In analytical mechanics, the term generalized coordinates refers to the parameters that describe the configuration of the system relative to some reference configuration. These parameters must uniquely define the configuration of the system relative to the reference configuration.,

p. 397, §7.2.1 Selection of generalized coordinates

/ref> This is done assuming that this can be done with a single chart. The generalized velocities are the time derivatives of the generalized coordinates of the system. An example of a generalized coordinate is the angle that locates a point moving on a circle. The adjective "generalized" distinguishes these parameters from the traditional use of the term coordinate to refer to Cartesian coordinates: for example, describing the location of the point on the circle using x and y coordinates. Although there may be many choices for generalized coordinates for a physical system, parameters that are convenient are usually selected for the specification of the configuration of the system and which make the solution of its equations of motion easier. If these parameters are independent of one another, the number of independent generalized coordinates is defined by the number of degrees of freedom of the system. Generalized coordinates are paired with generalized momenta to provide canonical coordinates on phase space.

Constraints and degrees of freedom

Generalized coordinates are usually selected to provide the minimum number of independent coordinates that define the configuration of a system, which simplifies the formulation of Lagrange's equations of motion. However, it can also occur that a useful set of generalized coordinates may be ''dependent'', which means that they are related by one or more constraint equations.

Holonomic constraints

For a system of ''N'' particles in 3D real coordinate space, the position vector of each particle can be written as a 3-tuple in Cartesian coordinates: :$\backslash mathbf\_1\; =\; (x\_1,y\_1,z\_1)\; \backslash ,,\; \backslash quad\; \backslash mathbf\_2\; =\; (x\_2,y\_2,z\_2)\; \backslash ,,\; \backslash ldots\; \backslash ,,\; \backslash mathbf\_N\; =\; (x\_N,y\_N,z\_N)\backslash ,.$ Any of the position vectors can be denoted r_{''k''} where ''k'' = 1, 2, ..., ''N'' labels the particles. A ''holonomic constraint'' is a ''constraint equation'' of the form for particle ''k''Some authors set the constraint equations to a constant for convenience with some constraint equations (e.g. pendulums), others set it to zero. It makes no difference because the constant can be subtracted to give zero on one side of the equation. Also, in Lagrange's equations of the first kind, only the derivatives are needed.
:$f(\backslash mathbf\_k,\; t)\; =\; 0$
which connects all the 3 spatial coordinates of that particle together, so they are not independent. The constraint may change with time, so time ''t'' will appear explicitly in the constraint equations. At any instant of time, any one coordinate will be determined from the other coordinates, e.g. if ''x_{k}'' and ''z_{k}'' are given, then so is ''y_{k}''. One constraint equation counts as ''one'' constraint. If there are ''C'' constraints, each has an equation, so there will be ''C'' constraint equations. There is not necessarily one constraint equation for each particle, and if there are no constraints on the system then there are no constraint equations.
So far, the configuration of the system is defined by 3''N'' quantities, but ''C'' coordinates can be eliminated, one coordinate from each constraint equation. The number of independent coordinates is ''n'' = 3''N'' − ''C''. (In ''D'' dimensions, the original configuration would need ''ND'' coordinates, and the reduction by constraints means ''n'' = ''ND'' − ''C''). It is ideal to use the minimum number of coordinates needed to define the configuration of the entire system, while taking advantage of the constraints on the system. These quantities are known as generalized coordinates in this context, denoted ''q_{j}''(''t''). It is convenient to collect them into an ''n''-tuple
:$\backslash mathbf(t)\; =\; (q\_1(t),\; q\_2(t),\; \backslash ldots,\; q\_n(t))$
which is a point in the ''configuration space'' of the system. They are all independent of one other, and each is a function of time. Geometrically they can be lengths along straight lines, or arc lengths along curves, or angles; not necessarily Cartesian coordinates or other standard orthogonal coordinates. There is one for each degree of freedom, so the number of generalized coordinates equals the number of degrees of freedom, ''n''. A degree of freedom corresponds to one quantity that changes the configuration of the system, for example the angle of a pendulum, or the arc length traversed by a bead along a wire.
If it is possible to find from the constraints as many independent variables as there are degrees of freedom, these can be used as generalized coordinates. The position vector r_{''k''} of particle ''k'' is a function of all the ''n'' generalized coordinates (and, through them, of time),Some authors e.g. Hand & Finch take the form of the position vector for particle ''k'', as shown here, as the condition for the constraint on that particle to be holonomic.
:$\backslash mathbf\_k\; =\; \backslash mathbf\_k(\backslash mathbf(t))\; \backslash ,,$
and the generalized coordinates can be thought of as parameters associated with the constraint.
The corresponding time derivatives of q are the generalized velocities,
:$\backslash dot\; =\; \backslash frac\; =\; (\backslash dot\_1(t),\; \backslash dot\_2(t),\; \backslash ldots,\; \backslash dot\_n(t))$
(each dot over a quantity indicates one time derivative). The velocity vector v_{''k''} is the total derivative of r_{''k''} with respect to time
:$\backslash mathbf\_k\; =\; \backslash dot\_k\; =\; \backslash frac\; =\; \backslash sum\_^n\; \backslash frac\backslash dot\_j\; \backslash ,.$
and so generally depends on the generalized velocities and coordinates. Since we are free to specify the initial values of the generalized coordinates and velocities separately, the generalized coordinates ''q_{j}'' and velocities ''dq_{j}''/''dt'' can be treated as ''independent variables''.

