Chapter 6

Some Methods in the Calculus of Variations

 

 

      Euler’s Equation

      Consider the two-dimensional plane shown in Figure 1.  Our initial position is specified by (x1,y1) and the final position is specified by (x2,y2). 

 

 

Figure 1.  Path y(x) that is used to move from position 1 to position 2.

 

Consider that we are asked to minimize the path integral of a function f between position 1 and position 2.  Suppose the path integral is minimized when the use path y(x).  If we change the path slightly by adding a second function n(x) then we expect that the path integral is expected to increase.  Consider the following path:

 

 

The function h(x) is a arbitrary function of x and is used to make small changes to the path.  The only requirements of h(x) are that h(x) has a continuous first derivative and that h(x) vanishes at the end points of the path, that is h(x1) = h(x2) = 0.

      Since the path integral is minimized when we follow the path, the path integral must have an extreme value when a = 0.  This requires that

 

 

The left hand side of this equation can be rewritten by differentiating the argument of the integral with respect to a:

 

 

Using our definition of y(a, x) we can easily show that

 

 

and

 

 

The requirement that the path integral has an extreme is equivalent to requiring that

 

 

The second term in the integrant can be rewritten as

 

 

This path integral is an extreme if

 

 

Since h(x) is an arbitrary function, this equation can only satisfied if

 

 

This equation is known as Euler’s equation.

 

 

      Example: Problem 6.4

      Show that the geodesic on the surface of a right circular cylinder is a segment of a helix.

 

      The element of distance along the surface of a cylinder is

 

                                                                                                      (6.4.1)

 

In cylindrical coordinates (x, y, z) are related to (r, f, z) by

 

                                                                                                                            (6.4.2)

 

Since we consider motion on the surface of a cylinder, the radius r is constant.  The expression for x, y, and z can be used to express dx, dy, and dz in cylindrical coordinates:

 

                                                                                                                    (6.4.3)

 

Substituting (6.4.3) into (6.4.1) and integrating along the entire path, we find

 

                                                    (6.4.4)

 

If S is to be a minimum,  must satisfy the Euler equation:

 

                                                                                                                         (6.4.5)

 

Since , the Euler equation becomes

 

                                                                                                                       (6.4.6)

 

This condition will be satisfied if

 

                                                          = constant º C                                              (6.4.7)

 

or,

 

                                                                                                                           (6.4.8)

 

Since r is constant, (6.4.8) implies that

 

                                                                  

 

and for any point along the path, z and f change at the same rate.  The curve described by this condition is a helix.

 

 

      Example: Problem 6.7

      Consider light passing from one medium with index of refraction n1 into another medium with index of refraction n2 (see Figure x).  Using Fermat’s principle to minimize time, and derive the law of refraction: n1 sin q1 = n2 sin q2.

 

Figure x.  Problem 6.7

 

      The time to travel the path shown in Figure x is

 

                                                   (6.7.1)

 

The velocity v = v1 when y > 0 and v = v2 when y < 0.  The velocity is thus a function of y, v = v(y), and dv/dy = 0 for all values of y, except for y = 0.  The function f is given by

 

                                                                                                               (6.7.2)

 

The Euler equation tells us

 

                                                                                     (6.7.3)

 

Now use v = c/n and y¢ = -tanq  to obtain

 

    (6.7.4)

 

This proves the assertion.

 

 

      Second Form of Euler’s Equation

      In some applications, the function f may not depend explicitly on x: f/x = 0.  In order to benefit from this constraint, it would be good to try to rewrite Euler’s equation with a term f/x instead of a term f/y.

      The first step in this process is to examine df/dx.  The general expression for df/dx is

 

 

Note that we have not assume that f/x = 0 in order to derive this expression for df/dx.  This equation can be rewritten as

 

 

We also know that

 

 

Applying Euler’s theorem to the term in the parenthesis on the right hand side, we can rewrite this equation as

 

 

or

 

 

This equation is called the second form of Euler’s equation.  If f does not depend explicitly on x we conclude that

 

 

 

 

      Example: Problem 6.4 – Part II

      Show that the geodesic on the surface of a right circular cylinder is a segment of a helix.

 

      We have already solved this problem using the “normal” form of Euler’s equation.  However, looking back at the solution we realize that the expression for f, , does not depend explicitly on f.  We thus should be able to use the second form of Euler’s equation to solve this problem.

 

 

This equation requires that

 

 

Since r is constant, this equation implies that

 

 

and for any point along the path, z and f change at the same rate.  The curve described by this condition is a helix.

 

 

      Euler’s Equation with Several Dependent Variables

      Consider a situation where the function f depends on several dependent variables y1, y2, y3, … etc., each of which depends on the independent variable x.  Each dependent variable yi(a, x) is related to the solution yi(0, x) in the following manner:

 

 

If the independent functions y1, y2, y3, etc. minimize the path integral of f, they must satisfy the following condition:

 

 

for i = 1, 2, 3, ….  The procedure to find the optimum paths is similar to the procedures we have discussed already, except that we need to solve the Euler equation for each dependent variable yi.

