PHYS 1443 – Section 001
Lecture #15
Tuesday, June 27, 2006
Dr. Jaehoon Yu
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Tuesday, June 27, 2006
Angular Momentum & Its Conservation
Similarity of Linear and Angular Quantities
Conditions for Equilibrium
Mechanical Equilibrium
How to solve equilibrium problems?
Elastic properties of solids
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
1
Announcements
• Reading assignments
– CH12 – 7 and 12 – 8
• Last quiz tomorrow
– Early in the class
– Covers Ch. 10 – 12
• Final exam
–
–
–
–
Date and time: 8 – 10am, Friday, June 30
Location: SH103
Covers: Ch 9 – what we cover tomorrow
No class this Thursday
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
2
Angular Momentum of a Rotating Rigid Body
z
Let’s consider a rigid body rotating about a fixed axis
Each particle of the object rotates in the xy plane about the z-axis
at the same angular speed, w
L=rxp
O
y
m
r
f
x
Magnitude of the angular momentum of a particle of mass mi
about origin O is miviri
2
Li  mi ri vi  mi ri w
p
Summing over all particle’s angular momentum about z axis
Lz   Li   mi ri 2w 
i
i
Since I is constant for a rigid body
Thus the torque-angular momentum
relationship becomes
What do
you see?
Lz 
ext
2
i i
i
dL z
dw
 I
dt
dt

 m r w  Iw
 I
 is angular
acceleration
dLz

 I
dt
Thus the net external torque acting on a rigid body rotating about a fixed axis is equal to the moment
of inertia about that axis multiplied by the object’s angular acceleration with respect to that axis.
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
3
Example for Rigid Body Angular Momentum
A rigid rod of mass M and length l is pivoted without friction at its center. Two particles of mass
m1 and m2 are attached to either end of the rod. The combination rotates on a vertical plane with
an angular speed of w. Find an expression for the magnitude of the angular momentum.
y
m2
l

m2 g
O
m1
m1 g
x
The moment of inertia of this system is
1
1
1
2
2

I
+
I
+
I

Ml
+
m
l
+
m2l 2
I
rod
m1
m2
1
12
4
4
2
l2  1
 L  Iw  wl  1 M + m + m 
  M + m1 + m2 

1
2
4
3


4 3

Find an expression for the magnitude of the angular acceleration of the
system when the rod makes an angle  with the horizon.
If m1 = m2, no angular
momentum because net
torque is 0.
If +/-p/2, at equilibrium
so no angular momentum.
Tuesday, June 27, 2006
First compute net
external torque
   m1 g
l
cos
2
 ext   +  2

 2  -m2 g
gl cos  m1 - m2 
2
1
m1 - m1 gl cos 
2



Thus 
ext


2
l
1


I
 M + m1 + m2 
becomesPHYS 1443-001, Summer
4 3
2006

Dr. Jaehoon Yu
l
cos
2
2m1 - m1  cos 
g /l
1

 M + m1 + m2 
3
4 
Conservation of Angular Momentum
Remember under what condition the linear momentum is conserved?
dp
F

0

Linear momentum is conserved when the net external force is 0. 
dt
p  const
By the same token, the angular momentum of a system
is constant in both magnitude and direction, if the
resultant external torque acting on the system is 0.
What does this mean?
dL


0
 ext
dt
L  const
Angular momentum of the system before and
after a certain change is the same.
r
r
Li  L f  constant
Three important conservation laws K i + U i  K f + U f
r
r
for isolated system that does not get p

p
i
f
affected by external forces
r
r
Li  L f
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
Mechanical Energy
Linear Momentum
Angular Momentum
5
Example for Angular Momentum Conservation
A star rotates with a period of 30 days about an axis through its center. After the star
undergoes a supernova explosion, the stellar core, which had a radius of 1.0x104km, collapses
into a neutron star of radius 3.0km. Determine the period of rotation of the neutron star.
What is your guess about the answer?
Let’s make some assumptions:
The period will be significantly shorter,
because its radius got smaller.
1. There is no external torque acting on it
2. The shape remains spherical
3. Its mass remains constant
Li  L f
Using angular momentum
conservation
I iw  I f w f
The angular speed of the star with the period T is
Thus
w
I iw
mri 2 2p


f
If
mrf2 Ti
Tf 
2p
wf
 r f2
 2
r
 i
Tuesday, June 27, 2006
2p
w
T
2

3
.
0


-6
Ti  

2
.
7

10
days  0.23s

30
days

4

1
.
0

10



PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
6
Similarity Between Linear and Rotational Motions
All physical quantities in linear and rotational motions show striking similarity.
Quantities
Mass
Length of motion
Speed
Acceleration
Force
Work
Power
Momentum
Kinetic Energy
Tuesday, June 27, 2006
Linear
Mass
Rotational
Moment of Inertia
M
Distance
r
t
v
a
t
I  mr 2
Angle  (Radian)
L

