PHYS 1441 – Section 501
Lecture #13
Wednesday, July 14, 2004
Dr. Jaehoon Yu
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Rolling Motion
Torque
Moment of Inertia
Rotational Kinetic Energy
Angular Momentum and Its Conservation
Conditions for Mechanical Equilibrium
Today’s homework is #6 due 7pm, Friday, July 23!!
Remember the second term exam, Monday, July 19!!
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
1
Angular Displacement, Velocity, and Acceleration
Using what we have learned in the previous slide, how

 i



f
would you define the angular displacement?
 f  i
How about the average angular speed?
Unit? rad/s

And the instantaneous angular speed?
Unit? rad/s
  lim
By the same token, the average angular
acceleration
Unit? rad/s2
And the instantaneous angular
acceleration? Unit? rad/s2

t f  ti
t 0
t f  ti
t 0

t
 d

t
dt
 f  i
  lim


f
i

t
 d

dt
t
When rotating about a fixed axis, every particle on a rigid object rotates through
the same angle and has the same angular speed and angular acceleration.
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
2
How about the acceleration?
How many different linear accelerations do you see
in a circular motion and what are they? Two
Tangential, at, and the radial acceleration, ar.
Since the tangential speed v is
v  r
The magnitude of tangential a  v   r  r   r
 
t
acceleration at is
t
t t
What does this
relationship tell you?
Although every particle in the object has the same angular
acceleration, its tangential acceleration differs proportional to its
distance from the axis of rotation.
2
v2


r

The radial or centripetal acceleration ar is ar 

r
r
 r 2
What does The father away the particle is from the rotation axis, the more radial
this tell you? acceleration it receives. In other words, it receives more centripetal force.
Total linear acceleration is
Wednesday, July 14, 2004
a  at2  ar2

r 
2
 r
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu

2 2
 r  2 4
3
Rolling Motion of a Rigid Body
What is a rolling motion?
A more generalized case of a motion where the
rotational axis moves together with the object
A rotational motion about the moving axis
To simplify the discussion, let’s
make a few assumptions
1.
2.
Limit our discussion on very symmetric
objects, such as cylinders, spheres, etc
The object rolls on a flat surface
Let’s consider a cylinder rolling without slipping on a flat surface
Under what condition does this “Pure Rolling” happen?
The total linear distance the CM of the cylinder moved is
s  R
R  s
s=R
Wednesday, July 14, 2004
Thus the linear
speed of the CM is
vCM 
Condition
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
ds
d
R
 R
dt
dt
for “Pure Rolling”
4
More Rolling Motion of a Rigid Body
The magnitude of the linear acceleration of the CM is
P’
CM
aCM
vCM


R
 R
t
t
2vCM As we learned in the rotational motion, all points in a rigid body
moves at the same angular speed but at a different linear speed.
vCM
P
At any given time the point that comes to P has 0 linear
speed while the point at P’ has twice the speed of CM
CM is moving at the same speed at all times.
Why??
A rolling motion can be interpreted as the sum of Translation and Rotation
P’
CM
P
vCM
P’
vCM
CM
v=0
vCM
Wednesday, July 14, 2004
+
v=R
v=R
2vCM
P’
=
P
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
CM
vCM
P
5
Torque
Torque is the tendency of a force to rotate an object about an axis.
Torque, t, is a vector quantity.
F
f
r
P
Line of
Action
d2
d
Moment
arm
F2
Consider an object pivoting about the point P
by the force F being exerted at a distance r.
The line that extends out of the tail of the force
vector is called the line of action.
The perpendicular distance from the pivoting point
P to the line of action is called Moment arm.
Magnitude of torque is defined as the product of the force
exerted on the object to rotate it and the moment arm.
When there are more than one force being exerted on certain
points of the object, one can sum up the torque generated by each
force vectorially. The convention for sign of the torque is positive if
rotation is in counter-clockwise and negative if clockwise.
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
t  rF sin f  Fd
t  t
1
t 2
 F1d1  F2 d2
6
Example for Torque
A one piece cylinder is shaped as in the figure with core section protruding from the larger
drum. The cylinder is free to rotate around the central axis shown in the picture. A rope
wrapped around the drum whose radius is R1 exerts force F1 to the right on the cylinder,
and another force exerts F2 on the core whose radius is R2 downward on the cylinder. A)
What is the net torque acting on the cylinder about the rotation axis?
R1
F1
The torque due to F1
t1  R1F1 and due to F2
So the total torque acting on
the system by the forces is
R2
t  t
1
t 2  R2 F2
 t 2  R1F1  R2 F2
F2
Suppose F1=5.0 N, R1=1.0 m, F2= 15.0 N, and R2=0.50 m. What is the net torque
about the rotation axis and which way does the cylinder rotate from the rest?
Using the
above result
Wednesday, July 14, 2004
t  R F  R F
1 1
2 2
 5.0 1.0 15.0  0.50  2.5N  m
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
The cylinder rotates in
counter-clockwise.
7
Moment of Inertia
Measure of resistance of an object to
changes in its rotational motion.
Equivalent to mass in linear motion.
Rotational Inertia:
For a group
of particles
I   mi ri
i
What are the dimension and
unit of Moment of Inertia?
2
For a rigid
body
ML 
2
I   r 2 dm
kg m
2
Determining Moment of Inertia is extremely important for
computing equilibrium of a rigid body, such as a building.
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
8
Ft
Torque & Angular Acceleration
m Let’s consider a point object with mass m rotating on a circle.
r F
What forces do you see in this motion?
r
The tangential force Ft and radial force Fr
Ft  mat  mr
The tangential force Ft is
The torque due to tangential force Ft is t  Ft r  mat r  mr 2  I
What do you see from the above relationship?
What does this mean?
What law do you see from
this relationship?
Wednesday, July 14, 2004
t  I
Torque acting on a particle is proportional to
the angular acceleration.
Analogs to Newton’s 2nd law of motion
in rotation.
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
9
Rotational Kinetic Energy
y
vi
mi
ri

