classical mechanics fall 2011 chapter 8: two-body central...
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Classical Mechanics Fall 2011
Chapter 8: Two-Body Central Force Problems
1. Center of Mass and Relative Coordinates
The basic problem is that of two particles interacting via a central force such as the gravitational force or the electric force. For such forces, the potential energy (and force) is a function of the position of one particle relative to the other. First, let us use the positions
of the two particles relative to the origin to describe the system. Each position has three components so the total number of generalized coordinates needed is six. The total kinetic energy is
T = 1
2m1r1
2 + 12m2 r2
2. (8.1)
The potential energy is
U =U r1 −r2( ). (8.2)
Defining the relative position
r = r1 −
r2, (8.3)
the potential energy becomes
U =U(r). (8.4)
Thus, if we use the relative position vector r (representing 3 generalized coordinates) to
describe the system, the potential energy will be a function of r (magnitude of r ) only,
which is very convenient. We now need another position vector to have the six coordinates needed for the complete description of the system. The center of mass is a characteristic of the system as a whole and its position is a possible choice for the other coordinate. What makes the CM position the best choice is that the total momentum of the system is
O
1
2
2
conserved because there are no external forces acting on it. Further, the total momentum of the system is equal to the momentum of the CM:
m1r1 +m2
r2 = MrCM , (8.5)
where
M = m1 +m2 (8.6)
The conservation of momentum therefore implies that
MrCM = constant, (8.7)
i.e.,
rCM = constant. (8.8)
Thus, the CM moves with constant velocity. We can therefore choose an inertial reference frame in which the CM is at rest to analyze the motion. This frame is called the CM frame. In the CM frame, only the relative motion of the particles is important; the CM is at rest. To see this more quantitatively, let us convert from (
r1,r2 ) to (
rCM ,r ) coordinates. Recall that
rCM ≡R = m1
r1 +m2r2
M, (8.9)
where we have changed notation for the position of the CM for convenience. Thus,
R = m1r1 +m2
r2M
. (8.10)
Further,
r = r1 −
r2. (8.11)
Eqs. (8.10) and (8.11) yield
r1 =R + m2
Mr (8.12)
and
r2 =R − m1
Mr . (8.13)
The total kinetic energy is then
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T = 12m1r1
2 + 12m2 r2
2
= 12MR + 1
2m1m2
Mr 2. (8.14)
In-‐class Problem: Prove Eq. (8.14).
Defining the reduced mass
µ = m1m2
m1 +m2
, (8.15)
we have
T == 1
2MR + 1
2µr 2 . (8.16)
The Lagrangian for the two-‐body system is therefore
L == 1
2MR + 1
2µr 2 −U(r). (8.17)
The Lagrange equation for the R coordinate (really three equations – one for for each component) is
ddt
MR( ) = 0, (8.18)
i.e.,
MR = constant. (8.19)
Thus, the momentum of the CM is conserved as predicted above. This also means that R is
constant. Shifting to the CM frame (where R = 0), the Lagrangian becomes
L = 1
2µr 2 −U(r). (8.20)
Eq. (8.20) looks like the Lagrangian for a single particle with mass equal to the reduced mass. Thus, with the change in coordinates, we have transformed the two-‐body problem to what is effectively a one-‐body problem. This greatly simplifies the analysis. Note that if m1 m2, µ ≈ m2. In this case, particle 1 hardly moves and the problem is, to good approximation, a one-‐body problem in which particle 2 moves relative to a fixed force center located at the position of particle 1.
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2. Angular Momentum
The total angular momentum of the system is
L = r1 ×m1r1 +r2 ×m2
r2. (8.21)
Substituting for r1 and
r2 using Eqs. (8.12) and (8.13), along with R = 0 in the CM frame,
we obtain
L = r × µr . (8.22)
In-‐class Problem: Prove Eq. (8.22).
Thus, the total angular momentum is that of a single body with mass µ at position r
relative to the origin, which is consistent with the effective one-‐body Lagrangian we obtained above in the CM frame.
