##### Generalized Coordinates

The name generalized coordinates is given to any set of quantities
which completely specifies as the state of system. The generalized
coordinates are customarily written as $q_{1},q_{2},q_{3},\ldots$
or simply as the $q_{j}$. A set of independent generalized coordinates
whose number equals the number $s$ degrees of freedom of the system
and which are not restricted by the constraints will be called a proper
set of generalized coordinates. In certain instances it may be advantageous
to use generalized coordinates whose number exceeds the number of
degrees through the use of the Lagrange undetermined multipliers.
We shall consider a general mechanical system which consists of a
collection of $n$ discrete, point particles. In order to specify
the state of such a system at a given time, it is necessary to use
$n$ radius vectors. If there exist equation of constraint which relate
some of these coordinates to others, then not all of the $3n$ coordintes
are independent. In fact, if there are $m$ equations of constraint,
then $3n-m$ coordinats are independent, and the system is said to
possess $3n-m$ degrees of freedom.
In addition to the generalized coordinates, we may define a set of
quantities which consists of the time derivatives of the $q_{j}$
such as $\dot{q}_{1},\dot{q}_{2},\dot{q}_{3},\ldots$ or simply $\dot{q}_{j}$.
In analogy with the nomenclature for rectangular coordinates we call
$\dot{q}_{j}$'s the generalized velocities.
We may represent the state of such a system by a point in an $s$-dimensional
space called configuration space, each point specifying the configuration
of the system at a particular instant. A dynamical path in a configuration
space consisting of proper generalized coordinates is automatically
consistent with the constraints on the system.

##### Principle of least action

In physics, the principle of least action is a variational principle
that, when applied to the action of a mechanical system, can be used
to obtain the equations of motion for that system. The principle of
least action is defined by the statement that for each mechanical
system there exists a certain integral $S$, called the action. It
has a minimum value for the actual motion, so that its variation $\delta S$
is zero. To determine the action integral for a free material particle,
the integrand must be a differential of the first order . But only
scalar of this kind that one can construct for a free particle is
the intercal $ds$, or $\alpha ds$ , where $\alpha$ is some constant.
So for a free particle the action must have the form-
\begin{equation}
S=\alpha\int_{a}^{b}ds\label{eq:2-1}
\end{equation}
The $\int_{a}^{b}$ is an integral along the world line of the particle
at the initial position and at the final position at definite times
$t_{1}$and $t_{2}$ and $\alpha$ is some constant characterizing
the particle. So the dynamics of a system is governed by the stationarity
of the action integral can be represent as an integral with respect
to the time-
\begin{equation}
S=\int_{a}^{b}L(q_{i}(t),\dot{q}_{i}(t))dt\label{eq:2-2}
\end{equation}
where the Lagrangian function $L=L(q,\dot{q})$ is the Lagrange function
of the mechanical system in the generalized coordinates $q_{i}(i=1,2,\ldots N)$
and the velocities $\dot{q_{i}}=\frac{dq_{i}}{dt}$. We assume that
the system has $N$ degrees of freedom. Now if $q_{i}(t)=q_{i}^{classical}(t)+\varepsilon\delta q(t)$
then as $S=S(\varepsilon)$ has minimum at $\varepsilon=0$ we have-
\begin{eqnarray}
\frac{dS(0)}{d\varepsilon} & = & 0\nonumber \\
& = & \int_{t_{i}}^{t_{f}}\left(\frac{\partial L}{\partial q_{i}}\delta q_{i}+\frac{\partial L}{\partial\dot{q_{i}}}\delta\dot{q_{i}}\right)dt\nonumber \\
& = & \int_{t_{i}}^{t_{f}}\left(\frac{\partial L}{\partial q_{i}}+\frac{d}{dt}\frac{\partial L}{\partial\dot{q_{i}}}\right)dt\;\text{ (Integration by parts)}\label{eq:2-3}
\end{eqnarray}
As $\delta q_{i}$ is arbitrary the variation of the action integral
leads to the Lagrange equation of motion-
\begin{equation}
\frac{d}{dt}\left(\frac{\partial L}{\partial\dot{q}_{i}}\right)-\frac{\partial L}{\partial q_{i}}=0\label{eq:2-4}
\end{equation}

