frictional forces are broken the
two surface begin to move relative to each other
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Friction
• Kinetic Friction:
– When the static frictional forces are broken the
two surface begin to move relative to each other
Fk = FN (μk)
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Friction
• Kinetic Friction:
– When the static frictional forces are broken the
two surface begin to move relative to each other
Fk = FN (μk)
Kinetic friction between two given surfaces is
usually weaker than the static friction between
the same two surfaces
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Static Friction vs. Kinetic Friction
• Kinetic Friction:
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Friction
• The applied force required to overcome the
static friction will usually be greater than the
kinetic friction.
• As a result the applied force will cause the
object to accelerate once the static frictional
bonds are broken
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Common Coefficients of Friction
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Friction Example
Wood Block
10 kg wooden block being pulled by a
rope
Fs = FN μs = 98N(0.58)= 56.8N
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Friction Example
Wood Block
10 kg wooden block being pulled by a rope
 When 56.8N of force is applied the wooden
block , the static friction is broken
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Friction Example
Wood Block
 The static friction is immediately replaced by
kinetic friction
Fk = FN μk = 98N ( 0.40) = 39.2N
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Friction Example
Wood Block
56.8 N > 39.2N
FNet = 56.8 N – 39.2N =17.6N
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Friction Example
Wood Block
FNet = 56.8 N – 39.2N =17.6N
FNet = m a
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Friction Example
Wood Block
FNet = 56.8 N – 39.2N =17.6N
a =Fnet /m = 17.6N/10Kg =1.76m/s2
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Figure 4-20
Forces of static and kinetic friction
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Figure 4-21
Pulling at an angle: a closer look at the normal force
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The pulling force is divided into
1) lifting force
2 horizontal force
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lifting force = sinθ(F)
horizontal force = cosθ(F)
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 lifting force reduces the normal
force
 this results in a reduction in the
frictional force
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Figure 4-21
Pulling at an angle: a closer look at the normal force