021 small oscillations - george mason universitycomplex.gmu.edu/ small oscillations.pdf · labeling...
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PHYS 705: Classical MechanicsSmall Oscillations
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Assumptions:
Formulation of the Problem
- A conservative system with depending on position only
- The transformation equation defining does not dep on time explicitly
jV q
Now, consider the situation when a system is near an equilibrium point
where the generalized forces acting on the system @ is ZERO, i.e.,
0
0 0,jj q
VQ q jq
jq
0 jq
(conservative force)
Our goal is to describe the motion of the system when it is slightly deviated
from this equilibrium point:
0j j jq q
0 jq
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Now, we expand the Lagrangian around :
Formulation of the Problem
1. The potential energy term: jV q
For small displacement , we can expand V in a Taylor expansion:
2
1 01 0
0 0
1, , ,2n n j j k
j j k
V VV q q V q qq q q
(E’s sum rule applies here and coefficients evaluated at .)
1j
0 jq
Note: - can be set at zero
- since is an equilibrium
01 0, , nV q q
0
0j
Vq
0 jq
0 jq
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Note: - is just a number (V evaluated at )
- Since V is assumed to be a smooth function and the order of the
partial derivatives can be switch, is symmetric:
So, near an equilibrium , can be well approximated by
a quadratic form:
Formulation of the Problem
jV q
12 jk j kV V where
0 jq
2
0
jkj k
VVq q
jkV
jkV jk kjV V
(E’s sum rule applies to all repeated indices)
0j jq q
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Recall that we can in general write the KE as (Ch. 1):
Formulation of the Problem
2. The kinetic energy term:
2 1 0
2
12
12
i ii j k
i j k j k
ii j
j
ii
i j i
i
t
T T T T m q qq q
m q mq t
r r
r r r
2
1
0
- quadratic in
- linear in
- independent of
j
j
j
T q
T q
T q
With the assumption that transformation equation
does not dependent on time explicitly, i.e.,
we will only have the quadratic term remained…
1, , ,i i nq q tr r
0i
t
r
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(recall & )
Formulation of the ProblemThis gives,
i ii
ik
jj
k
mq q
m
r r
Note: lq
12 jk j kT qm q where,
12 jk j kT T
0j j jq q
- in general can be a function of
- but we can also Taylor expand it around
- keeping only lowest order term (const term) in , we then have
jk lm q
0lq
01 010
, ,, , jkjk n l
ljk nm q q
mm q q
q
01 0, ,jk jk nT m q q where,
j
j jq
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- the deviation from the equilibrium is our generalized coords
- and evaluated at are constant square matrices
- dynamic near ANY equilibrium quadratic forms in T and V
- coupling information among diff dofs are encoded in the cross terms
- GOAL for the following analysis is to
Formulation of the Problem: Quadratic Form of LWith V and T given near , we can now form the Lagrangian:
j
12 jk j k jk j kL T V T V
0 jq
j kT n n
To find a coordinate transformation so that in the new coords
j kV0 jq
0 jq
and diagonalize simultaneously
the problem decoupled !
j kTj kV
(E’s sum rule)
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Formulation of the Problem: EOM near Eq.
Now we use the EL equation to get the EOM for :
12 ik i k ik i kL T V T V
j
(pick out only i=j)
1 12 2k k ji i
jj
L V V
(pick out only k=j)
j k kj
L V
(E’s sum rule)
are symmetric
jk kj
L T
Similarly, we have:
Then, EL eq gives EOM:
jk kj
d L Tdt
0jk k jk kT V (sum rule over k)There are j of these eqs and they are all coupled.
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Formulation of the Problem: EOM near Eq.
The EOM for the are a set of coupled 2nd order ODEs with constant
coefficients and the solution has the following general form:
j
(sum over k)
i tj jt Ca e
- is the complex amplitude for the generalized coordinate
0jk k jk kT V
jjCa
- Obviously, only the real part of contributes to the actual motionj
(We have chosen the form of the pre-factor for convenience later.)jCawith being real.ja
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Formulation of the Problem: Condition on Solution
Plugging the oscillatory solution into the 2nd order ODE, we have the following
matrix equation for the amplitudes :
For nontrivial solutions , we need the determinant of the
coefficients to vanish:
2 0jk jk kV T a
ja
0ja
(sum over k)
2det 0jk jkV T
2 211 11 1 1
2 21 1
0n n
n n nn nn
V T V T
V T V T
or
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- is the eigenvector which will determine the relative relations among
the generalized coords in the different eigenmodes (normal modes)
- are the eigenvalues or resonant frequencies of the problem
Formulation of the Problem: Eigenvalue Equation
One can think of this as the characteristic equation for the following
generalized eigenvalue problem:
a
2det 0 V T
Va Ta2
V and T are real
and symmetric
2 is real
a is real and
“orthogonal”
j
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Labeling the n eigenmodes by the index r, we can write the matrix
equation for the rth eigenmode as:
Formulation of the Problem: Eigenvalue Equation
Since V and T are square matrices, there are in general n distinct
eigenvalues and eigenvectors. (We will consider degeneracy later.)
