## Spherical polar pendulum for one and multiple masses (Take II)

Posted by peeterjoot on November 10, 2009

[Click here for a PDF of this post with nicer formatting]

# Motivation

Attempting the multiple spherical pendulum problem with a bivector parameterized Lagrangian has just been attempted ([1]), but did not turn out to be an effective approach. Here a variation is used, employing regular plain old scalar spherical angle parameterized Kinetic energy, but still employing Geometric Algebra to express the Hermitian quadratic form associated with this energy term.

The same set of simplifying assumptions will be made. These are point masses, zero friction at the pivots and rigid nonspringy massless connecting rods between the masses.

# The Lagrangian.

A two particle spherical pendulum is depicted in figure (\ref{fig:sPolarMultiPendulum:pendulumDouble})

\begin{figure}[htp]

\centering

\includegraphics[totalheight=0.4\textheight]{pendulumDouble}

\caption{Double spherical pendulum.}

\end{figure}

The position vector for each particle can be expressed relative to the mass it is connected to (or the origin for the first particle), as in

To express the Kinetic energy for any of the masses , we need the derivative of the incremental difference in position

Introducing a Hermitian conjugation , reversing and transposing the matrix, and writing

We can now write the relative velocity differential as

Observe that the inner product is Hermitian under this definition since . \footnote{Realized later, and being too lazy to adjust everything in these notes, the use of reversion here is not neccessary. Since the generalized coordinates are scalars we could use transposition instead of Hermitian conjugation. All the matrix elements are vectors so reversal doesn’t change anything.}

The total (squared) velocity of the th particle is then

(where the zero matrix in is a by one zero). Summing over all masses and adding in the potential energy we have for the Lagrangian of the system

There’s a few layers of equations involved and we still have an unholy mess of matrix and geometric algebra in the kernel of the kinetic energy quadratic form, but at least this time all the generalized coordinates of the system are scalars.

# Some tidy up.

Before continuing with evaluation of the Euler-Lagrange equations it is helpful to make a couple of observations about the structure of the matrix products that make up our velocity quadratic forms

Specifically, consider the products that make up the elements of the matrices . Without knowing anything about the grades that make up the elements of , since it is Hermitian (by this definition of Hermitian) there can be no elements of grade order two or three in the final matrix. This is because reversion of such grades inverts the sign, and the matrix elements in all equal their reverse. Additionally, the elements of the multivector column matrices are vectors, so in the product we can only have scalar and bivector (grade two) elements. The resulting one by one scalar matrix is a sum over all the mixed angular velocities , , and , so once this summation is complete any bivector grades of must cancel out. This is consistent with the expectation that we have a one by one scalar matrix result out of this in the end (i.e. a number). The end result is a freedom to exploit the convienence of explicitly using a scalar selection operator that filters out any vector, bivector, and trivector grades in the products . We will get the same result if we write

Pulling in the summation over we have

It appears justifiable to lable the factors of the angular velocity matrices as moments of inertia in a generalized sense. Using this block matrix form, and scalar selection, we can now write the Lagrangian in a slightly tidier form

After some expansion, writing , and so forth, one can find that the scalar parts of the block matrixes contained in are

The diagonal blocks are particularily simple and have no dependence

Observe also that , so the scalar matrix

is a real symmetric matrix. We have the option of using this explicit scalar expansion if desired for further computations associated with this problem. That completely eliminates the Geometric algebra from the problem, and is probably a logical way to formulate things for numerical work since one can then exploit any pre existing matrix algebra system without having to create one that understands non-commuting variables and vector products.

# Evaluating the Euler-Lagrange equations.

For the acceleration terms of the Euler-Lagrange equations our computation reduces nicely to a function of only

and

The last groupings above made use of , and in particular . We can now form a column matrix putting all the angular velocity gradient in a tidy block matrix representation

A small aside on Hamiltonian form. This velocity gradient is also the conjugate momentum of the Hamiltonian, so if we wish to express the Hamiltonian in terms of conjugate momenta, we require invertability of at the point in time that we evaluate things. Writing

and noting that , we get for the kinetic energy portion of the Hamiltonian

Now, the invertability of cannot be taken for granted. Even in the single particle case we do not have invertability. For the single particle case we have

so at this quadratic form is singlular, and the planar angular momentum becomes a constant of motion.

