Transcript Examples

Chapter 8
Rotational Motion
Rotational Inertia

An object rotating about an axis tends to
remain rotating unless interfered with by some
external influence.

This influence is called torque.

Rotation adds stability to linear motion.
– Examples:
» spinning football
» bicycle tires
» Frisbee
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The greater the distance between the bulk
of an object's mass and its axis of rotation,
the greater the rotational inertia.
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Examples:
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Tightrope walker
Inertia Bars
Ring and Disk on an Incline
Metronome
Torque
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Torque is the product of the force and leverarm distance, which tends to produce
rotation.
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Torque = force  lever arm
– Examples:
» wrenches
» see-saws
Center of Mass

The center of mass of an object is the
average position of mass.

Objects tend to rotate about their center of
mass.
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Examples:
» Meter stick
» Map of Texas
» Rotating Hammer
Stability
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For stability center of gravity must be over
area of support.

Examples:
» Tower of Pisa
» Touching toes with back to wall
» Meter stick over the edge
» Rolling Double-Cone

What is that force that throws you to the right
if you turn to the left in your car?
» It’s a “center-fleeing” force called centrifugal force.

What is that force that keeps you in your seat
when you turn left in your car?
» It’s a “center-seeking” force called centripetal force.
Direction of
Motion
Centripetal
Force
Centrifugal
Force
Centripetal Force
…is applied by some object.
 Centripetal means "center seeking".

Centrifugal Force
…results from a natural tendency.
 Centrifugal means "center fleeing".
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Examples
Centripetal
Force
Centrifugal
Force

water in bucket
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Bucket
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Nature
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moon and earth
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Earth’s gravity
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Nature
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car on circular path
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Road Friction
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Nature
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coin on a hanger
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Hanger
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Nature
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jogging in a space
station
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Space Station
Floor
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Nature
Conservation of Angular
Momentum
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

angular momentum = rotational inertia 
rotational velocity
L=Iw
Newton's first law for rotating systems:
– “A body will maintain its state of angular momentum
unless acted upon by an unbalanced external torque.”
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Examples:
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1.
2.
3.
4.
ice skater spin
cat dropped on back
Diving
Collapsing Stars (neutron stars)
End of Chapter 7
To compute your grade…
(This information is on the syllabus.)
Homework Average _____  40 = _______
Exam 1 _____  150 = _______
Exam 2 _____  150 = _______
Lab Exam 1 _____  50 = _______
Exam 3 _____  150 = _______
Final Exam _____  150 = _______
Lab Exam 2 _____  50 = _______
Lab Grades _____  100 = _______
Total = _________
8
Your Average = _________
Notice

The Physics 101 lab grades are posted outside of
your lab room.

You can pick up your old labs there as well.

Use your old labs and the notes on the study guide
to prepare for the lab exam.

You can pick up you homework and in-class
assignments outside of Dr. Bruton’s office
(room 330).
Circular Motion

Linear speed - the distance moved per unit
time. Also called simply speed.
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Rotational speed - the number of rotations
or revolutions per unit time.
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Rotational speed is often measured in
revolutions per minute (RPM).
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The linear speed is directly proportional to
both rotational speed and radial distance.
v=wr
What are two ways that you can increase
your linear speed on a rotating platform?
– Answers:
» Move away from the rotation axis.
» Have the platform spin faster.
Example Question

Two ladybugs are sitting on a phonograph record
that rotates at 33 1/3 RPM.
(a) Which ladybug has a great linear speed?
Answer: The one on the outside edge.
(b) Which ladybug has a great rotational speed?
Answer: Both have the same rotational speed.
Example Question
You sit on a rotating platform halfway between the
rotating axis and the outer edge.
You have a rotational speed of 20 RPM and a
tangential speed of 2 m/s.
What will be the linear speed of your friend who sit
at the outer edge?
Answer: 4 m/s
What will be his rotational speed?
Answer: 20 RPM