r r rVa Ta
n n
The general solution for will in general be a superposition (linear
combination) of all of these eigenmodes:
r ri t i tj jr r rt a C e C e
j
(sum over r)
1 , , Tr r nra aa where is the jth compoent of jra
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where and are real parameters and they will be determined by initial
conditions: and
As we mentioned previously, the actual physical motion is given by the real
part of the complex solution,
Formulation of the Problem: Real Physical Solution
r
Re cosj j r jr r rt t f a t
rf 0j 0j
0 cos 2 Rej r jr r jr rf a a C
0 sin 2 Imj r jr r r jr r rf a a C
(Here, we have taken the choice that for .) *
r r rC C C j t
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To show that are real, we will take the conjugate transpose of the
equation:
Formulation of the Problem: Props of Eigenmodes
r r rVa TaV and T are real
and symmetric† * †r r ra V a T
* †0 s r r s a Ta
†sr s sa Va Ta † * †
sr r r a V a aT--
(*)
r
Then,
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Consider the case when r = s, we have
Formulation of the Problem: Props of Eigenmodes
* †0 r r r r a Ta
For nontrivial solutions, we have and 0r a † 0r r a Ta
Then, the above condition gives * 0r r
2r r are real
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Conversely, this means that we can always find a real eigenvector
for if is real !
Formulation of the Problem: Props of Eigenmodes
Note: a real eigenvalue can be associated with a complex eigenvector.
Let call it, then straightforwardly gives
r r rVa Ta
r
Thus, are also two real eigenvector for the real eigenvalue
r r ri a α β
r r rVα Tα r r ri iVβ Tβand
and r rα β r
r
From now on, we will only consider real for real
r r rVa Ta
ra r
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Formulation of the Problem: Props of Eigenmodes
We look back at our Eq. (*) (eigen-system condition) with and
being real: 0s r r s a Ta
r
With distinct eigenvalues for , we then have r s r s
0r s a Ta r s
Lastly, to show the orthogonality of the eigenvectors,
ra
With , the matrix product in Eq. (**) is indeterminate. We will now show
that it is nonzero and in fact positive definite from a physical argument, i.e.,
r s
(**)
0r r a Ta
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To do that, we will exploit the fact that the Kinetic energy is a positive
definite quantity, i.e.,
Formulation of the Problem: Props of Eigenmodes
0T
jNow, we put in the expansion for in terms of its eigenmodes:
0q q
sum over r
12 jk j kT 1
2 jk j kT m q q
1 sin sin 02 jk r r jr r r s s ks s sT T f a t f a t
sum over s
sum over j and k
cosj r jr r rf a t
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We have just shown that for , we have the orthogonality condition:
Formulation of the Problem: Props of Eigenmodes
So, the previous Q-triple sum for the kinetic energy T collapses with only
terms surviving:
2 2 21 sin 02 jk jr kr r r r rT T a a f t
, ,sum over r j and k
r s
0r s a Ta or 0jk jr ksT a a
r s
Notice that the [] term are all squares and it will generically be positive
The matrix product or
when we are considering nontrivial solutions .
0jk jr krT a a 0r a
0r r a Ta
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So, we have the following orthogonality condition for the matrix product:
Formulation of the Problem: Props of Eigenmodes
Recall that the eigenvalue equation, ,determines the
direction of the eigenvector and we do have a freedom in choosing
its magnitude.
r s00r s
a Tar s
We now impose the normalization of the eigenvectors such that:
r r rVa Tara
r s10r s
a Tar s
or jk jr ks rsT a a
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Now, we will define a new set of coordinates (normal modes) so that the
oscillations decoupled !
Normal Modes
Recall we have the general expansion for our generalized coords:
We will define the normal coordinates as the term inside the ( ),
*r ri t i tj jr r rt a C e C e (sum over r)
1 , , Tr r nra aa where is the jth component of jra
*r ri t i tr r rt C e C e
r t
and the complex amplitude will be determined by IC.rC
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- This is a linear transformation between two coordinate systems
and this particular linear transformation is specified by the set of
eigenvectors: or .
Then, we can write,
Normal Modes
- Each of the jth generalized coordinate is now written as a linear
combination of the normal coordinates .
- The rth normal coordinates is purely periodic depending on
the rth eigenfrequency only.
(sum over r)
jra
r t
- This linear transformation can be inverted to give in terms of
r
j jr rt a t
j
r
1 | | rA a a
r j
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Now, we are going to explicitly show that the system decoupled in the
normal coordinates !
