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

6. 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?
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.

8. Direction of
Motion
Centripetal
Force
Centrifugal
Force

9. Centripetal Force Centrifugal Force
…is applied by some object.
Centripetal means "center seeking".
Centrifugal Force
…results from a natural tendency.
Centrifugal means "center fleeing".

10. 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
Earth’s gravity
Road Friction
Hanger
Space Station Floor
Nature

11. 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.”

12. Examples: 1. ice skater spin 2. cat dropped on back 3. Diving
4. Collapsing Stars (neutron stars)

13. End of Chapter 7

14. 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 = _________

15. 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).

16. 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).

17. 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.

18. 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.

19. Example Question Answer: 4 m/s Answer: 20 RPM
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