Returning to the evaluation of the Euler-Lagrange equations, the problem is now reduced to calculating the right hand side of the following system

With back substituition of 22, and 24 we have a complete non-multivector expansion of the left hand side. For the right hand side taking the and derivatives respectively we get

So to procede we must consider the partials. A bit of thought shows that the matrices of partials above are mostly zeros. Illustrating by example, consider , which in block matrix form is

Observe that the diagonal term has a scalar plus its reverse, so we can drop the one half factor and one of the summands for a total contribution to of just

Now consider one of the pairs of off diagonal terms. Adding these we contributions to of

This has exactly the same form as the diagonal term, so summing over all terms we get for the position gradient components of the Euler-Lagrange equation just

The only thing that remains to do is evaluate the matrixes.

It should be possible but it is tedious to calculate the block matrix derivative terms from the partials using

However multiplying this out and reducing is a bit tedious and would be a better job for a symbolic algebra package. With 22 available to use, one gets easily

The right hand side of the Euler-Lagrange equations now becomes

Can the matrices be factored out, perhaps allowing for expression as a function of ? How to do that if it is possible is not obvious. The driving reason to do so would be to put things into a tidy form where things are a function of the system angular velocity vector , but this is not possible anyways since the gradient is non-linear.

# Hamiltonian form and linearization.

Having calculated the Hamiltonian equations for the multiple mass planar pendulum in [2], doing so for the spherical pendulum can now be done by inspection. With the introduction of a phase space vector for the system using the conjugate momenta (for angles where these conjugate momenta are non-singlular)

we can write the Hamiltonian equations

The position gradient is given explicitly in 39, and that can be substituted here. That gradient is expressed in terms of and not the conjugate momenta, but the mapping required to express the whole system in terms of the conjugate momenta is simple enough

It is apparent that for any sort of numerical treatment use of a angular momentum and angular position phase space vector is not prudent. If the aim is nothing more than working with a first order system instead of second order, then we are probably better off with an angular velocity plus angular position phase space system.

This eliminates the requirement for inverting the sometimes singular matrix , but one is still left with something that is perhaps tricky to work with since we have the possibility of zeros on the left hand side. The resulting equation is of the form

where is a possibly singular matrix, and is a non-linear function of the components of , and . This is concievably linearizable in the neighbourhood of a particular phase space point . If that is done, resulting in an equation of the form

where and is an appropriate matrix of partials (the specifics of which don’t really have to be spelled out here). Because of the possible singularities of the exponentiation techniques applied to the linearized planar pendulum may not be possible with such a linearization. Study of this less well formed system of LDEs probably has interesting aspects, but is also likely best tackled independently of the specifics of the spherical pendulum problem.

## Thoughts about the Hamiltonian singularity.

The fact that the Hamiltonian goes singular on the horizontal in this spherical polar representation is actually what I think is the most interesting bit in the problem (the rest being a lot mechanical details). On the horizontal or radians/sec makes no difference to the dynamics. All you can say is that the horizontal plane angular momentum is a constant of the system. It seems very much like the increasing uncertaintly that you get in the corresponding radial QM equation. Once you start pinning down the angle, you loose the ability to say much about .

It is also kind of curious how the energy of the system is never ill defined but a choice of a particular orientation to use as a reference for observations of the momenta introduces the singularity as the system approaches the horizontal in that reference frame.

Perhaps there are some deeper connections relating these classical and QM similarity. Would learning about symplectic flows and phase space volume invariance shed some light on this?

# A summary.

A fair amount of notation was introduced along the way in the process of formulating the spherical pendulum equations. It is worthwhile to do a final consise summary of notation and results before moving on for future reference.

The positions of the masses are given by

With the introduction of a column vector of vectors (where we multiply matrices using the Geometric vector product),

and a matrix of velocity components (with matrix multiplication of the vector elements using the Geometric vector product), we can form the Lagrangian

An explicit scalar matrix evaluation of the (symmetric) block matrix components of was evaluated and found to be

These can be used if explicit evaluation of the Kinetic energy is desired, avoiding redundant summation over the pairs of skew entries in the quadratic form matrix

We utilize angular position and velocity gradients

and use these to form the Euler-Lagrange equations for the system in column vector form

For the canonical momenta we found the simple result

For the position gradient portion of the Euler-Lagrange equations 63 we found in block matrix form

A set of Hamiltonian equations for the system could also be formed. However, this requires that one somehow restrict attention to the subset of phase space where the canonical momenta matrix is non-singular, something not generally possible.

# References

[1] Peeter Joot. {Spherical polar pendulum for one and multiple masses, and multivector Euler-Lagrange formulation.} [online]. http://sites.google.com/site/peeterjoot/math2009/sPolarMultiPendulum.pdf.

[2] Peeter Joot. Hamiltonian notes. [online]. http://sites.google.com/site/peeterjoot/math2009/hamiltonian.pdf.

## Leave a Reply