Non-holonomic constraints

A mechanical system can involve constraints on both the generalized coordinates and their derivatives. Constraints of this type are known as non-holonomic. First-order non-holonomic constraints have the form :$g(\backslash mathbf,\; \backslash dot,\; t)\; =\; 0\backslash ,,$ An example of such a constraint is a rolling wheel or knife-edge that constrains the direction of the velocity vector. Non-holonomic constraints can also involve next-order derivatives such as generalized accelerations.

Physical quantities in generalized coordinates

Kinetic energy

The total kinetic energy of the system is the energy of the system's motion, defined as :$T\; =\; \backslash frac\; \backslash sum\_^N\; m\_k\; \backslash dot\_k\; \backslash cdot\; \backslash dot\_k\backslash ,,$ in which · is the dot product. The kinetic energy is a function only of the velocities v_{''k''}, not the coordinates r_{''k''} themselves. By contrast an important observation is
:$\backslash dot\_k\; \backslash cdot\; \backslash dot\_k\; =\; \backslash sum\_^n\; \backslash left(\backslash frac\backslash cdot\backslash frac\backslash right)\backslash dot\_i\backslash dot\_j\; ,$
which illustrates the kinetic energy is in general a function of the generalized velocities, coordinates, and time if the constraints also vary with time, so ''T'' = ''T''(q, ''d''q/''dt'', ''t'').
In the case the constraints on the particles are time-independent, then all partial derivatives with respect to time are zero, and the kinetic energy is a homogeneous function of degree 2 in the generalized velocities.
Still for the time-independent case, this expression is equivalent to taking the line element squared of the trajectory for particle ''k'',
:$ds\_k^2\; =\; d\backslash mathbf\_k\backslash cdot\; d\backslash mathbf\_k\; =\; \backslash sum\_^n\; \backslash left(\backslash frac\backslash cdot\backslash frac\backslash right)\; dq\_i\; dq\_j\; \backslash ,,$
and dividing by the square differential in time, ''dt''^{2}, to obtain the velocity squared of particle ''k''. Thus for time-independent constraints it is sufficient to know the line element to quickly obtain the kinetic energy of particles and hence the Lagrangian.
It is instructive to see the various cases of polar coordinates in 2d and 3d, owing to their frequent appearance. In 2d polar coordinates (''r'', ''θ''),
:$\backslash left(\backslash frac\backslash right)^2\; =\; \backslash dot^2\; +\; r^2\backslash dot^2\; \backslash ,,$
in 3d cylindrical coordinates (''r'', ''θ'', ''z''),
:$\backslash left(\backslash frac\backslash right)^2\; =\; \backslash dot^2\; +\; r^2\backslash dot^2\; +\; \backslash dot^2\; \backslash ,,$
in 3d spherical coordinates (''r'', ''θ'', ''φ''),
:$\backslash left(\backslash frac\backslash right)^2\; =\; \backslash dot^2+r^2\backslash dot^2\; +r^2\backslash sin^2\backslash theta\; \backslash ,\; \backslash dot^2\; \backslash ,.$

