Udwadia–Kalaba equation

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In theoretical physics, the Udwadia–Kalaba equation [1] is a method for deriving the equations of motion of a constrained mechanical system. This equation was discovered by Firdaus E. Udwadia and Robert E. Kalaba in 1992. The fundamental equation is the simplest and most comprehensive equation so far discovered [2] for writing down the equations of motion of a constrained mechanical system. The approach is based on Gauss's principle and it is the simplest and most general so far discovered for mechanical systems within the framework of classical mechanics. The Udwadia–Kalaba equation applies to a wide class of constraints, both holonomic constraints and nonholonomic ones, as long as they are linear with respect to the accelerations. The equation even generalizes to constraint forces that do not obey D'Alembert's principle.[3][4][5]


In theoretical physics, the Udwadia–Kalaba equation addresses the problem of constrained motion of mechanical systems. The problem of finding a simple closed form expression for mechanical systems that are subjected to equality constraints has been a foundational issue in analytical dynamics. It has been a problem of central importance in theoretical physics since the time it was first explicitly stated by Lagrange, and it has been vigorously worked on by numerous physicists, scientists, mathematicians, and engineers over the last 200 years or more.

The centrality of the problem stems for the fact that the difference between the motion of a set of particles subjected to forces and that of a system of particles is that in the latter the particles are cognizant of the motion of one another, and are therefore subjected to constraints that each particle poses on some (or all) of its neighbors. The standard method of handling this problem is through the use of Lagrange multipliers, first introduced by Lagrange. This approach however, is unsuitable when the number of degrees of freedom of a system becomes large, since the multipliers are required to be found usually through the solution of nonlinear sets of equations. Other approaches such as the Gibbs-Appell approach, and its variants, suffer from similar difficulties. There has therefore been, for a considerable length of time in analytical dynamics, a quest for obtaining a simple, explicit equation that describes the motion of a constrained mechanical system that is subjected to equality constraints.

After a research effort lasting about 15 years, in 1992 Professors Firdaus E. Udwadia and Robert E. Kalaba discovered an explicit and elegant equation of motion for constrained systems in their remarkable three-page paper that appeared in the Proceedings of the Royal Society of London. This equation is nowadays called the Udwadia-Kalaba (UK) equation.

The Udwadia–Kalaba equation has several advantages over the available (Lagrangian) approaches for describing the motion of mechanical systems, especially when dealing with nonlinear systems that have many degrees of freedom (that is, large-scale mechanical systems). The UK equation: (1) is explicit, (2) is simple and elegant, (3) provides a new, simpler, and deeper understanding of the manner in which Nature executes motion, (4) handles with equal ease constraints that are holonomic and/or nonholonomic, and then some, (5) handles constraints that are nonlinear in position and in velocity, which go well beyond the so called Pffafian constraints handled by the theory using Lagrange multipliers, and (6) is computationally efficient and simple to implement.

The breakthrough in arriving at this fundamental equation of motion for mechanical systems came about because Udwadia and Kalaba abandoned the ‘received’ view’ of mechanics that originated from Lagrange. The Lagrangian view of analytical dynamics (motion of bodies subjected to forces) has been paramount in theoretical physics since the late 18th century, and it is widely held even today. It relies on the determination of the Lagrange multipliers to describe the motion of constrained mechanical systems. Udwadia and Kalaba, in a starkly contrarian move, relinquished this ubiquitously held Lagrangian view of mechanics, and instead used and developed a perspective on mechanics, first proposed by Carl F. Gauss, which attempts to understand the motion, and, in particular, the way in which Nature orchestrates constrained motion, in a very different manner. This led to their discovery of the Udwadia-Kalaba (UK) equation, which rests on what is today called Gauss’s Principle. Remarkably, nowhere in the UK equation or in its derivation is the notion of a Lagrange multiplier ever invoked.

The physical interpretation of the UK equation leads to a simple, new, and totally different interpretation (from that provided by the Lagrangian view) of the manner in which Nature brings about the motion of systems of bodies. Besides the benefit of being explicit, it is this new interpretation of the UK equation that has been of great importance in areas that go beyond theoretical physics, such as, the precision control of highly nonlinear general dynamical systems.