##### The Hamiltonian

We consider a dynamical system of $N$ degrees of freedom, described
in terms of generalized coordinates $q_{n}(n=1,2,\ldots,N)$ and velocities
$\dot{q}_{n}$ or $\dfrac{dq_{n}}{dt}$. We assume a Lagrangian $L$,
which for the present can be any function of the coordinates and velocities.
\begin{equation}
L\equiv L(q,\dot{q})\label{eq:2-5}
\end{equation}
If the Lagrangian is expressed in generalized coordinates, we define
generalized momenta according to-
\begin{equation}
p_{i}=\frac{\partial L}{\partial\dot{q}_{i}}\label{eq:2-6}
\end{equation}
For the development of the theory we introduce a variation procedure,varying
each of the quantities $q_{n}$,$\dot{q}_{n}$and $p_{n}$ independently
by small quantity $\delta q_{n}$,$\delta\dot{q}_{n}$ and $\delta p_{n}$
of order $\epsilon$ and working to the accuracy of $\epsilon$. As
a result of this variation procedure equation \eqref{eq:2-6} will
get violated, as its left-hand side will be made to differ from its
right-hand side by a quantity of order $\epsilon$. The Hamiltonian
$H$ is defined by the equation-
\begin{equation}
H\equiv p_{i}\dot{q}_{i}-L\label{eq:2-7}
\end{equation}
where a summation is understood over all values for a repeated suffix
in a term. From the equation..... it is clear that the hamitonian
$H$ is the function of both position and velocities. The $p_{i}(q,\dot{q})$
is defined by the equation \eqref{eq:2-6}. Since $H$ doesnot explicitly
depend on $\dot{q}_{i}$, we have-
\begin{align}
\frac{\partial H}{\partial\dot{q}_{i}} & =p_{i}-\frac{\partial L}{\partial\dot{q}_{i}}\nonumber \\
& =p_{i}-p_{i}\quad[\text{using \eqref{eq:2-6}}]\nonumber \\
& =0\label{eq:2-8}
\end{align}
For a slight change of hamiltonian $\delta H$, we have
\begin{eqnarray}
\delta H & = & \delta(p_{i},\dot{q_{i}})-\delta L\nonumber \\
& = & \dot{q_{i}}\delta p_{i}+p_{i}\delta\dot{q_{i}}-\frac{\partial L}{\partial q_{i}}\delta q_{i}-\frac{\partial L}{\partial\dot{q_{i}}}\delta\dot{q_{i}}\nonumber \\
& = & \dot{q_{i}}\delta p_{i}-\frac{\partial L}{\partial q_{i}}\delta q_{i}+\left(p_{i}-\frac{\partial L}{\partial\dot{q_{i}}}\right)\delta\dot{q_{i}}\nonumber \\
& = & \dot{q_{i}}\delta p_{i}-\frac{\partial L}{\partial q_{i}}\delta q_{i}\label{eq:2-9}
\end{eqnarray}
From \eqref{eq:2-9} we can see that $\delta H$ does not depend on
the $\delta\dot{q}$'s.