Generalized momentum

The ''generalized momentum'' "canonically conjugate to" the coordinate ''q_{i}'' is defined by
:$p\_i\; =\backslash frac.$
If the Lagrangian ''L'' does ''not'' depend on some coordinate ''q_{i}'', then it follows from the Euler–Lagrange equations that the corresponding generalized momentum will be a conserved quantity, because the time derivative is zero implying the momentum is a constant of the motion;
:$\backslash dot\_i\; =\; \backslash frac\backslash frac\; =\; \backslash frac=0\backslash ,.$

Examples

Bead on a wire

For a bead sliding on a frictionless wire subject only to gravity in 2d space, the constraint on the bead can be stated in the form ''f''(r) = 0, where the position of the bead can be written r = (''x''(''s''), ''y''(''s'')), in which ''s'' is a parameter, the arc length ''s'' along the curve from some point on the wire. This is a suitable choice of generalized coordinate for the system. Only ''one'' coordinate is needed instead of two, because the position of the bead can be parameterized by one number, ''s'', and the constraint equation connects the two coordinates ''x'' and ''y''; either one is determined from the other. The constraint force is the reaction force the wire exerts on the bead to keep it on the wire, and the non-constraint applied force is gravity acting on the bead. Suppose the wire changes its shape with time, by flexing. Then the constraint equation and position of the particle are respectively :$f(\backslash mathbf,\; t)\; =\; 0\; \backslash ,,\backslash quad\; \backslash mathbf\; =\; (x(s,\; t),\; y(s,\; t))$ which now both depend on time ''t'' due to the changing coordinates as the wire changes its shape. Notice time appears implicitly via the coordinates ''and'' explicitly in the constraint equations.

Simple pendulum

thumb|Dynamic model of a simple pendulum. The relationship between the use of generalized coordinates and Cartesian coordinates to characterize the movement of a mechanical system can be illustrated by considering the constrained dynamics of a simple pendulum. A simple pendulum consists of a mass M hanging from a pivot point so that it is constrained to move on a circle of radius L. The position of the mass is defined by the coordinate vector r=(x, y) measured in the plane of the circle such that y is in the vertical direction. The coordinates x and y are related by the equation of the circle :$f(x,\; y)\; =\; x^2+y^2\; -\; L^2=0,$ that constrains the movement of M. This equation also provides a constraint on the velocity components, :$\backslash dot(x,\; y)=2x\backslash dot\; +\; 2y\backslash dot\; =\; 0.$ Now introduce the parameter θ, that defines the angular position of M from the vertical direction. It can be used to define the coordinates x and y, such that :$\backslash mathbf=(x,\; y)\; =\; (L\backslash sin\backslash theta,\; -L\backslash cos\backslash theta).$ The use of θ to define the configuration of this system avoids the constraint provided by the equation of the circle. Notice that the force of gravity acting on the mass m is formulated in the usual Cartesian coordinates, :$\backslash mathbf=(0,-mg),$ where g is the acceleration of gravity. The virtual work of gravity on the mass m as it follows the trajectory r is given by :$\backslash delta\; W\; =\; \backslash mathbf\backslash cdot\backslash delta\; \backslash mathbf.$ The variation $\backslash delta$r can be computed in terms of the coordinates x and y, or in terms of the parameter θ, :$\backslash delta\; \backslash mathbf\; =(\backslash delta\; x,\; \backslash delta\; y)\; =\; (L\backslash cos\backslash theta,\; L\backslash sin\backslash theta)\backslash delta\backslash theta.$ Thus, the virtual work is given by :$\backslash delta\; W\; =\; -mg\backslash delta\; y\; =\; -mgL\backslash sin\backslash theta\backslash delta\backslash theta.$ Notice that the coefficient of $\backslash delta$y is the y-component of the applied force. In the same way, the coefficient of $\backslash delta$θ is known as the generalized force along generalized coordinate θ, given by :$F\_\; =\; -mgL\backslash sin\backslash theta.$ To complete the analysis consider the kinetic energy T of the mass, using the velocity, :$\backslash mathbf=(\backslash dot,\; \backslash dot)\; =\; (L\backslash cos\backslash theta,\; L\backslash sin\backslash theta)\backslash dot,$ so, :$T=\; \backslash frac\; m\backslash mathbf\backslash cdot\backslash mathbf\; =\; \backslash frac\; m\; (\backslash dot^2+\backslash dot^2)\; =\; \backslash frac\; m\; L^2\backslash dot^2.$ D'Alembert's form of the principle of virtual work for the pendulum in terms of the coordinates x and y are given by, :$\backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_\; +\; \backslash lambda\; \backslash frac,\backslash quad\; \backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_\; +\; \backslash lambda\; \backslash frac.$ This yields the three equations :$m\backslash ddot\; =\; \backslash lambda(2x),\backslash quad\; m\backslash ddot\; =\; -mg\; +\; \backslash lambda(2y),\backslash quad\; x^2+y^2\; -\; L^2=0,$ in the three unknowns, x, y and λ. Using the parameter θ, those equations take the form :$\backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_,$ which becomes, :$mL^2\backslash ddot\; =\; -mgL\backslash sin\backslash theta,$ or :$\backslash ddot\; +\; \backslash frac\backslash sin\backslash theta=0.$ This formulation yields one equation because there is a single parameter and no constraint equation. This shows that the parameter θ is a generalized coordinate that can be used in the same way as the Cartesian coordinates x and y to analyze the pendulum.