The central problem of constrained motion[edit]

In the study of the dynamics of mechanical systems, the configuration of a given system S is, in general, completely described by n generalized coordinates so that its generalized coordinate n-vector is given by

where T denotes matrix transpose. Using Newtonian or Lagrangian dynamics, the unconstrained equations of motion of the system S under study can be derived as a matrix equation (see matrix multiplication):

Udwadia–Kalaba equations of motion (Unconstrained)

where the dots represent derivatives with respect to time:

It is assumed that the initial conditions q(0) and are known. We call the system S unconstrained because may be arbitrarily assigned.

The n-vector Q denotes the total generalized force acted on the system by some external influence; it can be expressed as the sum of all the conservative forces as well as non-conservative forces.

The n-by-n matrix M is symmetric, and it can be positive definite or semi-positive definite . Typically, it is assumed that M is positive definite; however, it is not uncommon to derive the unconstrained equations of motion of the system S such that M is only semi-positive definite; i.e., the mass matrix may be singular (it has no inverse matrix).[6][7]


We now assume that the unconstrained system S is subjected to a set of m consistent equality constraints given by

where A is a known m-by-n matrix of rank r and b is a known m-vector. We note that this set of constraint equations encompass a very general variety of holonomic and non-holonomic equality constraints. For example, holonomic constraints of the form

can be differentiated twice with respect to time while non-holonomic constraints of the form

can be differentiated once with respect to time to obtain the m-by-n matrix A and the m-vector b. In short, constraints may be specified that are

  1. nonlinear functions of displacement and velocity,
  2. explicitly dependent on time, and
  3. functionally dependent.

As a consequence of subjecting these constraints to the unconstrained system S, an additional force is conceptualized to arise, namely, the force of constraint. Therefore, the constrained system Sc becomes

Udwadia–Kalaba equations of motion (Constrained)

where Qc—the constraint force—is the additional force needed to satisfy the imposed constraints. The central problem of constrained motion is now stated as follows:

  1. given the unconstrained equations of motion of the system S,
  2. given the generalized displacement q(t) and the generalized velocity of the constrained system Sc at time t, and
  3. given the constraints in the form as stated above,

find the equations of motion for the constrained system—the acceleration—at time t, which is in accordance with the agreed upon principles of analytical dynamics.

Equation of motion[edit]

The solution to this central problem is given by the Udwadia–Kalaba equation. When the matrix M is positive definite, the equation of motion of the constrained system Sc, at each instant of time, is[2][8]

where the '+' symbol denotes the pseudoinverse of the matrix . The force of constraint is thus given explicitly as

and since the matrix M is positive definite the generalized acceleration of the constrained system Sc is determined explicitly by

In the case that the matrix M is semi-positive definite , the above equation cannot be used directly because M may be singular. Furthermore, the generalized accelerations may not be unique unless the (n + m)-by-n matrix

has full rank (rank = n).[6][7] But since the observed accelerations of mechanical systems in nature are always unique, this rank condition is a necessary and sufficient condition for obtaining the uniquely defined generalized accelerations of the constrained system Sc at each instant of time. Thus, when has full rank, the equations of motion of the constrained system Sc at each instant of time are uniquely determined by (1) creating the auxiliary unconstrained system[7]

and by (2) applying the fundamental equation of constrained motion to this auxiliary unconstrained system so that the auxiliary constrained equations of motion are explicitly given by[7]

Moreover, when the matrix has full rank, the matrix is always positive definite. This yields, explicitly, the generalized accelerations of the constrained system Sc as

This equation is valid when the matrix M is either positive definite or positive semi-definite! Additionally, the force of constraint that causes the constrained system Sc—a system that may have a singular mass matrix M—to satisfy the imposed constraints is explicitly given by

Non-ideal constraints[edit]

At any time during the motion we may consider perturbing the system by a virtual displacement δr consistent with the constraints of the system. The displacement is allowed to be either reversible or irreversible. If the displacement is irreversible, then it performs virtual work. We may write the virtual work of the displacement as

The vector describes the non-ideality of the virtual work and may be related, for example, to friction or drag forces (such forces have velocity dependence). This is a generalized D'Alembert's principle, where the usual form of the principle has vanishing virtual work with .

