Hence the typical Lagrange’s equation may be now written in the form

or, again,

It has been proposed by Helmholtz to give the name *kinetic potential* to the combination .

As shown under Mechanics, § 22, we derive from (10)

and therefore in the case of a conservative system free from extraneous force,

which is the equation of energy. For examples of the application of the formula (13) see Mechanics, § 22.

3. *Constrained Systems.*

It has so far been assumed that the geometrical relations, if any, which exist between the various parts of the system Case of varying relations. are of the type § 1 (1), and so do not contain t explicitly. The extension of Lagrange’s equations to the case of “varying relations” of the type

was made by J. M. L. Vieille. We now have

so that the expression § 1 (8) for the kinetic energy is to be replaced by

where

and the forms of are as given by § 1 (7). It is to be remembered that the coefficients will in general involve explicitly as well as implicitly through the co-ordinates . Again, we find

where is defined as in § 1 (13). The derivation of Lagrange’s equations then follows exactly as before. It is to be noted that the equation § 2 (15) does not as a rule now hold. The proof involved the assumption that is a homogeneous quadratic function of the velocities .

It has been pointed out by R. B. Hayward that Vieille’s case can be brought under Lagrange’s by introducing a new co-ordinate () in place of , so far as it appears explicitly in the relations (1). We have then

The equations of motion will be as in § 2 (10), with the additional equation

where is the force corresponding to the co-ordinate . We may suppose to be adjusted so as to make , and in the remaining equations nothing is altered if we write for before, instead of after, the differentiations. The reason why the equation § 2 (15) no longer holds is that we should require to add a term on the right-hand side; this represents the rate at which work is being done by the constraining forces required to keep constant.

As an example, let be the co-ordinates of a particle relative to axes fixed in a solid which is free to rotate about the axis of . If be the angular co-ordinate of the solid, we find without difficulty

where is the moment of inertia of the solid. The equations of motion, viz.

and

become

and

If we suppose adjusted so as to maintain , or (again) if we suppose the moment of inertia to be infinitely great, we obtain the familiar equations of motion relative to moving axes, viz.

where has been written for . These are the equations which we should have obtained by applying Lagrange’s rule at once to the formula

which gives the kinetic energy of the particle referred to axes rotating with the constant angular velocity . (See Mechanics, § 13.)

More generally, let us suppose that we have a certain group of co-ordinates whose absolute values do not affect the expression for the kinetic energy, and that by suitable forces of the corresponding types the velocity-components are maintained constant. The remaining co-ordinates being denoted by , we may write

where is a homogeneous quadratic function of the velocities of the type § 1 (8), whilst is a homogeneous quadratic function of the velocities alone. The remaining terms, which are bilinear in respect of the two sets of velocities, are indicated more fully. The formulae (10) of § 2 give <mathn\!</math> equations of the type

where

These quantities are subject to the relations

The remaining dynamical equations, equal in number to the co-ordinates , yield expressions for the forces which must be applied in order to maintain the velocities constant; they need not be written down. If we follow the method by which the equation of energy was established in § 2, the equations (17) lead, on taking account of the relations (19), to

or, in case the forces depend only on the co-ordinates and are conservative,

The conditions that the equations (17) should be satisfied by zero values of the velocities are

or in the case of conservative forces

*i.e.* the value of must be *stationary*.

We may apply this to the case of a system whose configuration relative to axes rotating with constant angular velocity () is defined by means of the n co-ordinates . Rotating axes. This is important on account of its bearing on the kinetic theory of the tides. Since the Cartesian co-ordinates of any particle of the system relative to the moving axes are functions of , of the form § 1 (1), we have, by (15)

whence

The conditions of relative equilibrium are given by (23).

It will be noticed that this expression , which is to be stationary, differs from the true potential energy by a term which represents the potential energy of the system in relation to fictitious “centrifugal forces.” The question of stability of relative equilibrium will be noticed later (§ 6).

It should be observed that the remarkable formula (20) may in the present case be obtained directly as follows. From (15) and (14) we find