Double pendulum

The benefits of generalized coordinates become apparent with the analysis of a double pendulum. For the two masses m_{i}, i=1, 2, let r_{i}=(x_{i}, y_{i}), i=1, 2 define their two trajectories. These vectors satisfy the two constraint equations,
:$f\_1\; (x\_1,\; y\_1,\; x\_2,\; y\_2)\; =\; \backslash mathbf\_1\backslash cdot\; \backslash mathbf\_1\; -\; L\_1^2\; =\; 0$
and
:$f\_2\; (x\_1,\; y\_1,\; x\_2,\; y\_2)\; =\; (\backslash mathbf\_2-\backslash mathbf\_1)\; \backslash cdot\; (\backslash mathbf\_2-\backslash mathbf\_1)\; -\; L\_2^2\; =\; 0.$
The formulation of Lagrange's equations for this system yields six equations in the four Cartesian coordinates x_{i}, y_{i} i=1, 2 and the two Lagrange multipliers λ_{i}, i=1, 2 that arise from the two constraint equations.
Now introduce the generalized coordinates θ_{i} i=1,2 that define the angular position of each mass of the double pendulum from the vertical direction. In this case, we have
:$\backslash mathbf\_1\; =\; (L\_1\backslash sin\backslash theta\_1,\; -L\_1\backslash cos\backslash theta\_1),\; \backslash quad\; \backslash mathbf\_2\; =\; (L\_1\backslash sin\backslash theta\_1,\; -L\_1\backslash cos\backslash theta\_1)\; +\; (L\_2\backslash sin\backslash theta\_2,\; -L\_2\backslash cos\backslash theta\_2).$
The force of gravity acting on the masses is given by,
:$\backslash mathbf\_1=(0,-m\_1\; g),\backslash quad\; \backslash mathbf\_2=(0,-m\_2\; g)$
where g is the acceleration of gravity. Therefore, the virtual work of gravity on the two masses as they follow the trajectories r_{i}, i=1,2 is given by
:$\backslash delta\; W\; =\; \backslash mathbf\_1\backslash cdot\backslash delta\; \backslash mathbf\_1\; +\; \backslash mathbf\_2\backslash cdot\backslash delta\; \backslash mathbf\_2.$
The variations δr_{i} i=1, 2 can be computed to be
:$\backslash delta\; \backslash mathbf\_1\; =\; (L\_1\backslash cos\backslash theta\_1,\; L\_1\backslash sin\backslash theta\_1)\backslash delta\backslash theta\_1,\; \backslash quad\; \backslash delta\; \backslash mathbf\_2\; =\; (L\_1\backslash cos\backslash theta\_1,\; L\_1\backslash sin\backslash theta\_1)\backslash delta\backslash theta\_1\; +(L\_2\backslash cos\backslash theta\_2,\; L\_2\backslash sin\backslash theta\_2)\backslash delta\backslash theta\_2$
Thus, the virtual work is given by
:$\backslash delta\; W\; =\; -(m\_1+m\_2)gL\_1\backslash sin\backslash theta\_1\backslash delta\backslash theta\_1\; -\; m\_2gL\_2\backslash sin\backslash theta\_2\backslash delta\backslash theta\_2,$