The Udwadia–Kalaba equation is modified by an additional non-ideal constraint term to


Inverse Kepler problem[edit]

The method can solve the inverse Kepler problem of determining the force law that corresponds to the orbits that are conic sections.[9] We take there to be no external forces (not even gravity) and instead constrain the particle motion to follow orbits of the form

where , is the eccentricity, and l is the semi-latus rectum. Differentiating twice with respect to time and rearranging slightly gives a constraint

We assume the body has a simple, constant mass. We also assume that angular momentum about the focus is conserved as

with time derivative

We can combine these two constraints into the matrix equation

The constraint matrix has inverse

The force of constraint is therefore the expected, central inverse square law

Inclined plane with friction[edit]

Consider a small block of constant mass on an inclined plane at an angle above horizontal. The constraint that the block lie on the plane can be written as

After taking two time derivatives, we can put this into a standard constraint matrix equation form

The constraint matrix has pseudoinverse

We allow there to be sliding friction between the block and the inclined plane. We parameterize this force by a standard coefficient of friction multiplied by the normal force

Whereas the force of gravity is reversible, the force of friction is not. Therefore, the virtual work associated with a virtual displacement will depend on C. We may summarize the three forces (external, ideal constraint, and non-ideal constraint) as follows:

Combining the above, we find that the equations of motion are

This is like a constant downward acceleration due to gravity with a slight modification. If the block is moving up the inclined plane, then the friction increases the downward acceleration. If the block is moving down the inclined plane, then the friction reduces the downward acceleration.

See also[edit]


  1. ^ Udwadia, F.E.; Kalaba, R.E. (1996). Analytical Dynamics: A New Approach. Cambridge University Press. ISBN 0-521-04833-8
  2. ^ a b Udwadia, F.E.; Kalaba, R.E. (1992). "A new perspective on constrained motion" (PDF). Proceedings of the Royal Society of London, Series A. 439 (1906): 407–410. Bibcode:1992RSPSA.439..407U. doi:10.1098/rspa.1992.0158. 
  3. ^ Udwadia, F. E.; Kalaba, R. E. (2002). "On the Foundations of Analytical Dynamics" (PDF). Intl. Journ. Nonlinear Mechanics. 37 (6): 1079–1090. Bibcode:2002IJNLM..37.1079U. doi:10.1016/S0020-7462(01)00033-6. 
  4. ^ Calverley, Bob (2001). "Constrained or Unconstrained, That Is the Equation". USC News. 
  5. ^ Udwadia, Firdaus; Kalaba, Robert (2002). "What is the General Form of the Explicit Equations of Motion for Constrained Mechanical Systems?" (PDF). Journal of Applied Mechanics. 69: 335–339. Bibcode:2002JAM....69..335U. doi:10.1115/1.1459071. 
  6. ^ a b Udwadia, F.E.; Phohomsiri, P. (2006). "Explicit equations of motion for constrained mechanical systems with singular mass matrices and applications to multi-body dynamics" (PDF). Proceedings of the Royal Society of London, Series A. 462 (2071): 2097–2117. Bibcode:2006RSPSA.462.2097U. doi:10.1098/rspa.2006.1662. 
  7. ^ a b c d Udwadia, F.E.; Schutte, A.D. (2010). "Equations of motion for general constrained systems in Lagrangian mechanics" (PDF). Acta Mechanica. 213 (1): 111–129. doi:10.1007/s00707-009-0272-2. 
  8. ^ Udwadia, F.E.; Kalaba, R.E. (1993). "On motion" (PDF). Journal of the Franklin Institute. 330 (3): 571–577. doi:10.1016/0016-0032(93)90099-G. 
  9. ^ Zhang, Bingzhan; Zhen, Shengchao; Zhao, Han; Huang, Kang; Deng, Bin; Chen, Ye-Hwa (2015). "A novel study on Kepler's law and inverse square law of gravitation". Eur. J. Phys. 36. Bibcode:2015EJPh...36c5018Z. doi:10.1088/0143-0807/36/3/035018.