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

mvr = mvr

Quiz Next Class!