and the generalized forces are
:$F\_\; =\; -(m\_1+m\_2)gL\_1\backslash sin\backslash theta\_1,\backslash quad\; F\_\; =\; -m\_2gL\_2\backslash sin\backslash theta\_2.$
Compute the kinetic energy of this system to be
:$T=\; \backslash fracm\_1\; \backslash mathbf\_1\backslash cdot\backslash mathbf\_1\; +\; \backslash fracm\_2\; \backslash mathbf\_2\backslash cdot\backslash mathbf\_2\; =\; \backslash frac(m\_1+m\_2)L\_1^2\backslash dot\_1^2\; +\; \backslash fracm\_2L\_2^2\backslash dot\_2^2\; +\; m\_2L\_1L\_2\; \backslash cos(\backslash theta\_2-\backslash theta\_1)\backslash dot\_1\backslash dot\_2.$
Euler–Lagrange equation yield two equations in the unknown generalized coordinates θ_{i} i=1, 2, given by
:$(m\_1+m\_2)L\_1^2\backslash ddot\_1+m\_2L\_1L\_2\backslash ddot\_2\backslash cos(\backslash theta\_2-\backslash theta\_1)\; +\; m\_2L\_1L\_2\backslash ddot^2\backslash sin(\backslash theta\_1-\backslash theta\_2)\; =\; -(m\_1+m\_2)gL\_1\backslash sin\backslash theta\_1,$
and
:$m\_2L\_2^2\backslash ddot\_2+m\_2L\_1L\_2\backslash ddot\_1\backslash cos(\backslash theta\_2-\backslash theta\_1)\; +\; m\_2L\_1L\_2\backslash ddot^2\backslash sin(\backslash theta\_2-\backslash theta\_1)=-m\_2gL\_2\backslash sin\backslash theta\_2.$
The use of the generalized coordinates θ_{i} i=1, 2 provides an alternative to the Cartesian formulation of the dynamics of the double pendulum.

Spherical pendulum

For a 3d example, a spherical pendulum with constant length ''l'' free to swing in any angular direction subject to gravity, the constraint on the pendulum bob can be stated in the form :$f(\backslash mathbf)\; =\; x^2\; +\; y^2\; +\; z^2\; -\; l^2\; =\; 0\; \backslash ,,$ where the position of the pendulum bob can be written :$\backslash mathbf\; =\; (x(\backslash theta,\backslash phi),y(\backslash theta,\backslash phi),z(\backslash theta,\backslash phi))\; \backslash ,,$ in which (''θ'', ''φ'') are the spherical polar angles because the bob moves in the surface of a sphere. The position r is measured along the suspension point to the bob, here treated as a point particle. A logical choice of generalized coordinates to describe the motion are the angles (''θ'', ''φ''). Only two coordinates are needed instead of three, because the position of the bob can be parameterized by two numbers, and the constraint equation connects the three coordinates ''x'', ''y'', ''z'' so any one of them is determined from the other two.

