Rotational Motion

1
Chapter 8

• Rotational Motion

2
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

3

• The greater the distance between the bulk of an
  object's mass and its axis of rotation, the
  greater the rotational inertia.
• Examples
• Tightrope walker
• Inertia Bars
• Ring and Disk on an Incline
• Metronome

4
Torque

• Torque is the product of the force and lever-arm
  distance, which tends to produce rotation.
• Torque force lever arm
• Examples
• wrenches
• see-saws

5
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.
• Examples
• Meter stick
• Map of Texas
• Rotating Hammer

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Stability

• 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

7

• What is that force that throws you to the right
  if you turn to the left in your car?
• Its a center-fleeing force called centrifugal
  force.
• What is that force that keeps you in your seat
  when you turn left in your car?
• Its a center-seeking force called centripetal
  force.

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Direction of Motion
Centrifugal Force
Centripetal Force
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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
• moon and earth
• car on circular path
• coin on a hanger
• jogging in a space station

• Bucket
• Earths gravity
• Road Friction
• Hanger
• Space Station Floor

• Nature
• Nature
• Nature
• Nature
• Nature

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Conservation of Angular Momentum

• angular momentum rotational inertia
  
• 
  rotational velocity
• L I w
• 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
• 1. ice skater spin
• 2. cat dropped on back
• 3. Diving
• 4. Collapsing Stars (neutron stars)

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End of Chapter 7
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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 _________

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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. Brutons office
  (room 330).

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Circular Motion

• Linear speed - the distance moved per unit time.
  Also called simply speed.
• Rotational speed - the number of rotations or
  revolutions per unit time.
• 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 w r
• 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.

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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.

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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