Generalized coordinates



In analytical mechanics, generalized coordinates are a set of parameters used to represent the state of a system in a configuration space. These parameters must uniquely define the configuration of the system relative to a reference state.,
p. 397,  §7.2.1 Selection of generalized coordinates
/ref> The generalized velocities are the time
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
s of the generalized coordinates of the system. The adjective "generalized" distinguishes these parameters from the traditional use of the term "coordinate" to refer to
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
An example of a generalized coordinate would be to describe the position of a pendulum using the angle of the pendulum relative to vertical, rather than by the x and y position of the pendulum. Although there may be many possible choices for generalized coordinates for a physical system, they are generally selected to simplify calculations, such as the solution of the
equations of motion In physics, equations of motion are equations that describe the behavior of a physical system in terms of its motion as a function of time.''Encyclopaedia of Physics'' (second Edition), R.G. Lerner, G.L. Trigg, VHC Publishers, 1991, ISBN (Verl ...
for the system. If the coordinates 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 In dynamical system theory, a phase space is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usuall ...

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 particles in 3D
real coordinate space In mathematics, the real coordinate space of dimension , denoted ( ) or is the set of the -tuples of real numbers, that is the set of all sequences of real numbers. With component-wise addition and scalar multiplication, it is a real vect ...
, the position vector of each particle can be written as a 3-
tuple In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
Cartesian coordinates A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
: :\begin & \mathbf_1 = (x_1,y_1,z_1), \\ & \mathbf_2 = (x_2,y_2,z_2), \\ & \qquad \qquad \vdots \\ & \mathbf_N = (x_N,y_N,z_N) \end Any of the position vectors can be denoted where labels the particles. A '' holonomic constraint'' is a ''constraint equation'' of the form for particle :f(\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 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 and are given, then so is . One constraint equation counts as ''one'' constraint. If there are constraints, each has an equation, so there will be 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 quantities, but coordinates can be eliminated, one coordinate from each constraint equation. The number of independent coordinates is . (In dimensions, the original configuration would need coordinates, and the reduction by constraints means ). 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 . It is convenient to collect them into an -
tuple In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
:\mathbf(t) = (q_1(t),\ q_2(t),\ \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
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s along curves, or angles; not necessarily Cartesian coordinates or other standard
orthogonal coordinates In mathematics, orthogonal coordinates are defined as a set of ''d'' coordinates q = (''q''1, ''q''2, ..., ''q'd'') in which the coordinate hypersurfaces all meet at right angles (note: superscripts are indices, not exponents). A coordinate su ...
. There is one for each degree of freedom, so the number of generalized coordinates equals the number of degrees of freedom, . 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 of particle is a function of all the generalized coordinates (and, through them, of time),Some authors e.g. Hand & Finch take the form of the position vector for particle , as shown here, as the condition for the constraint on that particle to be holonomic. :\mathbf_k = \mathbf_k(\mathbf(t)) \,, and the generalized coordinates can be thought of as parameters associated with the constraint. The corresponding time derivatives of are the generalized velocities, :\dot = \frac = (\dot_1(t),\ \dot_2(t),\ \ldots,\ \dot_n(t)) (each dot over a quantity indicates one
time derivative A time derivative is a derivative of a function with respect to time, usually interpreted as the rate of change of the value of the function. The variable denoting time is usually written as t. Notation A variety of notations are used to denote t ...
). The velocity vector is the
total derivative In mathematics, the total derivative of a function at a point is the best linear approximation near this point of the function with respect to its arguments. Unlike partial derivatives, the total derivative approximates the function with res ...
of with respect to time :\mathbf_k = \dot_k = \frac = \sum_^n \frac\dot_j \,. 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 and velocities 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(\mathbf, \dot, t) = 0\,, 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 In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accel ...
of the system is the energy of the system's motion, defined as :T = \frac \sum_^N m_k \dot_k \cdot \dot_k\,, in which · is the
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an algeb ...
. The kinetic energy is a function only of the velocities , not the coordinates themselves. By contrast an important observation is :\dot_k \cdot \dot_k = \sum_^n \left(\frac\cdot\frac\right)\dot_i\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 . 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 In mathematics, a homogeneous function is a function of several variables such that, if all its arguments are multiplied by a scalar, then its value is multiplied by some power of this scalar, called the degree of homogeneity, or simply the ''d ...
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 , :ds_k^2 = d\mathbf_k\cdot d\mathbf_k = \sum_^n \left(\frac\cdot\frac\right) dq_i dq_j \,, and dividing by the square differential in time, , to obtain the velocity squared of particle . 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 In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to the o ...
, :\left(\frac\right)^2 = \dot^2 + r^2\dot^2 \,, in 3D cylindrical coordinates , :\left(\frac\right)^2 = \dot^2 + r^2\dot^2 + \dot^2 \,, in 3D spherical coordinates , :\left(\frac\right)^2 = \dot^2+r^2\dot^2 +r^2\sin^2\theta \, \dot^2 \,.

