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Circular Motion
Which horse on a merry-go-round moves faster…
One on the outside rail
One near the inside rail?
Or…how about a turntable?
More than one kind of speed…
Linear speed (velocity) Distance time Greater for outside horse AKA tangential speed
Rotational speed (velocity) # of rotations per unit of time Same for all horses AKA angular speed
Interesting application…
Rims of train wheels are tapered
Washer on a string…
The washer’s speed does not change, but its velocity does Centripetal acceleration
What would Isaac say?
Newton’s 1st Law Newton’s 2nd Law
Force needed to cause an acceleration
A new force…
Centripetal force – force that causes an object to follow a circular path
What’s wrong with this picture?
Be careful…
Centripetal force sometimes called “centrifugal” force A “fictitious” force
Centripetal Acceleration
ac = v2/r
Circular Motion Problems
#1 a - d
Back to Isaac...
F = ma
Fc = m (v2/r)
Fc = mv2
r
Circular Motion Practice
#1 e #2
Centripetal Force Lab
Fill in the blank…
A ____________ causes acceleration.
A ____________ causes rotation.
force
torque
Let’s pretend...
You need to loosen a stuck bolt with a wrench...
You aren’t strong enough...what do you do?
When opening a door...
Is it easier to open if you push close to or far away from the hinges?
Torque
Produces rotation
= Fr Long arm, less force More force, shorter arm
Ex. wrench, screwdriver, doorknob
Circular Motion Practice
#3
Rotational Inertia
Resistance to change in rotational motion
Greater distance from center of mass to rotation point = greater inertia
Why do baseball players “choke up” on the bat?
“Choking up” on a baseball bat will increase speed of swing
Why do giraffes have a slower gait than
Chihuahuas?
Why do longer pendulums swing more
slowly than shorter ones?
Angular Momentum
mvr Ice skater pulls in arms – r decreases Momentum is conserved