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Physics 101: Lecture 15, Pg 1

Momentum Angular Momentum p = mV L = I  conserved if  F ext
= 0 conserved if  ext
=0
Vector Vector!
units: kg-m/s units: kg-m 2 /s
Physics 101: Lecture 15, Pg 2 15

 Right Hand Rule
 Wrap fingers of right hand around direction of rotation, thumb gives direction of angular momentum.
Physics 101: Lecture 15, Pg 3

z
 A disk of mass M and radius R rotates around the z axis with angular velocity
 i
. A second identical disk, initially not rotating, is dropped on top of the first. There is friction between the disks, and eventually they rotate together with angular velocity
 f
. Find
 f
.
z
 i
 f
Physics 101: Lecture 15, Pg 4 20

Four students (mass 70kg) are riding on a merry-go-round (solid disk of mass = 90kg rotating with angular velocity

=3 rad/s. Initially all four students are on the outer edge. Suddenly 3 of students pull themselves to within 0.25m of the center. What is the final angular velocity of the merry-go-round?
Before
After
1.2m
.25
Physics 101: Lecture 15, Pg 5 17

Before After
1.2m
.25
Physics 101: Lecture 15, Pg 6 17

What is the centripetal acceleration felt by each of the students?
Before After
1.2m
.25
Physics 101: Lecture 15, Pg 7 17

You are sitting on a freely rotating bar-stool with your arms stretched out and a weight in each hand. Your professor gives you a twist and you start rotating around a vertical axis though the center of the stool. You can assume that the bearing the stool turns on is frictionless, and that there is no net external torque present once you have started spinning.
You now pull your arms and hands (and weights) close to your body.
Physics 101: Lecture 15, Pg 8 22

What happens to the angular momentum as you pull in your arms?
1. it increases
2. it decreases
3. it stays the same
L
1
L
2
What happens to your angular velocity as you pull in your arms?
1. it increases
2. it decreases
3. it stays the same
Physics 101: Lecture 15, Pg 9 25

What happens to your kinetic energy as you pull in your arms?
1. it increases
2. it decreases
3. it stays the same
K

1
2
I
 
1
2 I
I
2  2 
1
2 I
L
2

1
(using L = I

)

2
I
1
I
2
L L
Physics 101: Lecture 15, Pg 10 29

A student sits on a barstool holding a bike wheel. The wheel is initially spinning CCW in the horizontal plane (as viewed from above) L= 25 kg m 2 /s She now turns the bike wheel over. What happens?
A. She starts to spin CCW.
B. She starts to spin CW.
C. Nothing
Start w/ angular momentum L pointing up from wheel. When wheel is flipped, no more angular momentum from it pointing up, so need to spin person/stool to conserve L!
Physics 101: Lecture 15, Pg 11 34

 Suppose you have a spinning gyroscope in the configuration shown below:
 If the left support is removed, what will happen??
pivot support g

Physics 101: Lecture 15, Pg 12 48

 Suppose you have a spinning gyroscope in the configuration shown below:
 If the left support is removed, what will happen?
 The gyroscope does not fall down!
pivot g

Physics 101: Lecture 15, Pg 13 45

Bicycle wheel
 ... instead it precesses around its pivot axis !
 pivot
Physics 101: Lecture 15, Pg 14



= I a
 L = I

 Right Hand Rule gives direction
 If

= 0, L is conserved
Physics 101: Lecture 15, Pg 15