Normal Modes
We have and (sum over r) j jr rt a j jr rt a
Now, calculate T :
2
1212121 12 2
jk j k
jk jr r k
jk jr ks
s s
r
rs
s
r s r
T
T T
a
a
T
a
a
r s rsa Ta
-the are
orthogonal !
'r sa
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Normal Modes
To simplify the matrix product (blue term), we will use the eigenvalue
equation for a particular rth eigenmode,
Now, calculate V:
121212
jk j k
j
jk jr k
k jr r s
ss
ks
r
V V
V a a
V a a
jk jr r jk jrV a T a (sum over j)
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Normal Modes
Substituting this back into our equation for the potential energy, we have,
Multiply a different eigenvector on both sides, we have,
2
121 12 2
r s
r s r
jk jr ks
rs rr
V aV a
jk jr r jk jrV a T a
sa
jk jr ks r jk jr ksV a a T a a
Using the orthogonality condition again, we have
jk jr ks r rsV a a ( also diagonalizes !)ra jkV
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Normal Modes
Forming the Lagrangian,
2 212 r r rL T V
Calculating the EOM using EL equation for each rth normal mode ,
(NO sum over r)
0r r
d L Ldt
2 0r r r
r
-EOM for each of the normal mode is decoupled !
-Each normal mode evolutes in time as a simple harmonic oscillator with a
single eigenfrequency .r
2r r
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the general solution will be a linear combination of these normal modes .
Normal Modes
Note:
In general, for a system with n generalized coordinates, we will have n
normal modes and using the linear coordinate transformation
r j jr rt a
*r ri t i tj jr r jr r rt a a C e C e
(sum over r)
(sum over r)
Recall that the complex coefficients are determined by IC.rC
For typical IC, all normal modes will present, i.e., 0rC r
so that ALL normal modes will participate in the motion.
r
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- Let say, we have all except ,
- Then, the motion is very simple with all generalize coordinates proportional to
ONE non-zero normal mode
Normal Modes- However, one can imagine situations in which the IC will only lead to
the excitation of a certain normal mode.
1
1 1j jt a (for all j)
in proportions given by .
0rC 1 0C
1ja
- As an example in 2D: if , then (symmetric motion)a aa a
A 1 2 1a
if , then (anti-sym motion)a aa a
A 1 2 1a
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We start with our Lagrangian:
To ease in visualization later, let consider a 2 dofs case:
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
The reduction of the small oscillation problem into a set of equations
for its normal modes has an elegant geometric interpretation…
(matrix notation)
12 jk j k jk j kL T V
12
L ηTη ηVη
2 2 2 211 1 12 1 2 22 2 11 1 12 1 2 22 2
1 12 22 2
L T T T V V V
both T and V are symmetric
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The reduction to the normal modes equation corresponds to a linear
transformation A which simultaneously diagonalizes the two real-
symmetric quadratic forms: T and V.
Now, we will try to visualize this geometrically in 2D.
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
2 2 2 211 1 12 1 2 22 2 11 1 12 1 2 22 2
1 12 22 2
L T T T V V V
2 2 2 21 2 1 1 2 2
1 12 2
L
j jr rt a η Aζ
(no crossed terms)
(in going to the
normal coords)
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For any positive definite quadratic forms:
Note:
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
2 2,F x y ax bxy cy
, 1F x y
x
y
- If and the ellipse is a circle
a c
- If , the ellipse will be rotated
0b
Matrix
Diagonalization
semi-major &
semi-minor axes
coordinate
axes
x
y
Geometrically:
One can visualize F as the contour curve of the function: in
2D as an ellipse.
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Finding the normal modes corresponds to the simultaneous
diagonalization of both T and V. Let see now, how this is done
geometrically in 3 steps:
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
Step 0: in original generalized coordinates: and forT forVη η
Both quadratic forms
are NOT the same and
the ellipses are
oriented differently1
2T
1
2V
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Rotation, Squeeze, and Simultaneous Diagonalization of V and T
' η R η
Step 1: Rotate T so that it is aligned with coord axes
1'
2'T
1'
2'V
' η R η
22 21 1 2
222 1 1 2
1 ' ' ' ' '' ' ' '2
12
a cL b
aligns T in space
and rotates V in space
but V is still tilted.
R 'η'η
cos sinsin cos
R
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2 21 1 2 2
1 ' ' ' ' ' ' '2
a b c
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Rotation, Squeeze, and Simultaneous Diagonalization of V and T
Step 2: Squeeze (expand and contract) T so that it becomes a circle with
unit radius
1''
2''T
1''
2''V
1,2'' 'η G η
2 21
2 21 2 1 2 2
1 '' '' '' '1 '' ' ' '' '' ''2
'2
L a b c
G expands and contracts
T so that it becomes a
circle. G applies the
same scaling to V.
