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Physics 3550, Fall 2011 Two Body, Central-Force Problem Relevant Sections in Text: §8.1 – 8.7

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Physics 3550, Fall 2011 Two Body, Central-Force Problem Relevant Sections in Text: §8.1 – 8.7 Two Body, Central-Force Problem. Physics 3550, Fall 2011 Two Body, Central-Force Problem Relevant Sections in Text: x8.1 { 8.7 Two Body, Central-Force Problem { Introduction. I have already mentioned the two body central force problem several times. This is, of course, an important dynamical system since it represents in many ways the most fundamental kind of interaction between two bodies. For example, this interaction could be gravitational { relevant in astrophysics, or the interaction could be electromagnetic { relevant in atomic physics. There are other possibilities, too. For example, a simple model of strong interactions involves two-body central forces. Here we shall begin a systematic study of this dynamical system. As we shall see, the conservation laws admitted by this system allow for a complete determination of the motion. Many of the topics we have been discussing in previous lectures come into play here. While this problem is very instructive and physically quite important, it is worth keeping in mind that the complete solvability of this system makes it an exceptional type of dynamical system. We cannot solve for the motion of a generic system as we do for the two body problem. The two body problem involves a pair of particles with masses m1 and m2 described by a Lagrangian of the form: 1 2 1 2 L = m ~r_ + m ~r_ − V (j~r − ~r j): 2 1 1 2 2 2 1 2 Reflecting the fact that it describes a closed, Newtonian system, this Lagrangian is in- variant under spatial translations, time translations, rotations, and boosts.* Thus we will have conservation of total energy, total momentum and total angular momentum for this system. The EL equations are equivalent to (exercise) ¨ 0 ~r2 − ~r1 m1~r1 = V (j~r1 − ~r2j) ; j~r1 − ~r2j ¨ 0 ~r1 − ~r2 m2~r2 = V (j~r1 − ~r2j) : j~r1 − ~r2j * \Boosts" are transformations corresponding to changing to a reference frame moving with constant relative velocity. The totality of these symmetry transformations (spacetime translations, rotations, boosts) in non-relativistic Newtonian mechanics forms a group known as the Galileo group. 1 Two Body, Central-Force Problem. Here the prime on the potential energy function denotes a derivative with respect to its argument. The equations of motion consist of 6 coupled ODEs corresponding to the 6 degrees of freedom. In general these equations are non-linear. There is one exceptional case where the equations are linear. Can you see what this case is? From these equations we see that the force exerted on each particle (i) is equal in magni- tude and opposite in direction to that on the other particle (i.e., this force obeys Newton's third law); (ii) is directed along a line joining the (instantaneous) particle positions, and (iii) has a magnitude depending only on the distance between the particles. Of course, the standard example of such a two body problem arises in the motion of gravitating bodies (\Kepler problem"), or in the motion of a pair of electric charges ignoring magnetic and radiative effects. In each case the potential energy is of the form 1 V / : j~r1 − ~r2j The constant of proportionality depends upon the masses, charges, gravitational constant, etc. Reduction to relative motion Our first step toward solving the two body problem is to reduce the problem to that of the relative motion of the two bodies. The physical idea is simple. Given the homogeneity of space, the absolute position of either of the particles has no relevant meaning. Only the relative position should matter. This idea manifests itself as follows. Because the system is invariant under spatial translations, total momentum will be conserved. This means the motion of the center of mass will be at constant velocity in any inertial reference frame (IRF) (exercise). So, this conservation law tells us that 3 degrees of freedom move as a free particle. The remaining 3 degrees of freedom describe the relative motion of the particles; it is here where all the interesting dynamics lie. This effective reduction in degrees of freedom from 6 to 3 as arises from the conservation of total momentum (which is the center of mass momentum) of the system. Let us now make the above comments mathematically precise. Define six generalized coordinates which are to replace the 6 Cartesian coordinates labeling the position of each of the bodies: m ~r + m ~r R~ = 1 1 2 2 ; m1 + m2 ~r = ~r1 − ~r2: We call R~ the center of mass position and we call ~r the relative position. This