O
x
What do you think the kinetic energy of a rigid object
that is undergoing a circular motion is?
1
1
K i  mi vi2  mi ri 2 
Kinetic energy of a masslet, mi,
2
2
moving at a tangential speed, vi, is
Since a rigid body is a collection of masslets, the total kinetic energy of the
rigid object is
1
1
2 
2 
K R   Ki   mi ri     mi ri 
2 i
2 i

i
Since moment of Inertia, I, is defined as
I   mi ri2
i
The above expression is simplified as
Wednesday, July 14, 2004
1 
K R  I
2
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
10
Example for Moment of Inertia
In a system consists of four small spheres as shown in the figure, assuming the radii are
negligible and the rods connecting the particles are massless, compute the moment of
inertia and the rotational kinetic energy when the system rotates about the y-axis at .
y
m
Since the rotation is about y axis, the moment of
inertia about y axis, Iy, is
b
l
M
O
l
M
x
b
m
I   mi ri2  Ml2 Ml 2 m  02  m  02  2Ml 2
i
This is because the rotation is done about y axis,
and the radii of the spheres are negligible.
1 2 1
K R  I  2 Ml 2  2  Ml 2 2
2
2
Why are some 0s?

Thus, the rotational kinetic energy is

Find the moment of inertia and rotational kinetic energy when the system rotates on
the x-y plane about the z-axis that goes through the origin O.

2
2
2
I   mi ri 2  Ml 
Ml 2 mb2 mb 2  2 Ml  mb
i
Wednesday, July 14, 2004

1
1
K R  I 2  2 Ml 2  2mb2  2  Ml 2  mb2  2
2
2
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
11
Kinetic Energy of a Rolling Sphere
Let’s consider a sphere with radius R
rolling down a hill without slipping.
R

h

vCM
Since vCM=R
What is the speed of the
CM in terms of known
quantities and how do you
find this out?
1
1
2
2 2
K  I CM   MR 
2
2
2
1
 vCM   1 Mv 2
 I CM 
CM

2
2
 R 
1I
 2
  CM2  M vCM
2 R

Since the kinetic energy at the bottom of the hill must
be equal to the potential energy at the top of the hill
1  I CM
 2


M
 2
vCM  Mgh
K
2 R

vCM 
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
2 gh
1  I CM / MR 2
12
Angular Momentum and Its Conservation
ur
ur
L  I
Angular Momentum: Tendency to keep the rotational Motion
Remember under what condition the linear momentum is conserved?
Linear momentum is conserved when the net external force is 0.  F  0 
dp
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
t

ext
0

dt
L  const
Angular momentum of the system before and
after a certain change is the same.
ur
ur
L i  L f  constant
Three important conservation laws
for isolated system that does not get
affected by external forces
Wednesday, July 14, 2004
Ki  U i  K f  U f
ur
ur
pi  p f
ur
ur
Li  L f
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
Mechanical Energy
Linear Momentum
Angular Momentum
13
Effect of Angular Momentum Conservation
Large I
Small 
Small I
Large 
Small I
Large 
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
14
Example for Angular Momentum Conservation
A star rotates with a period of 30days 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 start 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:
Using angular momentum
conservation
The period will be significantly shorter,
because its radius got smaller.
1. There is no torque acting on it
2. The shape remains spherical
3. Its mass remains constant
Li  L f
I i  I f  f
The angular speed of the star with the period T is
Thus

