Physics College Lecture Series Centrifugal “Force”: The Fake F orce
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Transcript of Physics College Lecture Series Centrifugal “Force”: The Fake F orce
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Physics College Lecture SeriesCentrifugal “Force”:
The Fake Force
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Objectives• Explain why the net force acting on
an object experiencing uniform circular motion is directed inward.
• Define and describe centripetal acceleration.
• Define and describe centripetal force.• Define centrifugal force as an
imaginary force resulting from inertia.
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Common misconceptions
“Objects slide across the
dashboard to the outside of a curve
in the road because of centrifugal
force.”
“People stick to the outer wall of an amusement
park ride because of centrifugal
force.”
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3 Examples from prior knowledge to refute idea of centrifugal force
• Definition of force
• Newton’s first law
• Acceleration of an object undergoing uniform circular motion
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Definition of force• A force is a push or pull upon an
object resulting from one object’s interaction with another object.
• So…where’s the push or pull? Where’s the interaction? Is it towards the center or towards the edge?
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Newton’s First Law• An object will resist a change in its
state (either at rest or in motion) unless acted upon by an unbalanced force
• What is the current state of an object experiencing uniform circular motion?
• What will it “want” to continue doing?• Inertia in action
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Acceleration of an object undergoing uniform circular motion
• Acceleration is directed inward, how can the net force be outward?
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So, if centrifugal “force” doesn’t exist, how do we explain the misconceptions?
• People “stick” to the outer wall of an amusement park ride.
• Objects slide across the dashboard towards the outside of a curve in the road.
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The truth!•Centripetal force: Center seeking!
• Centripetal force is INWARD• Centripetal acceleration is INWARD• Tangential velocity is “straight”
ahead!
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IntermissionAnswer the questions:• What is ‘centrifugal force’?• How do we know that centrifugal force is
not actually a force? • For an object moving in a circular path, in
which direction is it actually trying to go?
Write in the cue column:• At least one question to ask the professor
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Rotating Drum Ride• Vertical drum rotates, you’re
pressed against wall– Friction force against wall matches
gravity– Seem to stick to wall, feel very heavyThe forces real and perceived
Real Forces:
Friction; upCentripetal; inwardsGravity (weight); down
Perceived Forces:
Centrifugal; outwardsGravity (weight); down
Perceived weight; down and out
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Objects on dashboard• Tape Deck Demo
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Connections to yesterday’s lab• You saw that Force depended on
mass, velocity and radius…
• You had to PULL the mass to keep it in a circle—you needed a force towards the CENTER!
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Velocity and Acceleration in Uniform Circular Motion
•
• Centripetal acceleration depends on velocity and radius
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Centripetal Force (“center seeking”)
• Centripetal force depends on velocity, radius and mass
• Since acceleration is directed inward, force is directed inward as well!
rmvmaF cc
2
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Conclusions• Centrifugal “force” is non-existent:
Newton’s Laws explain motion in a circle and in a straight line
• Inertia explains why object want to remain on a straight path
• Actual net force is inward and is called centripetal force!