Generalized coordinates and virtual work

The ''principle of virtual work'' states that if a system is in static equilibrium, the virtual work of the applied forces is zero for all virtual movements of the system from this state, that is, $\backslash delta$W=0 for any variation $\backslash delta$r. When formulated in terms of generalized coordinates, this is equivalent to the requirement that the generalized forces for any virtual displacement are zero, that is ''F''_{i}=0.
Let the forces on the system be F_{j}, ''j=1, ..., m'' be applied to points with Cartesian coordinates r_{j}, j=1,..., m, then the virtual work generated by a virtual displacement from the equilibrium position is given by
:$\backslash delta\; W\; =\; \backslash sum\_^m\; \backslash mathbf\_j\backslash cdot\; \backslash delta\backslash mathbf\_j.$
where δr_{j}, ''j=1, ..., m'' denote the virtual displacements of each point in the body.
Now assume that each δr_{j} depends on the generalized coordinates ''q''_{i}, ''i=1, ..., n'', then
:$\backslash delta\; \backslash mathbf\_j\; =\; \backslash frac\; \backslash delta\_1\; +\; \backslash ldots\; +\; \backslash frac\; \backslash delta\_n,$
and
:$\backslash delta\; W\; =\; \backslash left(\backslash sum\_^m\; \backslash mathbf\_j\backslash cdot\; \backslash frac\backslash right)\; \backslash delta\_1\; +\; \backslash ldots\; +\; \backslash left(\backslash sum\_^m\; \backslash mathbf\_j\backslash cdot\; \backslash frac\backslash right)\; \backslash delta\_n.$
The ''n'' terms
:$F\_i\; =\; \backslash sum\_^m\; \backslash mathbf\_j\backslash cdot\; \backslash frac,\backslash quad\; i=1,\backslash ldots,\; n,$
are the generalized forces acting on the system. KaneT. R. Kane and D. A. Levinson, Dynamics: theory and applications, McGraw-Hill, New York, 1985 shows that these generalized forces can also be formulated in terms of the ratio of time derivatives,
:$F\_i\; =\; \backslash sum\_^m\; \backslash mathbf\_j\backslash cdot\; \backslash frac,\backslash quad\; i=1,\backslash ldots,\; n,$
where v_{j} is the velocity of the point of application of the force F_{j}.
In order for the virtual work to be zero for an arbitrary virtual displacement, each of the generalized forces must be zero, that is
:$\backslash delta\; W\; =\; 0\; \backslash quad\; \backslash Rightarrow\; \backslash quad\; F\_i\; =0,\; i=1,\backslash ldots,\; n.$

See also

*Canonical coordinates *Hamiltonian mechanics *Virtual work *Orthogonal coordinates *Curvilinear coordinates *Mass matrix *Stiffness matrix *Generalized forces

Notes

References

Bibliography of cited references

* *{{cite book |last1=Kibble |first1=T.W.B |author1-link=Tom Kibble |last2=Berkshire |first2=F.H. |year=2004 |title=Classical Mechanics |edition=5th |publisher=Imperial College Press |location=River Edge NJ |isbn=1860944248 Category:Dynamical systems Category:Rigid bodies

p. 397, §7.2.1 Selection of generalized coordinates

/ref> This is done assuming that this can be done with a single chart. The generalized velocities are the time derivatives of the generalized coordinates of the system. An example of a generalized coordinate is the angle that locates a point moving on a circle. The adjective "generalized" distinguishes these parameters from the traditional use of the term coordinate to refer to Cartesian coordinates: for example, describing the location of the point on the circle using x and y coordinates. Although there may be many choices for generalized coordinates for a physical system, parameters that are convenient are usually selected for the specification of the configuration of the system and which make the solution of its equations of motion easier. If these parameters are independent of one another, the number of independent generalized coordinates is defined by the number of degrees of freedom of the system. Generalized coordinates are paired with generalized momenta to provide canonical coordinates on phase space.

Constraints and degrees of freedom

Generalized coordinates are usually selected to provide the minimum number of independent coordinates that define the configuration of a system, which simplifies the formulation of Lagrange's equations of motion. However, it can also occur that a useful set of generalized coordinates may be ''dependent'', which means that they are related by one or more constraint equations.

