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 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.”

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 heavyThe forces real and perceived

Real Forces:

Friction; upCentripetal; inwardsGravity (weight); down

Perceived Forces:

Centrifugal; outwardsGravity (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

•

• Centripetal acceleration depends on velocity and radius

2

22

222

2 442

T

r

rTr

rTr

r

vac

T

r

time

ntdisplacemevelocity

2

Centripetal Force (“center seeking”)

• Centripetal force depends on velocity, radius and mass

• Since acceleration is directed inward, force is directed inward as well!

r

mvmaF cc

2

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!