11,2
2
1 0
0 1
G 1,2'' 'η G η
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Rotation, Squeeze, and Simultaneous Diagonalization of V and T
Step 3: Rotate again but this time we rotate so that V is in
1
2T
1
2V
''ζ R η
2 21 2
2 21 1 2 2
1122
L
aligns V in space.
Since T is a circle, the
rotation does NOT affect T.
R ζ
1 1,
''ζ R ηcos sinsin cos
R
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Rotation, Squeeze, and Simultaneous Diagonalization of V and T
1 1,
2 2, T
1 1,
2 2, V
A R GR
2 2 2 21 2 1 1 2 2
1 12 2
L
ζ Aη
- So the sequence of linear transformations needed to diagonalize both T
and V consists of a rotation followed by a squeeze and by another rotation.
ζ Aη
- The key step is the squeeze so that T becomes a circle before the last
rotation to align V along its axes.
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Unlike the standard eigenvalue problem, the linear transformation
needed to simultaneously diagonalize both real-symmetry quadratic forms
(T and V) is NOT orthogonal in the traditional sense, i.e.,
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
A R GR
(the squeeze matrix is not
orthogonal)AA 1
However, it is orthogonal in a more generalized sense
A is orthogonal with respect to the metric tensor T
ATA 1r s rsa Tarecall
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A metric tensor is just a generalization of our Euclidean distance
measure.
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
With regular Euclidean space, the distance between two points is,
2 22 2jk j kd d dx dyds d qdz q d s s
2ds
For other non-Euclidean space, distance in general can be measured by a
metric tensor G give by,
2jk j kds g dq dq
where is the matrix element of G.jkg
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Other familiar examples of metrix tensors:
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
Mikowski space (Lorentzian metric): (non-positive definite)
2 2 2 2 2ds dr r d dz 2
1 0 00 00 0 1
G r
Cylindrical coord:
Spherical coord:
2 2 2 2 2 2 2sinds dr r d r d 2
2 2
1 0 00 00 0 sin
G rr
2 2 2 2 2 2ds dx dy dz c dt
Posi
tive
def
init
e m
etri
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So, in our discussion for small oscillations, the condition for the kinetic
energy matrix given by
Rotation, Squeeze, and Simultaneous Diagonalization of V and T
simply means that the set of eigenvectors are orthonormal with
respect to a different metric given by the metric tensor T or .
ATA 1
rajkT
AND, the set of eigenvectors from T diagonalizes simultaneously the
potential energy matrix V,
AVA λ
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When one or more of the roots from the eigenvalue characteristic equation,
is repeated i.e., , , then our argument in showing that
Degeneracy
from 0s r r s a Ta
r s r s0r s a Ta
does not follow directly.
However, one can still construct a FULL set of orthonormal eigenvectors
from the degenerate set of allowed vectors.
To see how to do that, let consider a simple 3D case when we have 1
distinct eigenvalue and a double root for the other two eigenvalues3
1 2 3
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There is an infinite set of eigenvectors associated with in the
degenerate plane to
Let be a normalized eigenvector associate with
Degeneracy
r r rVa Ta
3 3 1a Ta 3
3a
1a
2a3a3 1 0a Ta
3 2 0a Ta
1, 2,3r
is orthogonal to both 3a
1 2 and a a
But, might NOT be orthogonal to 1a 2a
3a
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Check:
Fortunately, one can always construct an orthogonal pair from any
randomly chosen vectors in the plane , e.g.,
Degeneracy
1 2 22 1' c c VaV Vaa
1a
2a3a
1 2 and a a
Let be a new eignenvector given by,
2 1 1 2 2' c c a a a
2'a
2'a- Any linear combination of will
also be an eigenvector for 1 2 and a a
1 2
2
1 1 1 2 2 2
1 1 2 2 'c c
c c
Ta TaT a a Ta
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Now, we enforce the orthogonality between
Degeneracy
1 2 and 'a a
1 2 1 1 1 2 1 2' 0c c a Ta a Ta a Ta
1a
2a3a 2'a
This new vector must also be properly normalized,
is normalized1 2 1 2 0c c a Ta1a
11 2
2
cc
a Ta
2 2 1 1 2 2 1 1 2 21 ' ' c c c c a Ta a a T a a 2 21 1 1 1 2 1 2 2 2 22c c c c a Ta a Ta a Ta
1 2 2 1a Ta a Ta T is symmetric
2 211 1 2 2
2
1 2 cc c c cc
2 22 1 1c c
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Thus by solving the two equations
for , we have successful construct a set
of orthonormal eigenvectors for the degenerate
eigenvalue
Degeneracy
1a
2a3a 2'a
11 2
2
cc
a Ta and 2 22 1 1c c
1 2 and c c
1 2
The standard Gram-Schmidt orthogonalization procedure can be used
when the degeneracy space is more than 2 dimensions.
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