Download Static friction

Thursday, October 4, 2012
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Lesson 9 HW due TODAY
Lesson 10 HW due TOMORROW
Newton’s 2nd Law lab report due TODAY
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UPDATE – new due date!
 Lab is due MONDAY for an A, B, or F
the resistive force that arises between an object and a surface as they attempt to move past one another.
 CLASS:
contact force
 AGENT: the surface
 SYMBOL:
f
 DIRECTION: parallel to the surface, opposing an applied force.
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Friction is the force that opposes a sliding motion.
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Friction is highly useful. It enables us to walk and drive a car, among other things.
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Friction is dissipative. Friction causes mechanical energy to be converted to heat. N
(friction)
f
Big view:
Surfaces look
perfectly smooth.
Fpush
W
Small view:
Microscopic
irregularities resist
movement.
Friction may or may not exist between two surfaces. If it exists, it opposes the
direction object “wants” to slide. It is parallel to the surface.
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The friction that exists between two surfaces is directly proportional to the normal force. This has several implications, such as…
 Friction on a sloping surface is less than friction on a flat surface (since the normal force is less on a slope).
 Increasing weight of an object increases the friction between the object and the surface it is resting on.
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Weighting down a car over the drive wheels increases the friction between the drive wheels and the road (which increases the car’s ability to accelerate).
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Occurs between two surfaces that are not slipping relative to each other. Static friction is tricky. It can range from zero up to a maximum allowed value for two surfaces.
fs  sN
 fs : static frictional force (N)
 s: coefficient of static friction
 N: normal force (N)
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Static friction between two surfaces is zero unless there is a force trying to make the surfaces slide on one another.
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Static friction can increase as the force trying to push an object increases until it reaches its maximum allowed value as defined by s.
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Once the maximum value of static friction has been exceeded by an applied force, the surfaces begin to slide and the friction is no longer static friction.
Force Diagram
N
Physics
W
surface
There is no static friction since there is no applied horizontal
force trying to slide the book on the surface.
Force Diagram
N
fs
Physics
W
F
surface
Static friction is equal to the applied horizontal force, and there is
no movement of the book since F = 0 (and the object is at rest).
Force Diagram
N
fs
Physics
W
F
surface
Static friction is at its maximum value! It is still equal to F, but if
F increases any more, the book will slide.
Force Diagram
N
fk
Physics
W
F
surface
Static friction cannot increase any more! The book accelerates to the
right. Friction becomes kinetic friction, which is usually a smaller force.
Without friction, the book will
slide down the ramp.
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With friction it stays in place,
if there is sufficient static
friction holding it there.
At maximum angle before the book slides, let’s prove
that s = tan 
y
x

ΣFy = may
N – mg cos θ = 0
N = mg cos θ
θ
ΣFx = max
fs – mg sin θ = 0
sN – mg sin θ = 0
s mg cos θ = mg sin θ
s = sin θ/ cos θ
s = tan θ
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Force that opposes the motion as an object slides across a surface.
The direction is always opposite of the moving object.
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This type of friction occurs between surfaces that are slipping past each other.
fk = kN
 fk : kinetic frictional force (N)
 k: coefficient of kinetic friction
 N: normal force (N)
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Kinetic friction (sliding friction) is generally less than static friction (motionless friction) for most surfaces.
Sample Problem 10.1:
A 10‐kg box rests on a ramp that is laying flat. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30.
a) What is the maximum horizontal force that can be applied to the box before it begins to slide?
b) What force is necessary to keep the box sliding at constant velocity?
Sample Problem 10.1:
A 10‐kg box rests on a ramp that is laying flat. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30.
a) What is the maximum horizontal force that can be applied to the box before it begins to slide?
b) What force is necessary to keep the box sliding at constant velocity?
Sample problem 10.2
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
a) no force horizontal force is applied to the box?
Sample problem 10.2
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
a) no force horizontal force is applied to the box?
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
b) 20 N horizontal force is applied to the box?
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
b) 20 N horizontal force is applied to the box?
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
c)
a 60 N horizontal force is applied to the box?
A 10‐kg wooden box rests on a wooden floor. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and floor if
c)
a 60 N horizontal force is applied to the box?
Sample Problem 10.3:
A 10‐kg wooden box rests on a wooden ramp. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and ramp if
a) the ramp is at a 25o angle?
Sample Problem 10.3:
A 10‐kg wooden box rests on a wooden ramp. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and ramp if
a) the ramp is at a 25o angle?
A 10‐kg wooden box rests on a wooden ramp. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and ramp if
b)
the ramp is at a 45o angle?
c)
what is the acceleration of the box when the ramp is at 45o? A 10‐kg wooden box rests on a wooden ramp. The coefficient of static friction is 0.50, and the coefficient of kinetic friction is 0.30. What is the friction force between the box and ramp if
b)
the ramp is at a 45o angle?
c)
what is the acceleration of the box when the ramp is at 45o?