t
w

t
v
w
Force F  ma
r r
Work W  F  d
Torque   I
Work W  
P  w
P  F v
p  mv
Kinetic
K
1
mv 2
2
L  Iw
Rotational
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
KR 
1
Iw 2
2
7
Conditions for Equilibrium
What do you think the term “An object is at its equilibrium” means?
The object is either at rest (Static Equilibrium) or its center of mass
is moving with a constant velocity (Dynamic Equilibrium).
When do you think an object is at its equilibrium?
Translational Equilibrium: Equilibrium in linear motion
Is this it?
The above condition is sufficient for a point-like particle to be at its
translational equilibrium. However for object with size this is not
sufficient. One more condition is needed. What is it?
Let’s consider two forces equal magnitude but in opposite direction acting
on a rigid object as shown in the figure. What do you think will happen?
F
d
d
F  0
CM
-F

The object will rotate about the CM. The net torque
 0
acting on the object about any axis must be 0.
For an object to be at its static equilibrium, the object should not
have linear or angular speed. vCM  0 w  0
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
8
More on Conditions for Equilibrium
To simplify the problem, we will only deal with forces acting on x-y plane, giving torque
only along z-axis. What do you think the conditions for equilibrium be in this case?
The six possible equations from the two vector equations turns to three equations.
F  0
F
F
x
y
0
0
AND
  0

z
0
What happens if there are many forces exerting on the object?
If an object is at its translational static equilibrium,
and if the net torque acting on the object is 0
about one axis, the net torque must be 0 about
r’
O
r5
O’
any arbitrary axis.
Why is this true?
Because the object is not moving, no matter what
the rotational axis is, there should not be in motion.
Tuesday, June 27, 2006
PHYS 1443-001,
Summer
It is simply
a matter
of 2006
mathematical calculation.9
Dr. Jaehoon Yu
Center of Gravity Revisited
When is the center of gravity of a rigid body the same as the center of mass?
Under the uniform gravitational field throughout the body of the object.
Let’s consider an arbitrary shaped object
The center of mass of this object is
CM
CoG
m x  m x
M
m
m y m y


yCM

m
M

xCM 
i i
i i
i
i
i
i
i
i
Let’s now examine the case with gravitational acceleration on
each point is gi
Since the CoG is the point as if all the gravitational force is exerted
on, the torque due to this force becomes
m1 g1 + m2 g 2 +   xCoG m1g1x1 + m2 g2 x2 +   
If g is uniform throughout the body
Tuesday, June 27, 2006
Generalized expression for
different g throughout the body
m1 + m2 +   gxCoG  m1x1 + m2 x2 +   g
x
PHYS 1443-001, Summer 2006CoG
Dr. Jaehoon Yu

m x  x
CM 10
m
i i
i
Example for Mechanical Equilibrium
A uniform 40.0 N board supports the father and the daughter each weighing 800 N and
350 N, respectively. If the support (or fulcrum) is under the center of gravity of the board
and the father is 1.00 m from CoG, what is the magnitude of normal force n exerted on
the board by the support?
1m
F
MFg
x
n
MBg
Since there is no linear motion, this system
is in its translational equilibrium
D
F
MDg
x
F
0
 n -M B g -M F g -M D g  0
n  40.0 + 800 + 350  1190N
y
Therefore the magnitude of the normal force
Determine where the child should sit to balance the system.
The net torque about the fulcrum
by the three forces are
Therefore to balance the system
the daughter must sit
Tuesday, June 27, 2006
  M B g  0 + n  0 + M F g 1.00 - M D g  x  0
x

MFg
800
1.00m 
1.00m  2.29m
MDg
350
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
11
Example for Mech. Equilibrium Cont’d
Determine the position of the child to balance the
system for different position of axis of rotation.
Rotational axis
1m
F
MFg

x
n
x/2
D
MFg
MBg
The net torque about the axis of
rotation by all the forces are
 M B g  x / 2 + M F g  1.00 + x / 2 - n x / 2 - M D g  x / 2  0
n  MBg + MF g + MDg
  M B g  x / 2 + M F g  1.00 + x / 2
- M B g + M F g + M D g  x / 2 - M D g  x / 2
Since the normal force is
The net torque can
be rewritten
 M F g 1.00 - M D g  x  0
Therefore
x
Tuesday, June 27, 2006
MFg
800

1.00m 
1.00m  2.29m
MDg
350
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
What do we learn?
No matter where the
rotation axis is, net effect of
the torque is identical.
12
Example 12 – 8
A 5.0 m long ladder leans against a wall at a point 4.0m above the ground. The ladder is
uniform and has mass 12.0kg. Assuming the wall is frictionless (but ground is not),
determine the forces exerted on the ladder by the ground and the wall.
FW
FBD
mg
FGy
O
FGx
First the translational equilibrium,
using components
 Fx FGx - FW  0
 F  -mg + F
y
0
Gy
Thus, the y component of the force by the ground is
FGy  mg  12.0  9.8N  118N
The length x0 is, from Pythagorian theorem
x0  5.02 - 4.02  3.0m
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
13
Example 12 – 8 cont’d
From the rotational equilibrium