Generalized momentum

The ''generalized momentum'' " canonically conjugate to" the coordinate is defined by :p_i =\frac. If the Lagrangian does ''not'' depend on some coordinate , then it follows from the Euler–Lagrange equations that the corresponding generalized momentum will be a
conserved quantity In mathematics, a conserved quantity of a dynamical system is a function of the dependent variables, the value of which remains constant along each trajectory of the system. Not all systems have conserved quantities, and conserved quantities are ...
, because the time derivative is zero implying the momentum is a constant of the motion; :\dot_i = \frac\frac = \frac=0\,.


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 , where the position of the bead can be written , in which is a parameter, the
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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, , and the constraint equation connects the two coordinates and ; 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(\mathbf, t) = 0 \,,\quad \mathbf = (x(s, t), y(s, t)) which now both depend on time 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

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 A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward th ...
consists of a mass hanging from a pivot point so that it is constrained to move on a circle of radius . The position of the mass is defined by the coordinate vector measured in the plane of the circle such that is in the vertical direction. The coordinates and are related by the equation of the circle :f(x, y) = x^2+y^2 - L^2=0, that constrains the movement of . This equation also provides a constraint on the velocity components, : \dot(x, y)=2x\dot + 2y\dot = 0. Now introduce the parameter , that defines the angular position of from the vertical direction. It can be used to define the coordinates and , such that : \mathbf=(x, y) = (L\sin\theta, -L\cos\theta). The use of 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 is formulated in the usual Cartesian coordinates, : \mathbf=(0,-mg), where is the acceleration due to gravity. The virtual work of gravity on the mass as it follows the trajectory is given by : \delta W = \mathbf\cdot\delta \mathbf. The variation can be computed in terms of the coordinates and , or in terms of the parameter , : \delta \mathbf =(\delta x, \delta y) = (L\cos\theta, L\sin\theta)\delta\theta. Thus, the virtual work is given by :\delta W = -mg\delta y = -mgL\sin(\theta)\delta\theta. Notice that the coefficient of is the -component of the applied force. In the same way, the coefficient of is known as the generalized force along generalized coordinate , given by : F_ = -mgL\sin\theta. To complete the analysis consider the kinetic energy of the mass, using the velocity, : \mathbf=(\dot, \dot) = (L\cos\theta, L\sin\theta)\dot, so, : T= \frac m\mathbf\cdot\mathbf = \frac m (\dot^2+\dot^2) = \frac m L^2\dot^2. D'Alembert's form of the principle of virtual work for the pendulum in terms of the coordinates and are given by, : \frac\frac - \frac = F_ + \lambda \frac,\quad \frac\frac - \frac = F_ + \lambda \frac. This yields the three equations :m\ddot = \lambda(2x),\quad m\ddot = -mg + \lambda(2y),\quad x^2+y^2 - L^2=0, in the three unknowns, , and . Using the parameter , those equations take the form :\frac\frac - \frac = F_, which becomes, : mL^2\ddot = -mgL\sin\theta, or : \ddot + \frac\sin\theta=0. This formulation yields one equation because there is a single parameter and no constraint equation. This shows that the parameter is a generalized coordinate that can be used in the same way as the Cartesian coordinates and to analyze the pendulum.