Now, for an isolated two-‐particle system in which the particles interact via a central force, the net external torque on the system is zero and so the total angular momentum is constant. The angular momentum vector therefore has a constant direction, which is perpendicular to the vectors
r and r according to Eq. (8.22). Thus, the relative position
and relative velocity vectors are always in the same plane. It follows that the motion must lie in one plane, i.e., the motion is two-‐dimensional.
3. Two-‐Dimensional Equations of Motion
Since the motion is confined to a plane and the potential energy is only a function of the relative position r, we will use 2D polar coordinates to describe the motion. The radial coordinate r is equivalent to the magnitude of the relative position vector. The Lagrangian is
L = 1
2µ( r2 + r2 φ 2 )−U(r). (8.23)
The coordinate φ is ignorable ( L is independent of it), therefore
∂L∂ φ
= µr2 φ = lz = l = constant. (8.24)
Thus, angular momentum is conserved.
The Lagrange equation for the radial coordinate is
ddt
∂L∂ r
= ∂L∂r, (8.25)
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which gives
µr = µr φ 2 − dU(r)
dr. (8.26)
The first term in Eq. (8.26) is the centrifugal force. Since the angular momentum is a constant, it is useful to substitute for φ in Eq. (8.26) using Eq. (8.24):
µr = l2
µr3− dU(r)
dr. (8.27)
Note that the centrifugal force is now a function of r only. (The other parameters are constants). We can define a fictitious centrifugal potential energy Ucf (r) such that
Fcf (r) = −dUcf (r)dr
. (8.28)
Using Fcf (r) = l2 µr3 and integrating Eq. (8.28), we obtain
Ucf =l2
2µr2, (8.29)
where we have taken Ucf (∞) = 0. Let us now define an effective potential energy Ueff (r) such that
Ueff (r) =Ucf (r)+U(r) =l2
2µr2+U(r). (8.30)
With this definition, Eq. (8.27) becomes
µr = −
dUeff
dr. (8.31)
Eq. (8.31) is equivalent to the equation of motion of a single particle moving with a potential energy Ueff .
Conservation of Energy
Multiplying both sides of Eq. (8.31) by r gives
ddt
12µ r2⎛
⎝⎜⎞⎠⎟ = −
dUeff (r)dt
, (8.32)
i.e.,
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ddt
12µ r2 +Ueff (r)
⎛⎝⎜
⎞⎠⎟ = 0. (8.33)
Integrating yields
12µ r2 +Ueff (r) = E = constant. (8.34)
If the actual potential energy U(r)derives from an attractive inverse-‐square force (F(r)∝−1 r2 ) such as gravity, then
U(r) = −αr
, (Attractive inverse square force) (8.35)
where α is a constant. In this case,
Ueff (r) =l2
2µr2− αr. (8.36)
For large r, the 1/r term dominates; therefore, Ueff (r) is negative. For small r, the 1/r2 (centrifugal) term dominates and Ueff (r) is positive. It follows that Ueff (r) must have a minimum. A sketch of Ueff (r) is shown in the figure. When E =Ueff , r = 0 , i.e., the radial velocity is instantaneously zero. The values of r at which this occurs are the radial turning points. At these r values, the equivalent one body is neither moving toward or away from
the force center. If the total energy E > 0, then there is only one such r value (rmin) where radial motion toward the force center ceases. Clearly for r > rmin , E >Ueff (r) ; therefore, the body can move to infinity. The motion is unbounded. For E < 0, in general the radial motion is bounded between rmin and rmax . An example of such bounded motion is an elliptical orbit of a planet in our solar system. When the planet is closest to the sun, r = rmin and when the planet is farthest from the sun, r = rmax . Note that a bounded orbit does not necessarily mean that the orbit is closed.
Question: At what value of E would a planet move in a circular orbit?
Ueff
r
rmin rmax
rmin E>0
E<0
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In-‐class Problem: Taylor, Problem 8.13
4. Equation of the Orbit
For ease of discussion and visualization, let us assume that µ ≈ m1 so that the motion of particle 1 is nearly identical to that of the equivalent one body. Particle 2 is nearly stationary (the CM is very close to its position) and particle 1 orbits particle 2.