is easily seen to be an invertible transformation. We have m2 m1 ~r1 = R~ + ~r; ~r2 = R~ − ~r: m1 + m2 m1 + m2 2 Two Body, Central-Force Problem. Substituting these expressions (and their time derivatives) into the Lagrangian, we get 1 _ 2 1 2 L = (m + m )R~ + m~r_ − V (j~rj); 2 1 2 2 where m m m = 1 2 m1 + m2 is called the reduced mass of the system. We see that, in these coordinates, the Lagrangian is of the form ~ _ ~_ ~_ _ L(~r; R; ~r; R) = Lcm(R) + Lrelative(~r; ~r); indicating that the center of mass motion R~(t) and relative motion ~r(t) are completely decoupled (exercise). We can solve for the motion of R~ and ~r separately. The motion of R~ is obvious: R~(t) = R~ 0 + V~ t: By a suitable choice of inertial reference frame we can even set R~(t) = 0 without affecting ~r(t) (exercise). Put differently, up to a boost | which is a symmetry for this system | all solutions to the equations of motion can be obtained by setting R~(t) = 0 and solving for the relative motion. Once we have determined ~r(t), we can easily reconstruct ~r1(t) and ~r2(t) from ~r(t) and R~(t) (exercise). In any case, the only non-trivial problem to solve is the relative motion problem. We henceforth focus on the Lagrangian 1 2 L = m~r_ − V (j~rj); 2 which describes the relative motion. Note that, mathematically speaking, this dynamical system is indistinguishable from a single particle moving in three dimensions subject to a central force field directed at fixed point in space. We have thus used the conservation of total momentum to reduce the analysis of the motion of two bodies interacting by a central force to the analysis of a system mathematically equivalent to that of a single particle moving in a central force field. All our subsequent analysis will apply to this physical system. Keep in mind that, in applications to the 2-body problem, the \particle" with mass m and position ~r is fictitious: its position is really the relative position of the two bodies, and the \particle" mass is really the reduced mass of the two body system. Exercise: Show that when m2 >> m1 we can work in a reference frame in which m2 is (approximately) at rest and we can view the reduced Lagrangian as describing the motion of m1 in the fixed force field of m2. 3 Two Body, Central-Force Problem. A small digression: Lagrangian Reduction By a clever change of variables in the 2 body problem, (~r1; ~r2) ! (R;~ ~r), we are able to obtain a Lagrangian in which three coordinates, namely (Rx;Ry;Rz), are cyclic. Thus the center of mass degrees of freedom \move" according to free particle EL equations and can be eliminated from the problem leaving us with a reduced problem involving 3 fewer degrees of freedom. You can see now why cyclic coordinates are also sometimes called \ignorable". The reduction of the 2 body problem to a problem involving only 3 degrees of freedom by using conservation of total momentum is an instance of what is usually called Lagrangian reduction. This term describes the effective reduction in degrees of freedom afforded by a conservation law. A trivial example of Lagrangian reduction occurs already for motion in one-dimension q described by a time independent Lagrangian. The conservation of energy allows us to completely solve for q(t) (up to quadrature)|thus leaving no more degrees of freedom to worry with. As we shall see, the complete integration of the EL equations for the 2-body problem is nothing but a sequence of Lagrangian reductions. There is a very general and powerful theory of reduction in mechanics (and field theory), but it will take us too far afield to develop it systematically. The best version of it takes place in the Hamiltonian formalism (to be discussed later), where the technique is called \symplectic reduction". Still, even though we will refrain from developing the general theory of Lagrangian reduction, because this is such an important tool in mechanics it is worth digressing for a moment to look at a couple of other examples of this phenomenon. Example: Spherical Pendulum The Lagrangian is 1 L = mR2(θ_2 + sin2 θφ_2) + mgR cos θ: 2 The coordinate φ is ignorable { it does not appear in L { and hence (exercise) mR2 sin2 θφ_ = l = constant: For any given l, this equation can be used to eliminate the φ degree of freedom in terms of an integral involving θ(t) (exercise). Thus we have a reduced problem involving a single degree of freedom θ. Once we find θ(t) we can easily obtain φ(t) by an integral, and the problem is solved. The motion in θ, i.e., θ(t), is determined by the equation of motion for θ with φ(t) eliminated using the conservation law above (exercise). This can be done because φ(t) does not appear in the equation, only φ_(t).
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