Double pendulum

The benefits of generalized coordinates become apparent with the analysis of a double pendulum. For the two masses , let define their two trajectories. These vectors satisfy the two constraint equations, :f_1 (x_1, y_1, x_2, y_2) = \mathbf_1\cdot \mathbf_1 - L_1^2 = 0 and :f_2 (x_1, y_1, x_2, y_2) = (\mathbf_2-\mathbf_1) \cdot (\mathbf_2-\mathbf_1) - L_2^2 = 0. The formulation of Lagrange's equations for this system yields six equations in the four Cartesian coordinates and the two Lagrange multipliers that arise from the two constraint equations. Now introduce the generalized coordinates that define the angular position of each mass of the double pendulum from the vertical direction. In this case, we have :\mathbf_1 = (L_1\sin\theta_1, -L_1\cos\theta_1), \quad \mathbf_2 = (L_1\sin\theta_1, -L_1\cos\theta_1) + (L_2\sin\theta_2, -L_2\cos\theta_2). The force of gravity acting on the masses is given by, :\mathbf_1=(0,-m_1 g),\quad \mathbf_2=(0,-m_2 g) where is the acceleration due to gravity. Therefore, the virtual work of gravity on the two masses as they follow the trajectories is given by : \delta W = \mathbf_1\cdot\delta \mathbf_1 + \mathbf_2\cdot\delta \mathbf_2. The variations can be computed to be : \delta \mathbf_1 = (L_1\cos\theta_1, L_1\sin\theta_1)\delta\theta_1, \quad \delta \mathbf_2 = (L_1\cos\theta_1, L_1\sin\theta_1)\delta\theta_1 +(L_2\cos\theta_2, L_2\sin\theta_2)\delta\theta_2 Thus, the virtual work is given by :\delta W = -(m_1+m_2)gL_1\sin\theta_1\delta\theta_1 - m_2gL_2\sin\theta_2\delta\theta_2, and the generalized forces are :F_ = -(m_1+m_2)gL_1\sin\theta_1,\quad F_ = -m_2gL_2\sin\theta_2. Compute the kinetic energy of this system to be : T= \fracm_1 \mathbf_1\cdot\mathbf_1 + \fracm_2 \mathbf_2\cdot\mathbf_2 = \frac(m_1+m_2)L_1^2\dot_1^2 + \fracm_2L_2^2\dot_2^2 + m_2L_1L_2 \cos(\theta_2-\theta_1)\dot_1\dot_2.
Euler–Lagrange equation In the calculus of variations and classical mechanics, the Euler–Lagrange equations are a system of second-order ordinary differential equations whose solutions are stationary points of the given action functional. The equations were discovered ...
yield two equations in the unknown generalized coordinates given by :(m_1+m_2)L_1^2\ddot_1+m_2L_1L_2\ddot_2\cos(\theta_2-\theta_1) + m_2L_1L_2\dot^2\sin(\theta_1-\theta_2) = -(m_1+m_2)gL_1\sin\theta_1, and :m_2L_2^2\ddot_2+m_2L_1L_2\ddot_1\cos(\theta_2-\theta_1) + m_2L_1L_2\dot^2\sin(\theta_2-\theta_1)=-m_2gL_2\sin\theta_2. The use of the generalized coordinates 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 free to swing in any angular direction subject to gravity, the constraint on the pendulum bob can be stated in the form :f(\mathbf) = x^2 + y^2 + z^2 - l^2 = 0 \,, where the position of the pendulum bob can be written :\mathbf = (x(\theta,\phi),y(\theta,\phi),z(\theta,\phi)) \,, in which are the spherical polar angles because the bob moves in the surface of a sphere. The position is measured along the suspension point to the bob, here treated as a
point particle A point particle (ideal particle or point-like particle, often spelled pointlike particle) is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension; being dimensionless, it does not take u ...
. A logical choice of generalized coordinates to describe the motion are the angles . 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 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, for any variation . 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 . Let the forces on the system be be applied to points with Cartesian coordinates , then the virtual work generated by a virtual displacement from the equilibrium position is given by :\delta W = \sum_^m \mathbf_j\cdot \delta\mathbf_j. where denote the virtual displacements of each point in the body. Now assume that each depends on the generalized coordinates then : \delta \mathbf_j = \frac \delta_1 + \ldots + \frac \delta_n, and : \delta W = \left(\sum_^m \mathbf_j\cdot \frac\right) \delta_1 + \ldots + \left(\sum_^m \mathbf_j\cdot \frac\right) \delta_n. The terms : F_i = \sum_^m \mathbf_j\cdot \frac,\quad i=1,\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 = \sum_^m \mathbf_j\cdot \frac,\quad i=1,\ldots, n, where is the velocity of the point of application of the force . In order for the virtual work to be zero for an arbitrary virtual displacement, each of the generalized forces must be zero, that is : \delta W = 0 \quad \Rightarrow \quad F_i =0, i=1,\ldots, n.

See also

* Canonical coordinates *
Hamiltonian mechanics Hamiltonian mechanics emerged in 1833 as a reformulation of Lagrangian mechanics. Introduced by Sir William Rowan Hamilton, Hamiltonian mechanics replaces (generalized) velocities \dot q^i used in Lagrangian mechanics with (generalized) ''moment ...
* Virtual work *
Orthogonal coordinates In mathematics, orthogonal coordinates are defined as a set of ''d'' coordinates q = (''q''1, ''q''2, ..., ''q'd'') in which the coordinate hypersurfaces all meet at right angles (note: superscripts are indices, not exponents). A coordinate su ...
Curvilinear coordinates In geometry, curvilinear coordinates are a coordinate system for Euclidean space in which the coordinate lines may be curved. These coordinates may be derived from a set of Cartesian coordinates by using a transformation that is locally inv ...
Mass matrix In analytical mechanics, the mass matrix is a symmetric matrix that expresses the connection between the time derivative \mathbf\dot q of the generalized coordinate vector of a system and the kinetic energy of that system, by the equation :T = ...
* Stiffness matrix * Generalized forces



Bibliography of cited references

* * * * * *{{cite book , last=Torby , first=Bruce , title=Advanced Dynamics for Engineers , series=HRW Series in Mechanical Engineering , year=1984 , publisher=CBS College Publishing , location=United States of America , isbn=0-03-063366-4 , chapter=Energy Methods Dynamical systems Rigid bodies