Tf 
I i
mri 2 2


f
If
mrf2 Ti
2
f
 r f2
 2
r
 i
Wednesday, July 14, 2004
2

T
2

3
.
0


6
Ti  

2
.
7

10
days  0.23s

30
days

4

1
.
0

10



PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
15
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
Wednesday, July 14, 2004
Linear
Mass
M
Distance
r
t
v
a
t
L
v
I  mr 2
Angle  (Radian)

t


t

P  F v
Torque t  I
Work W  t
P  t
p  mv
L  I
Force F  ma
Work W  Fd cos
Kinetic
Rotational
Moment of Inertia
K
1
mv 2
2
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
Rotational
KR 
1
I 2
2
16
Conditions for Equilibrium
What do you think does the term “An object is at its equilibrium” mean?
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 static
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 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
t 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. v
0  0
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004 CM
Dr. Jaehoon Yu
17
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
0
y
0
t  0 t
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 a motion.
Wednesday, July 14, 2004
PHYS 1441-501,
Summerof
2004
It is simply
a matter
mathematical calculation.18
Dr. Jaehoon Yu
Example for Mechanical Equilibrium
A uniform 40.0 N board supports a father and daughter 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
F
MFg
x
 MBg  MF g  MDg n
0
n  40.0  800  350  1190N
y
Therefore the magnitude of the normal force
0
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
Wednesday, July 14, 2004
t  M B g  0  M F g 1.00  M D g  x  0
x

MFg
800
1.00m 
1.00m  2.29m
MDg
350
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
19
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
t
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
t  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
Wednesday, July 14, 2004
MFg
800

1.00m 
1.00m  2.29m
MDg
350
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
What do we learn?
No matter where the
rotation axis is, net effect of
the torque is identical.
20
Example 9 – 9
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
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
21
Example 9 – 9 cont’d
From the rotational equilibrium
t
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 
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
22
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 t  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
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
23
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 vector 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.
Write down torque equation with proper signs
Solve the equations for unknown quantities
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
24
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
Wednesday, July 14, 2004
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 1441-501, Summer 2004
Dr. Jaehoon Yu
25
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
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
26
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.
Wednesday, July 14, 2004
F
P
Volume Stress  
A 
B
V
V
Volume Strain
Vi
V
i
Compressibility is the reciprocal of Bulk Modulus
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
27
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.
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
28
Density and Specific Gravity
Density, r (rho) , of an object is defined as mass per unit volume
3
kg / m
Unit?
3
[
ML
]
Dimension?
M
r 
V
Specific Gravity of a substance is defined as the ratio of the density
of the substance to that of water at 4.0 oC (rH2O=1.00g/cm3).
r substance
SG 
rH O
2
What do you think would happen of a
substance in the water dependent on SG?
Wednesday, July 14, 2004
Unit?
None
Dimension? None
SG  1 Sink in the water
SG  1 Float on the surface
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
29
Fluid and Pressure
What are the three states of matter?
Solid, Liquid, and Gas
By the time it takes for a particular substance to
How do you distinguish them?
change its shape in reaction to external forces.
A collection of molecules that are randomly arranged and loosely
What is a fluid? bound by forces between them or by the external container.
We will first learn about mechanics of fluid at rest, fluid statics.
In what way do you think fluid exerts stress on the object submerged in it?
Fluid cannot exert shearing or tensile stress. Thus, the only force the fluid exerts
on an object immersed in it is the forces perpendicular to the surfaces of the object.
This force by the fluid on an object usually is expressed in the form of P  F
A
the force on a unit area at the given depth, the pressure, defined as
Expression of pressure for an
dF Note that pressure is a scalar quantity because it’s
P

infinitesimal area dA by the force dF is
dA the magnitude of the force on a surface area A.
What is the unit and
Unit:N/m2
Special SI unit for
2
1
Pa

1
N
/
m
dimension of pressure?
pressure is Pascal
Dim.: [M][L-1][T-2]
Wednesday, July 14, 2004
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
30
Example for Pressure
The mattress of a water bed is 2.00m long by 2.00m wide and
30.0cm deep. a) Find the weight of the water in the mattress.
The volume density of water at the normal condition (0oC and 1 atm) is
1000kg/m3. So the total mass of the water in the mattress is
m  rW VM  1000  2.00  2.00  0.300  1.20 103 kg
Therefore the weight of the water in the mattress is
W  mg  1.20 103  9.8  1.18 10 4 N
b) Find the pressure exerted by the water on the floor when the bed
rests in its normal position, assuming the entire lower surface of the
mattress makes contact with the floor.
Since the surface area of the
mattress is 4.00 m2, the
pressure exerted on the floor is
Wednesday, July 14, 2004
F mg 1.18 10 4
3




2
.
95

10
P A A
4.00
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
31
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.
Wednesday, July 14, 2004
F
P
Volume Stress  
A 
B
V
V
Volume Strain
Vi
V
i
Compressibility is the reciprocal of Bulk Modulus
PHYS 1441-501, Summer 2004
Dr. Jaehoon Yu
32