Kinetic Friction

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Kinetic Friction
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Pre-test
Moves at constant velocity!
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Pretest 3(the crate in the snow)
?
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Friction

• Always acts parallel to the surface of contact
  (90o to the Normal Force)
• Comes in two flavors static and kinetic
• Static no relative motion of surfaces
• Kinetic relative motion of surfaces (sliding)

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Kinetic Friction

• Magnitude of force depends on
• types of surfaces in contact
• size of the normal force

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How do I measure kinetic friction?
Moves at constant velocity!
Compare the horizontal forces on the crate
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How do I measure kinetic friction?
Moves at constant velocity!
Compare the horizontal forces on the crate
Nperson,crate
ffloor,crate
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Simple Experiment
Constant Velocity
Lift or push to adjust Normal Force
Tension in cable
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Tension in Cable
Normal Force
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What if we double the speed?
Constant Velocity
Lift or push to adjust Normal Force
Tension in cable
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The frictional force does not depend on the
speed!
Tension in Cable
Normal Force
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The frictional force does not depend on the
speed!
Tension in Cable
fk constant x Normal Force
Normal Force
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The frictional force does not depend on the
speed!
Tension in Cable
fk ?kN
Normal Force
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What if I cover the bottom of the block with sand
paper?
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Tension in Cable
Normal Force
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Tension in Cable
Normal Force
What are the units of ?k?
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Is the normal force equal to the weight?
YES !
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Is the normal force equal to the weight?
W gt N
YES !
q
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370
A 70 kg skier skis down a slope that is angled
370 with respect to the horizontal. The
coefficient of kinetic friction between the skis
and the snow is 0.10.What is the skiers
acceleration?
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700 N
370
Which angle is 37??
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To find the angle, drive a spike
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To find the angle, drive a spike
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To find the angle, drive a spike
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To find the angle, drive a spike
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?snow,skier mk Nsnow,skier
560 N
56 N
560 N
37?
700 N
370
420 N
420 N - 56 N (70 kg) ax
ax 5.2 m/s2