##### Equations of motion

Equations of motion are equations that describe the behaviour of a
physical system in terms of its motion as a function of time. More
specifically, the equations of motion describe the behaviour of a
physical system as a set of mathematical functions in terms of dynamic
variables. Normally spatial coordinates and time are used, but others
are also possible, such as momentum components and time. The most
general choice are generalized coordinates which can be any convenient
variables characteristic of the physical system.If the dynamics of
a system is known, the equations are the solutions to the differential
equations describing the motion of the dynamics.
If the potential energy of a system is velocity-independent, then
the linear momentum components in rectangualr coordinates are gieven
by
\begin{equation}
p_{i}=\frac{\partial L}{\partial\dot{q}_{i}}\label{eq:2-10}
\end{equation}
By analogy we extend this result to the case in which the Lagrangian
is expressed in generalized coordinates and define the generalized
momenta according to
\begin{equation}
p_{i}=\frac{\partial L}{\partial\dot{q}_{i}}\label{eq:2-11}
\end{equation}
Now the Lagrange equation of motion are then expressed by-
\begin{equation}
\dot{p}_{i}=\frac{\partial L}{\partial q_{i}}\label{eq:2-12}
\end{equation}
And from \eqref{eq:2-7} we can define the Hamiltonian as-
\begin{equation}
H=\sum_{i}p_{i}\dot{q}_{i}-L\label{eq:2-13}
\end{equation}
Now the Lagrangian is considered to be a function of the generalized
coordinates, the generalized velocities, and possibly the time. The
dependence of $L$ on the time may arise either if the constraints
are time dependent or if the transformation equations connecting the
rectangualar and generalized coordinates explicitly contain the time.
We may solve \eqref{eq:2-11} for the generalized velocities and express
them as-
\begin{equation}
\dot{q}_{i}=\dot{q}_{i}(p_{j},q_{j},t)\label{eq:2-14}
\end{equation}
Thus in \eqref{eq:2-13} we may express the Hamiltonian as-
\begin{equation}
H(p_{i},q_{i},t)=\sum_{j}p_{j}\dot{q}_{j}-L(\dot{q}_{i},q_{i},t)\label{eq:2-15}
\end{equation}
This equation is written in a manner which stresses the fact that
the Hamiltonian is always considered as a function of the $(p_{i},q_{i},t)$.
Therefore the total differential of $H$ may be calculated by
\begin{equation}
dH=\sum_{k}\left(\frac{\partial H}{\partial q_{i}}dq_{i}+\frac{\partial H}{\partial p_{i}}dp_{i}\right)\label{eq:2-15-a}
\end{equation}
whereas the Lagrangian is a function of $(p_{i},q_{i},t)$ set.

##### Hamilton's equations of motion

From \eqref{eq:2-9} and \eqref{eq:2-15-a} if we identify the coefficients
of $\delta p_{i}$ and $\delta q_{i}$
\begin{align}
\dot{q}_{i} & =\frac{\partial H}{\partial p_{i}}\label{eq:2-16}\\
\dot{p}_{i} & =-\frac{\partial H}{\partial q_{i}}\label{eq:2-17}
\end{align}
where the dot denotes the ordinary derivative with respect to time
of generalized momenta $p_{i}=p_{i}(t)$ and the generalized coordinates
$q_{i}=q_{i}(t)$, where $i=1,2,...n$. Equation \eqref{eq:2-16}
and \eqref{eq:2-17} are Hamiton's equations of motion. Because of
their symmetrical appearance, they are also known as the canonical
equations of motion.

##### Poisson Bracket

In canonical coordinates on the phase space the poisson bracket of
two function $A(p_{i},q_{i},t)$ and $B(p_{i},q_{i},t)$ is defined
by
\begin{equation}
\{A,B\}_{PB}=\sum_{i=1}^{N}\left(\frac{\partial A}{\partial q_{i}}\frac{\partial B}{\partial p_{i}}-\frac{\partial A}{\partial p_{i}}\frac{\partial B}{\partial q_{i}}\right)\label{eq:2-18}
\end{equation}
Poisson brackets are antisymmetric and it also satisfies the Jacobi
identity. Possion brackets deform to quantum commutator in Hilbert
space.
The Hamilton's equations of motion have an equavalent expression in
terms of the Poisson bracket. This may be most directly demostrated
in an explicit coordinate frame. Suppose $A(p_{i},q_{i},t)$ is a
function on the mainfold. Then we have-
\begin{align}
\frac{d}{dt}A(p_{i},q_{i},t) & =\left(\frac{\partial A}{\partial q_{i}}\frac{dq_{i}}{dt}+\frac{\partial A}{\partial p_{i}}\frac{dp_{i}}{dt}\right)\nonumber \\
& =\left(\frac{\partial A}{\partial q_{i}}\dot{q}_{i}+\frac{\partial A}{\partial p_{i}}\dot{p}_{i}\right)\nonumber \\
& =\left(\frac{\partial A}{\partial q_{i}}\frac{\partial H}{\partial p_{i}}+\frac{\partial A}{\partial p_{i}}\frac{\partial H}{\partial q_{i}}\right)\nonumber \\
& =\{A,H\}\label{eq:2-19}
\end{align}
So in general \eqref{eq:2-19} implies
\begin{equation}
\frac{dA}{dt}=\dot{A}=\{A,H\}\label{eq:2-20}
\end{equation}
Further, by taking $p=p(t)$ and $q=q(t)$ to be solutions to Hamilton's
equations
\begin{align}
\dot{q}_{i} & =\frac{\partial H}{\partial p_{i}}=\{q_{i},H\}\label{eq:2-21}\\
\dot{p}_{i} & =-\frac{\partial H}{\partial q_{i}}=\{p_{i},H\}\label{eq:2-22}
\end{align}

