Download Centrifugal *force*: The fake force

Physics College Lecture Series
Centrifugal “Force”:
The Fake Force
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.
Common
“People stick to the
outer wall of an
amusement park
ride because of
centrifugal force.”
“Objects slide across
the dashboard to the
outside of a curve in
the road because of
centrifugal force.”
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
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?
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
Acceleration of an object
undergoing uniform circular motion
• Acceleration is directed inward, how can the
net force be outward?
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.
The truth!
• Centripetal force:
Center seeking!
• Centripetal force is INWARD
• Centripetal acceleration is INWARD
• Tangential velocity is “straight” ahead!
Intermission
Answer 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
Rotating Drum Ride
• Vertical drum rotates, you’re pressed
against wall
– Friction force against wall matches gravity
– Seem to stick to wall, feel very heavy
The forces real and perceived
Real Forces:
Friction; up
Centripetal; inwards
Gravity (weight); down
Perceived Forces:
Centrifugal; outwards
Gravity (weight); down
Perceived weight; down and
out
Objects on dashboard
• Tape Deck Demo
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!
Velocity and Acceleration
in Uniform Circular Motion
•
displaceme nt 2r
velocity 

time
T
• Centripetal acceleration depends on
velocity and radius
2 2
 2r 
4 r


2
2
2
v
4 r
T 

T
ac 


 2
r
r
r
T
2
Centripetal Force (“center seeking”)
• Centripetal force depends on velocity,
radius and mass
mv
Fc  mac 
r
2
• Since acceleration is directed inward,
force is directed inward as well!
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!