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Introduction to Linear Algebra with Mathematica
Dimensional Analysis is a technique based on two simple axioms about nature:
All relations between physical quantities must be dimensionally correct.
No physical relation should depend on any particular set of units.
Although these laws are quite simple, they are very important and serve as powerful instrument for correct mathematical/theoretical modelling. The dimension of a physical quantity (such as length, time, or mass) is given once and for all and will never change. When the unit changes, the numerical value of the quantity
also changes.
...
Quantity
Symbol
SI unit
Mass
m
Kilogram (kg)
Length
ℓ
meter (m)
Time
t
second (s)
Electric current
I
ampere (A)
Absolute temperature
Θ
kelvin (K)
A physical relation or equation is dimensionally correct if each side of an equality does
not have different dimensions. For instance, the well-known formula F = m 𝑎 is dimensionally correct because
[F]
newton (N)
[m]
kilogram (kg)
[𝑎]
m/s²
Degres of freedom
The important characteristi cs of a dynamical system is its number of degrees of freedom. It is customary to set this number equal to one half the number of independent variables that completely define the state of system, i. e., to one half the
dimension of the phase space of the syst m. Such a definition of the number of
degrees of freedom came from its first appearence in mechanics where the
one-dimensional motion of a material point is completely defined by two variables:
coordinate and velocity. According to this definition the number of degrees of freedom may not be an integer. For example, if a system is described by one differential equation of the third order or by three first order combined differential equations,
then its number of degrees of freedom equals one and a half. We note that if a
system is non-autonomous , i.e. the algorithm for its transition from one state to
another is explicitly dependent on tim , then it may be considered as autonomous
by means of the incorporation of time as one of the coordinates of its phase space.
In so doing a system described by a second order differential equation with external action must be considered as a system with one and a half degrees of freedom.
All dynamical systems havin g a physical meaning can be separated into two main
groups: systems with conservation of their phase volume and systems with decrease
of their phase volume, or dissipative sustems. If the autonomous system is described by differential equations of the type
\[
\dot{x}_j = f_j (x_1 , x_2 , \ldots , x_n ), \qquad j=1,2,\ldots , n.
\]
then it can be shown, based on the divergence theorem, that the variation of its phase volume dV in a time dt is
\[
{\text d}V = {\text d}t \int \left( \frac{{\text d}\dot{x}_1}{{\text d}x_1} + \frac{{\text d}\dot{x}_2}{{\text d}x_2} + \cdots + \frac{{\text d}\dot{x}_n}{{\text d}x_n} \right) {\text d}x_1 {\text d} x_2 \cdots {\text d}x_n = {\text d}t \int \mbox{div}\, \dot{\bf x}\,{\text d}{\bf x} ,
\]
where x is a vector with com ponents x1, …, xn. It follows from this that the
sufficient condition for the conservation of the phase volume is
\[
\mbox{div}\, \dot{\bf x} = 0.
\]
The 2n-dimensional space of points specified by the canonical coordinates and momenta is called phase space.
Hamiltonian evolution
When the dynamics is described by Hamilton's equations, the evolution in time is made by canonical transformations.
Jordan, T., Steppingstones in Hamiltonian dynamics, The American Journal of Physics, 2004, 72, No. 8, pp. 1095--1099. doi: 10.1119/1.1737394
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