##### Strong and Weak equation

We shall now have to distinguish between two kinds of equations. When
we apply the variation, equation \eqref{eq:2-5} remain valild to
the accuracy $\epsilon$. On the other hand equation \eqref{eq:2-6}
gets violated by a quantity of order $\epsilon$ under the variation.
The former kind of equation is called the strong equation. The latter
kind of equation is called weak equation. At this stage let us introduce
the weak equality sign '$\approx$' for constraints equations. And
for the strong equation we introduce the sign '$\equiv$'. We have
the following rules governing algebric work with weak and strong equations.
\begin{align}
\text{if }A & \equiv0\text{ then }\delta A=0;\label{eq:2-23}\\
\text{if }X & \approx0\text{ then }\delta X\neq0;\label{eq:2-24}
\end{align}
in general. From the relation $X\approx0$ emphasize that $X$ is
numerically restricted to be zero but does not identically vanish
throughout phase space. This means, in particular, that it has nonzero
Poisson brackets with the canonical variables. We can also deduce
that-
\begin{equation}
\delta X^{2}\approx2X\delta X\approx0\label{eq:2-25}
\end{equation}
On the other hand, the strong equation holds throughout phase space
and not just on the submanifold $X\approx0$ and can be written as-
\begin{equation}
X^{2}\equiv0\label{eq:2-26}
\end{equation}
Similarly from two weak equation $X_{1}\approx0$ and $X_{2}\approx0$
we can deduce the strong equation
\begin{equation}
X_{1}X_{2}\equiv0\label{eq:2-27}
\end{equation}
It may be that the $N$ quantities $\dfrac{\partial L}{\partial\dot{q}_{i}}$
on the right-hand side of \eqref{eq:2-6} are all independent functions
of the $N$ velocities $\dot{q}_{i}$. In this case equations \eqref{eq:2-6}
determinde each $\dot{q}$ as a function fo the $q$'s and $p$'s.
This case will be referred to as the standard case, and is the only
one usually considered in dynamical theory. If the $\dfrac{\partial L}{\partial\dot{q}}$
are not independent functions of the velocities, we can eliminate
the $\dot{q}$'s from the equations \eqref{eq:2-6} and obtain one
or more equations.
\begin{equation}
\phi(q,p)\approx0\label{eq:2-28}
\end{equation}
involving only $q$'s and $p$'s . We may suppose equation \eqref{eq:2-28}
to be written in such a way that the variation procedure changes $\phi$
by a quantity of order $\epsilon$,since if it changes $\phi$ by
a quantity of order $\epsilon^{k}$ ,we have only to replace $\phi$
by $\phi^{\frac{1}{k}}$in \eqref{eq:2-28} and the desired condition
will be fulfilled . We now have equation \eqref{eq:2-28} violated
by the order $\epsilon$ when we apply the variation, so it is correctly
written as weak equation. We shall need to use a complete set of independent
equations of the type \eqref{eq:2-28} say
\begin{equation}
\phi_{i}(q,p)\approx0\label{eq:2-29}
\end{equation}
where $i=1,2,\ldots,n$. The condition of the independence means that
none of the $\phi$'s is expressible linearly in terms of the others,
with functions of the $q$'s and $p$'s as coefficients. The condition
of completeness means that any function fot he $q$'s and $p$'s which
vnishes on account of equation \eqref{eq:2-6} and changes by the
order $\epsilon$ with the variation procedure is expressible as a
linear function of the $\phi_{i}$ with functions of the $q$'s and
$p$'s as coefficients.