4.9

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4.9 Static and Kinetic Frictional Forces
There are situations where friction helps us and
at other times it is a hindrance. Advantages of
friction
Disadvantages of friction wear and tear energy
loss.
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Microscopic view of Frictional Forces
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Frictional force VERSUS Applied force
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Frictional force VERSUS Applied force
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Static Frictional Force
The magnitude fs of the static frictional force
can have any value from zero up to a maximum
value of fsMAX, depending on the applied force.
fs fsMAX fsMAX m sFN
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Kinetic Frictional Force
The magnitude fk of the kinetic frictional force
is given by,
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Coefficients of Friction
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EXAMPLE 10 Sled Riding
A sled is traveling at 4.00 m/s along a
horizontal stretch of snow, as Figure 4.23a
illustrates. The coefficient of kinetic friction
is mk 0.0500. How far does the sled go before
stopping?
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4.10 The Tension Force
5 6. Part a of the drawing shows a bucket of water suspended from the pulley of a well the tension in the rope is 92.0 N. Part b shows the same bucket of water being pulled up from the well at a constant velocity. What is the tension in the rope in part b?
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Object on an Inclined Plane
43.   A Mercedes-Benz 300SL is parked on a road that rises above the horizontal. What are the magnitudes of (a) the normal force and (b) the static frictional force that the ground exerts on the tires?
57.    A worker stands still on a roof sloped at an angle of above the horizontal. He is prevented from slipping by a static frictional force of 390 N. Find the mass of the worker.