O
 -mg x0 2 + FW 4.0  0
Thus the force exerted on the ladder by the wall is
mg x0 2 118 1.5
FW 

 44 N
4.0
4.0
Tx component of the force by the ground is
F
x
 FGx - FW  0
Solve for FGx
FGx  FW  44 N
Thus the force exerted on the ladder by the ground is
FG  FGx2 + FGy2  442 + 1182  130N
The angle between the  tan -1  FGy 
-1  118 
o

tan

70





ladder and the wall is
 44 
 FGx 
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
14
Example for Mechanical Equilibrium
A person holds a 50.0N sphere in his hand. The forearm is horizontal. The biceps
muscle is attached 3.00 cm from the joint, and the sphere is 35.0cm from the joint. Find
the upward force exerted by the biceps on the forearm and the downward force exerted
by the upper arm on the forearm and acting at the joint. Neglect the weight of forearm.
FB
Since the system is in equilibrium, from
the translational equilibrium condition
F  0
O
l
mg
 F  F - F - mg  0
F
From the rotational equilibrium condition   F  0 + F  d - mg  l  0
d
x
U
y
B
U
U
B
FB  d  mg  l
mg  l 50.0  35.0

 583N
FB 
3.00
d
Force exerted by the upper arm is
FU  FB - mg  583 - 50.0  533N
Thus, the force exerted by
the biceps muscle is
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
15
How do we solve equilibrium problems?
1.
2.
3.
4.
5.
6.
Identify all the forces and their directions and locations
Draw a free-body diagram with forces indicated on it
Write down force equation for each x and y component with
proper signs
Select a rotational axis for torque calculations  Selecting
the axis such that the torque of one of the unknown forces
become 0 makes the problem easier to solve
Write down torque equation with proper signs
Solve the equations for unknown quantities
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
16
Elastic Properties of Solids
We have been assuming that the objects do not change their
shapes when external forces are exerting on it. It this realistic?
No. In reality, the objects get deformed as external forces act on it,
though the internal forces resist the deformation as it takes place.
Deformation of solids can be understood in terms of Stress and Strain
Stress: A quantity proportional to the force causing deformation.
Strain: Measure of degree of deformation
It is empirically known that for small stresses, strain is proportional to stress
The constants of proportionality are called Elastic Modulus Elastic Modulus 
Three types of
Elastic Modulus
Tuesday, June 27, 2006
1.
2.
3.
stress
strain
Young’s modulus: Measure of the elasticity in length
Shear modulus: Measure of the elasticity in plane
Bulk modulus: Measure of the elasticity in volume
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
17
Young’s Modulus
Let’s consider a long bar with cross sectional area A and initial length Li.
Li
Fex
After the stretch
F
Tensile Stress  ex
A
Young’s Modulus is defined as
Fex
Fex=Fin
A:cross sectional area
Tensile stress
Lf=Li+L
Tensile strain
Tensile Strain 
F
Y
ex
Tensile Stress
A


Tensile Strain L L
i
L
Li
Used to characterize a rod
or wire stressed under
tension or compression
What is the unit of Young’s Modulus?
Force per unit area
1. For fixed external force, the change in length is
Experimental
proportional to the original length
Observations
2. The necessary force to produce a given strain is
proportional to the cross sectional area
Elastic limit: Maximum stress that can be applied to the substance
before it becomes permanently
deformed
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
18
Bulk Modulus
F
Bulk Modulus characterizes the response of a substance to uniform
squeezing or reduction of pressure.
V
After the pressure change
F
F
V’
F
Normal Force
F
Volume stress
Pressure 

Surface Area the force applies
A
=pressure
If the pressure on an object changes by P=F/A, the object will
undergo a volume change V.
Bulk Modulus is
defined as
Because the change of volume is
reverse to change of pressure.
Tuesday, June 27, 2006
F
P
Volume Stress  A B
V
V
Volume Strain
Vi
V
i
Compressibility is the reciprocal of Bulk Modulus
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
19
Example for Solid’s Elastic Property
A solid brass sphere is initially under normal atmospheric pressure of 1.0x105N/m2. The
sphere is lowered into the ocean to a depth at which the pressures is 2.0x107N/m2. The
volume of the sphere in air is 0.5m3. By how much its volume change once the sphere is
submerged?
Since bulk modulus is
P
BV
Vi
The amount of volume change is
V  -
PVi
B
From table 12.1, bulk modulus of brass is 6.1x1010 N/m2
The pressure change P is
P  Pf - Pi  2.0 107 -1.0 105  2.0 107
Therefore the resulting
2.0 107  0.5
-4
3

V

V
V


1
.
6

10
m
f
i
volume change V is
6.11010
The volume has decreased.
Tuesday, June 27, 2006
PHYS 1443-001, Summer 2006
Dr. Jaehoon Yu
20