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L-9 Friction and Rotation
What is friction and what determines how big it is?How can friction keep us moving in a circle?What keeps us moving in circles ?center of gravity
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What is friction?
Friction is a force that acts between two surfaces that are in contactIt always acts to oppose motionIt is different depending on whether or there is motion or not.It is actually a force that occurs at the microscopic level.
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A closer look at friction
At the microscopic level even two smooth surfaces look bumpy this is what produces friction
Magnified view of surfaces
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Static friction
If we push on a block and it doesn’t move thenthe force we exert is less than the friction force.
push, P friction, f
This is the static friction force at work
If I push a little harder, the block may still not move the friction force can have any value upto some maximum value.
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Kinetic friction• If I keep increasing the pushing force, at
some point the block moves this occurs when the push P exceeds the maximum static friction force.
• When the block is moving it experiences a smaller friction force called the kinetic friction force
• It is a common experience that it takes more force to get something moving than to keep it moving.
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Homer discovers that kinetic friction is less than static friction!
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Measuring friction forcesfriction
gravity
At some point as the angle if the plane is increasedthe block will start slipping. At this point, the friction force and gravity are equal.
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Going in circles
Bart swings the tennis ball around his head in a circle. The ball is accelerating, what force makes it accelerate?
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Uniform circular motion
R v
The speed stays constant, but the direction changes
The acceleration in this case is called
centripetal acceleration
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Centripetal acceleration, aC
R
v
aC
The accelerationpoints toward thecenter of the circle
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Centripetal acceleration
toward the center
of the circle
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Magnitude of centripetal acceleration
• The centripetal acceleration depends on two factors the speed with which you take the turn and how tight the turn is
• More acceleration is required with a higher speed turn
• more acceleration is required with a tighter turn smaller radius of curvature
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big R
little R
for the same speed, the tighter turn requires more
acceleration
Wide turns and tight turns
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Centripetal acceleration
• centripetal acceleration
• for some turns, the “safe” speed is posted• a force is needed to produce this
centripetal acceleration• CENTRIPETAL FORCE• where does this force come from?
2
Cva =R
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Ball on a string
The tension in the stringprovides the necessarycentripetal force to keep the ball going in a circle.
path of ball if the stringbreaks
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Example• What is the tension in a string used to twirl a
0.3 kg ball at a speed of 2 m/s in a circle of 1 meter?
• Force = mass x acceleration [ m × aC ]• acceleration aC = v2 / R = (2 m/s)2/ 1 m
= 4 m/s2
• force = m aC = 0.3 × 4 = 1.2 N• If the string is not strong enough to handle
this tension it will break and the ball goes off in a straight line.
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Negotiating a flat (level) turn
• The centripetal force is provided by the frictionforce between the road and tires.
• this force is reduced if the road is wet or icy
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Banked turns • Since the road is banked (not horizontal) the force of the road on the box is not vertical
• Part of the force on the box from the road points toward the center of the circle
• This provides the centripetal force
• No friction is necessary to keep the box in the circle
R
N
FCENT
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What’s this Centrifugal force ? ?• The red object will make
the turn only if there is enough friction on it
• otherwise it goes straight• the apparent outward
force is called the centrifugal force
• it is NOT A REAL force!• it doesn’t go in a circle
until something makes it!
object onthe dashboard
straight lineobject naturally
follows
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Silly Silo (Rotor)
Bart’sweight
Friction betweenBart and wall
wall pushing in on Bart
The inward wall force keeps Bart in the circle.Friction keeps him from falling down.
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What makes something rotate in the first place?
TORQUE
AXEL
How do I apply a force to make the rod rotateabout the axel? Not just anywhere!
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TORQUE
• To make an object rotate, a force must be applied in the right place.
• the combination of force and point of application is called TORQUE
Force, F
lever arm, L
AXEL
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Torque = force times lever arm
Torque = F × L