Holonomic constraints

For a system of ''N'' particles in 3D real coordinate space, the position vector of each particle can be written as a 3-tuple in Cartesian coordinates: :$\backslash mathbf\_1\; =\; (x\_1,y\_1,z\_1)\; \backslash ,,\; \backslash quad\; \backslash mathbf\_2\; =\; (x\_2,y\_2,z\_2)\; \backslash ,,\; \backslash ldots\; \backslash ,,\; \backslash mathbf\_N\; =\; (x\_N,y\_N,z\_N)\backslash ,.$ Any of the position vectors can be denoted r

Non-holonomic constraints

A mechanical system can involve constraints on both the generalized coordinates and their derivatives. Constraints of this type are known as non-holonomic. First-order non-holonomic constraints have the form :$g(\backslash mathbf,\; \backslash dot,\; t)\; =\; 0\backslash ,,$ An example of such a constraint is a rolling wheel or knife-edge that constrains the direction of the velocity vector. Non-holonomic constraints can also involve next-order derivatives such as generalized accelerations.

Physical quantities in generalized coordinates

Kinetic energy

The total kinetic energy of the system is the energy of the system's motion, defined as :$T\; =\; \backslash frac\; \backslash sum\_^N\; m\_k\; \backslash dot\_k\; \backslash cdot\; \backslash dot\_k\backslash ,,$ in which · is the dot product. The kinetic energy is a function only of the velocities v

Generalized momentum

The ''generalized momentum'' "canonically conjugate to" the coordinate ''q

Examples

Bead on a wire

For a bead sliding on a frictionless wire subject only to gravity in 2d space, the constraint on the bead can be stated in the form ''f''(r) = 0, where the position of the bead can be written r = (''x''(''s''), ''y''(''s'')), in which ''s'' is a parameter, the arc length ''s'' along the curve from some point on the wire. This is a suitable choice of generalized coordinate for the system. Only ''one'' coordinate is needed instead of two, because the position of the bead can be parameterized by one number, ''s'', and the constraint equation connects the two coordinates ''x'' and ''y''; either one is determined from the other. The constraint force is the reaction force the wire exerts on the bead to keep it on the wire, and the non-constraint applied force is gravity acting on the bead. Suppose the wire changes its shape with time, by flexing. Then the constraint equation and position of the particle are respectively :$f(\backslash mathbf,\; t)\; =\; 0\; \backslash ,,\backslash quad\; \backslash mathbf\; =\; (x(s,\; t),\; y(s,\; t))$ which now both depend on time ''t'' due to the changing coordinates as the wire changes its shape. Notice time appears implicitly via the coordinates ''and'' explicitly in the constraint equations.

