Transcript Circular Motion

Circular Motion
New Unit
 Circular
Motion (Chapter 7,
section 3)
 Universal Gravitation
(Chapter 8)
Warm-Up
 Circular
 #1
a-c
Motion Practice
Which horse on a merry-go-round
moves faster…

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One on the
outside rail
One near the
inside rail?
Or…how about a turntable?
More than one kind of speed…

Linear speed (velocity)
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
Distance  time
Greater for outside horse
Can be called tangential speed
Rotational speed (velocity)



# of rotations per unit of time
Same for all horses
Also called angular speed
So…

In a rotating system –
All parts have same rotational speed
 Tangential speed varies with distance
from axis

Let’s try it…
Roll the soda can
 Roll the plastic cup

* Do they roll straight?
* Is tangential speed same for all parts of the
object?
* Is rotational speed the same…?
Let’s try it…Part 2.
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•
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•
Tape 2 cups together at the rims
Place on a “track” of two meter sticks
Center the cups and roll…what happens?
Roll a bit off center…what happens?
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
st
1
Law
 Newton’s 2nd Law

Force needed to cause an
acceleration
A new force…
force – force that
causes an object to follow a
circular path
 Centripetal
What’s wrong with this picture?
Be careful…

Centripetal force sometimes called
“centrifugal” force

A “fictitious” force
Simulating Gravity

Objects in free-fall in a rotating
object don’t feel “weightless” due to
centripetal force
To achieve a “g” approximate
to what we feel on Earth...

ac = v2
r
2km diameter structure for “tolerable”
RPMs!
 Toward center = less “g”

Circular Motion Practice

#1 d
Back to Isaac...
F = ma
Fc = m (v2/r)
Fc = mv2
r
Circular Motion Practice
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
#1 e
#2
Fill in the blank…

force
A ____________
causes acceleration.

torque
A ____________
causes rotation.
Let’s pretend...
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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
 Torque = force x length of lever arm

Long arm, less force
 More force, shorter arm


Ex. wrench, screwdriver, doorknob
Circular Motion Practice

#3
Rotational Inertia
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
Resistance to change in rotational
motion
Greater distance point of rotation is
from center of mass = 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?
Which will roll down a ramp with
more acceleration?
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A hollow cylinder or a solid cylinder
Same mass and radius
Solid cylinder –mass
concentrated closer to axis of
rotation
Angular Momentum
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mvr
Ice skater pulls in arms – r decreases
Momentum is conserved
mvr = mvr