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
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
Torque
Torque is the product of the force and leverarm distance, which tends to produce
rotation.
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.
Examples:
» Meter stick
» Map of Texas
» Rotating Hammer
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
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".
Examples
Centripetal
Force
Centrifugal
Force
water in bucket
Bucket
Nature
moon and earth
Earth’s gravity
Nature
car on circular path
Road Friction
Nature
coin on a hanger
Hanger
Nature
jogging in a space
station
Space Station
Floor
Nature
Conservation of Angular
Momentum
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.”
Examples:
–
–
–
–
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.
Rotational speed - the number of rotations
or revolutions per unit time.
Rotational speed is often measured in
revolutions per minute (RPM).
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