 

 

      Euler’s Equation with Boundary Conditions

      In many cases, the dependent variable y must satisfy certain boundary conditions.  For example, in problem 6.4 the function y must be located on the surface of the cylinder.  In this case, any point on y must satisfy the following condition:

 

 

In general, we can specify the constraint on the path(s) by using one or more functions g and requiring that g{yi; x} = 0.

      Let’s start with the case where we have two dependent variables y and z.  In this case, we can write the function f as

 

 

In this case, we can write the differential of J with respect to a as

 

 

Using our definition of yi(a, x) we can rewrite this relation as

 

 

For our choice of constraint we can immediately see that the derivative  of g must be zero:

 

 

Using our definition of y and z we can rewrite this equation as

 

 

This equation shows us a general relation between the functions hx and hy:

 

 

Using this relation we can rewrite our expression for the differential of J:

 

 

Since the function hx is an arbitrary function, the equation can only evaluate to 0 if the term in the brackets is equal to 0:

 

 

This equation can be rewritten as

 

 

This equation can only be correct if both sides are equal to a function that depends only on x:

 

 

or

 

 

 

Note that in this case, where we have one auxiliary condition, g{y, z; x} = 0, we end up with one Lagrange undetermined multiplier l(x).  Since we have three equations and three unknown, y, z, and l, we can determine the unknown.

      In certain problems the constraint can only be written in integral form.  For example, the constraint for problems dealing with ropes will be that the total length of the path is equal to the length of the rope L:

 

 

The curve y then must satisfy the following differential equation:

 

 

 

 

      Problem 6.12

      Repeat example 6.4, finding the shortest path between any two points on the surface of a sphere, but use the method of the Euler equation with an auxiliary condition imposed.

 

      The path length is given by

 

                                                                                                   (6.12.1)

 

and our equation of constraint is

 

                                                                                             (6.12.2)

 

The Euler equations with undetermined multipliers (6.69) tell us that

 

                                                                                           (6.12.3)

 

with a similar equation for z. Eliminating the factor l, we obtain

 

                                                                  (6.12.4)

 

This simplifies to

 

                   (6.12.5)

 

                                                                     (6.12.6)

 

and using the derivative of (2),

 

                                                                                                        (6.12.7)

 

This looks to be in the simplest form we can make it, but is it a plane?  Take the equation of a plane passing through the origin:

 

                                                                                                                                    (8)

 

and make it a differential equation by taking derivatives (giving A + By¢ = z¢ and By² = z²) and eliminating the constants.  The substitution yields (7) exactly.  This confirms that the path must be the intersection of the sphere with a plane passing through the origin, as required.

 

 

      Example: Problem 6.4 – Part III

      Show that the geodesic on the surface of a right circular cylinder is a segment of a helix.

 

      We have already solved this problem using the “normal” form of Euler’s equation.  However, this problem is a good example of how to approach problems with constraints.  Note: doing it in this way is NOT easier then the approaches we have used previously.

      Let us consider two points on the surface of the cylinder:

 

 

and

 

 

Consider an arbitrary path connecting the initial and final position.  The length of a tiny segment of this path is

 

 

The integral we want to minimize is

 

 

We immediately see that z is our independent variable and that x, and y are our dependent variables.  The function f is thus given by

 

 

The solution y is constrained to be on the surface of the cylinder and therefore, the following equation of constraint needs to be applied:

 

 

For this equation of constraint we know that

 

 

and

 

 

Note: if we had picked our function of constraint to be

 

 

we would get more complicated partial derivatives of g.

To solve the current problem we thus need to solve the following Euler equations:

 

 

 

These two equations can be rewritten as

 

 

 

Eliminating l we find that

 

 

This equation can be rewritten as

 

 

After simplifying this equation we obtain

 

 

Is this equation describing a helix?  Yes it is!  How do you see that?  Let's look at the definition of a helix:

 

 

We find that

 

 

and

 

 

Taking these relations and substituting them in the solution we obtained we find:

 

 

 

 

      The d notation

      It is common to use the d notation in the calculus of variations.  In order to use the d notation we use the following definitions:

 

 

 

In terms of these variables we find

 

 

Since dy is an arbitrary function, the requirement that dJ = 0 requires that the term in the parenthesis is 0.  This of course is the Euler equation we have encountered before!

      It is important to not that there is a significant difference between dy and dy.  Based on the definition of dy we see that dy tells us how y varies when we change a while keeping all other variables fixed (including for example the time t).

Chapter 6

In this Chapter we focus on an important method of solving certain problems in Classical Mechanics. 

In many problems we need to determine how a system evolves between an initial state and a final state.  For example, consider two locations on a two-dimensional plane.  Consider an object moving from one location to the other, and assume the object experiences a friction force when it moves over the surface.  One can ask questions such as “What is the path between the initial and the final condition that minimizes the work done by the friction force?” that can be most easily answered using the calculus of variations.  The evolution that can be studied using the calculus of variations is not limited to evolutions in real space.  For example, one can consider the evolution of a gas in a pV diagram and ask “What is the path between the initial and final state that maximizes the work by the gas?”