Physics 1501: Lecture 23, Pg 1 Physics 1501: Lecture 23 Today’s Agenda l Announcements çHW#8: due Oct. 28 l Honors’ students çsee me Wednesday at 2:30.

Physics 1501: Lecture 23, Pg 1

Physics 1501: Lecture 23Physics 1501: Lecture 23TodayToday’’s Agendas Agenda

AnnouncementsHW#8: due Oct. 28

Honors’ studentssee me Wednesday at 2:30 in P-114

TopicsReview of

» rolling motion

» Angular momentumGyroscope …


Angular Momentum ConservationAngular Momentum Conservation

A freely moving particle has a definite angular momentum about any given axis.

If no torques are acting on the particle, its angular momentum will be conserved.

In the example below, the direction of LL is along the z axis, and its magnitude is given by LZ = pd = mvd.

y

x

vv

d

m


Example: Throwing ball from stoolExample: Throwing ball from stool

A student sits on a stool which is free to rotate. The moment of inertia of the student plus the stool is I. She throws a heavy ball of mass M with speed v such that its velocity vector passes a distance d from the axis of rotation. What is the angular speed F of the student-stool

system after she throws the ball ?

top view: before after

d

vM

I I

F


Example: Throwing ball from stool...Example: Throwing ball from stool...

Conserve angular momentum (since there are no external torques acting on the student-stool system):LBEFORE = 0 , Lstool = IF

LAFTER = 0 = Lstool - Lball , Lball = Iballball = Md2 (v/d) = M d v

0 = IF - M d v

top view: before after

d

vM

I I

F

F = M v d / I


Summary of rotation:Summary of rotation:

Comparison between Rotation and Linear MotionComparison between Rotation and Linear Motion

Angular Linear

= x /R

= v / R

= a /R

x

v

a


ComparisonComparison KinematicsKinematics

Angular Linear


ComparisonComparison: : Dynamics Dynamics

Angular Linear

I = i mi ri2 m

F = a mr x F = I

L = r xp = I p = mv

W = W = F •x

K = WNET K = WNET


1. You are working with Marriott’s Great America and are given the task of designing their new roller coaster. Unfortunately, the cars have already been purchased, but you do get to design the track. The idea is to pull the cars up to the top of a hill, then release them from a standstill to go barreling down the hill, around a vertical loop, and on to the next thrill. You decide that this will all work best if the car falls completely under the influence of gravity and that the riders feel completely weightless at the top of the loop. The cars consist of a box for the passengers that when completely loaded have a mass of 1000kg. They also have four wheels, each of which is a solid cylinder of radius 0.25 m and a mass of 125 kg. The county regulations say that the maximum height of the entire rollercoaster can only be 30 meters. So how big is the loop you can make?

Lecture 23, Lecture 23, Act 1Act 1


Reconsider: Two DisksReconsider: Two Disks A disk of mass M and radius R rotates around the z axis

with angular velocity 0. A second identical disk, initially not rotating, is dropped on top of the first. The disks eventually they rotate together with angular velocity F.

0

z

F

z


Example: Two DisksExample: Two Disks Let’s use conservation of energy principle:

0

z

F

z

EINI EFIN

1/2 I 02 = 1/2 (I + I) F

2

F2 = 1/2 0

2

F = 0 / √2

EINI = EFIN


Example: Two DisksExample: Two Disks

Remember that using conservation of angular momentum:

LINI = LFIN we got a different answer !

F’ = 0 / 21/2 Conservation of energy !

Conservation of momentum !

F’ > F Which one is correct ?

F = 0 / 2

½M R2 0 = M R2 F F = ½ 0


Example: Two DisksExample: Two Disks

• Is the system conservative ?

• Are there any non-conservative forces involved ?

• In order for top disc to turn when in contact with the bottom one there has to be friction ! (non-conservative force !)

• So, we can not use the conservation of energy here.

• correct answer: F = 0/2

• We can calculate work being done due to this friction !

F

z

W = E = 1/2 I 02 - 1/2 (I+I) (0/2)2

= 1/2 I 02 (1 - 2/4)

= 1/4 I 02

= 1/8 MR2 02 This is 1/2 of initial Energy !


Lecture 23, Lecture 23, Act 2Act 2

A mass m=0.1kg is attached to a cord passing through a small hole in a frictionless, horizontal surface as in the Figure. The mass is initially orbiting with speed i = 5rad/s in a circle of radius ri = 0.2m. The cord is then slowly pulled from below, and the radius decreases to r = 0.1m. How much work is done moving the mass from ri to r ?

(A) 0.15 J (B) 0 J (C) - 0.15 J

ri

i


Gyroscopic Motion: a reviewGyroscopic Motion: a review

Suppose you have a spinning gyroscope in the configuration shown below:

If the left support is removed, what will happen ??

pivotsupport

g


Gyroscopic Motion...Gyroscopic Motion...

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



... instead it precessesprecesses around its pivot axis ! This rather odd phenomenon can be easily understood

using the simple relation between torque and angular momentum we derived in a previous lecture.

pivot


Gyroscopic Motion...Gyroscopic Motion... The magnitude of the torque about the pivot is = mgd. The direction of this torque at the instant shown is out of the page (using the right hand rule).

The change in angular momentum at the instant shown must also be out of the page!

LL pivot

d

mg



Consider a view looking down on the gyroscope. The magnitude of the change in angular momentum in a time

dt is dL = Ld.

So

where is the “precession frequency”

top view

LL(t)

LL(t+dt)

dLL d pivot



So

In this example = mgd and L = I:

The direction of precession is given by applying the right hand rule to find the direction of and hence of dLL/dt.

LL pivot

d

mg


Lecture 23, Lecture 23, Act 3Act 3StaticsStatics

Suppose you have a gyroscope that is supported on a gymbals such that it is free to move in all angles, but the rotation axes all go through the center of mass. As pictured, looking down from the top, which way will the gyroscope precess?

(a) clockwise (b) counterclockwise (c) it won’t precess

Physics 1501: Lecture 23, Pg 1 Physics 1501: Lecture 23 Today’s Agenda l Announcements çHW#8: due Oct. 28 l Honors’ students çsee me Wednesday at 2:30.

Documents

ff slide

z ff z slide

angular speed f

e fin slide

z axis

angular velocity

speed v

w net slide