Simple pendulum

thumb|Dynamic model of a simple pendulum. The relationship between the use of generalized coordinates and Cartesian coordinates to characterize the movement of a mechanical system can be illustrated by considering the constrained dynamics of a simple pendulum. A simple pendulum consists of a mass M hanging from a pivot point so that it is constrained to move on a circle of radius L. The position of the mass is defined by the coordinate vector r=(x, y) measured in the plane of the circle such that y is in the vertical direction. The coordinates x and y are related by the equation of the circle :$f(x,\; y)\; =\; x^2+y^2\; -\; L^2=0,$ that constrains the movement of M. This equation also provides a constraint on the velocity components, :$\backslash dot(x,\; y)=2x\backslash dot\; +\; 2y\backslash dot\; =\; 0.$ Now introduce the parameter θ, that defines the angular position of M from the vertical direction. It can be used to define the coordinates x and y, such that :$\backslash mathbf=(x,\; y)\; =\; (L\backslash sin\backslash theta,\; -L\backslash cos\backslash theta).$ The use of θ to define the configuration of this system avoids the constraint provided by the equation of the circle. Notice that the force of gravity acting on the mass m is formulated in the usual Cartesian coordinates, :$\backslash mathbf=(0,-mg),$ where g is the acceleration of gravity. The virtual work of gravity on the mass m as it follows the trajectory r is given by :$\backslash delta\; W\; =\; \backslash mathbf\backslash cdot\backslash delta\; \backslash mathbf.$ The variation $\backslash delta$r can be computed in terms of the coordinates x and y, or in terms of the parameter θ, :$\backslash delta\; \backslash mathbf\; =(\backslash delta\; x,\; \backslash delta\; y)\; =\; (L\backslash cos\backslash theta,\; L\backslash sin\backslash theta)\backslash delta\backslash theta.$ Thus, the virtual work is given by :$\backslash delta\; W\; =\; -mg\backslash delta\; y\; =\; -mgL\backslash sin\backslash theta\backslash delta\backslash theta.$ Notice that the coefficient of $\backslash delta$y is the y-component of the applied force. In the same way, the coefficient of $\backslash delta$θ is known as the generalized force along generalized coordinate θ, given by :$F\_\; =\; -mgL\backslash sin\backslash theta.$ To complete the analysis consider the kinetic energy T of the mass, using the velocity, :$\backslash mathbf=(\backslash dot,\; \backslash dot)\; =\; (L\backslash cos\backslash theta,\; L\backslash sin\backslash theta)\backslash dot,$ so, :$T=\; \backslash frac\; m\backslash mathbf\backslash cdot\backslash mathbf\; =\; \backslash frac\; m\; (\backslash dot^2+\backslash dot^2)\; =\; \backslash frac\; m\; L^2\backslash dot^2.$ D'Alembert's form of the principle of virtual work for the pendulum in terms of the coordinates x and y are given by, :$\backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_\; +\; \backslash lambda\; \backslash frac,\backslash quad\; \backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_\; +\; \backslash lambda\; \backslash frac.$ This yields the three equations :$m\backslash ddot\; =\; \backslash lambda(2x),\backslash quad\; m\backslash ddot\; =\; -mg\; +\; \backslash lambda(2y),\backslash quad\; x^2+y^2\; -\; L^2=0,$ in the three unknowns, x, y and λ. Using the parameter θ, those equations take the form :$\backslash frac\backslash frac\; -\; \backslash frac\; =\; F\_,$ which becomes, :$mL^2\backslash ddot\; =\; -mgL\backslash sin\backslash theta,$ or :$\backslash ddot\; +\; \backslash frac\backslash sin\backslash theta=0.$ This formulation yields one equation because there is a single parameter and no constraint equation. This shows that the parameter θ is a generalized coordinate that can be used in the same way as the Cartesian coordinates x and y to analyze the pendulum.

Double pendulum

The benefits of generalized coordinates become apparent with the analysis of a double pendulum. For the two masses m

Spherical pendulum

For a 3d example, a spherical pendulum with constant length ''l'' free to swing in any angular direction subject to gravity, the constraint on the pendulum bob can be stated in the form :$f(\backslash mathbf)\; =\; x^2\; +\; y^2\; +\; z^2\; -\; l^2\; =\; 0\; \backslash ,,$ where the position of the pendulum bob can be written :$\backslash mathbf\; =\; (x(\backslash theta,\backslash phi),y(\backslash theta,\backslash phi),z(\backslash theta,\backslash phi))\; \backslash ,,$ in which (''θ'', ''φ'') are the spherical polar angles because the bob moves in the surface of a sphere. The position r is measured along the suspension point to the bob, here treated as a point particle. A logical choice of generalized coordinates to describe the motion are the angles (''θ'', ''φ''). Only two coordinates are needed instead of three, because the position of the bob can be parameterized by two numbers, and the constraint equation connects the three coordinates ''x'', ''y'', ''z'' so any one of them is determined from the other two.

Generalized coordinates and virtual work

The ''principle of virtual work'' states that if a system is in static equilibrium, the virtual work of the applied forces is zero for all virtual movements of the system from this state, that is, $\backslash delta$W=0 for any variation $\backslash delta$r. When formulated in terms of generalized coordinates, this is equivalent to the requirement that the generalized forces for any virtual displacement are zero, that is ''F''

See also

*Canonical coordinates *Hamiltonian mechanics *Virtual work *Orthogonal coordinates *Curvilinear coordinates *Mass matrix *Stiffness matrix *Generalized forces

Notes

References

Bibliography of cited references

* *{{cite book |last1=Kibble |first1=T.W.B |author1-link=Tom Kibble |last2=Berkshire |first2=F.H. |year=2004 |title=Classical Mechanics |edition=5th |publisher=Imperial College Press |location=River Edge NJ |isbn=1860944248 Category:Dynamical systems Category:Rigid bodies