While much can be learned by examining the radial equation, to find the trajectory of the orbiting particle, we need to consider the variation of the angle φ as well. (Recall that the motion is two-‐dimensional, so only the two polar coordinates are needed to describe the orbit.) The trajectory is r(φ) , i.e., r as a function of φ . To determine the trajectory, we go back to the radial equation of motion:
µr = l2
µr3+ F(r), (8.37)
where
F(r) = − dU(r)dr
(8.38)
is the central force by which the particles interact. We will substitute
u = 1r (8.39)
into Eq. (8.37) and also rewrite d dt using the chain rule:
ddt
= ddφ
dφdt
= φ ddφ
= lµr2
ddφ
= lu2
µddφ. (8.40)
Thus,
r = dr
dt= lu
2
µddφ
1u
⎛⎝⎜
⎞⎠⎟ = − l
µdudφ. (8.41)
r = d r
dt= lu
2
µddφ
− lµdudφ
⎛⎝⎜
⎞⎠⎟= − l
2u2
µ2d 2udφ 2
. (8.42)
Substituting these results in Eq. (8.37) gives
− l2u2
µd 2udφ 2
= l2u3
µ+ F(r), (8.43)
i.e.,
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′′u (φ) = −u(φ)− µl2u2
F(u). (8.44)
Solving this equation for a given central force F will give the trajectory r(φ) by using the relation u = 1/ r .
5. Orbits for an Inverse Square Force
For an attractive inverse-‐square force,
F(r) = − αr2
= −αu2, (8.45)
where α is a proportionality constant. Substituting for F(r) in Eq. (8.44) gives
′′u (φ)+ u(φ) = αµl2. (8.46)
This is a second-‐order linear inhomogeneous differential equation similar to that of a forced harmonic oscillator with no damping and a constant applied force. The solution to the homogenous equation is sinusoidal (like the harmonic oscillator with ω = 1) and the particular solution is just a constant up =αµ l2 . Thus, the complete solution to Eq. (8.46) is
u(φ) = Acos(φ −δ )+ αµl2, (8.47)
where A and δ are constants of integration. We can make δ = 0 by an appropriate choice of the direction corresponding to φ = 0 . Recalling that r = 1/u, Eq. (8.47) becomes
r(φ) = c1+ ecosφ
, (8.48)
where e = Al2 /αµ is a positive dimensionless constant and
c = l2
αµ (8.49)
is also a positive quantity that has the dimensions of length. Eq. (8.48) is the equation of the orbit for an attractive inverse-‐square central force.
In Eq. (8.48), note that if e < 1, there is an upper bound and a lower bound on the values of r because −1≤ cosφ ≤1 . We have
rmin =c
1+ e and rmax =
c1− e
. (8.50)
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Thus, the orbit is bounded for e < 1. For e = 0, there is only one value of r, i.e., r = c. This corresponds to a circular orbit. For e ≥1 , there is a lower bound on r but no upper bound (note that r cannot be negative). Thus, in this case, the orbit is unbounded. The values of rmin and rmax can also be found by using energy considerations. Recall that radial turning points occur when r = 0 . Thus,
l2
2µr2+U(r) = E.
Taking U(r) = −α r for the potential energy corresponding to the inverse square force and substituting it in the equation above, we obtain the quadratic equation
2µEr2 + 2µαr − l2 = 0,
for which the solutions are
r± =α2E
−1± 1+ 2El2 µα 2( )⎡⎣⎢
⎤⎦⎥. (8.51)
We see that if −µα 2 2l2 < E < 0 , there are two solutions, corresponding to rmin ( r+ ) and rmax ( r− ). These are bounded orbits for which the eccentricity e < 1. For E > 0, there is only one positive solution, corresponding to rmin. Thus, these orbits are unbounded and the eccentricity e > 1. The explicit relationship between E and e can be shown by adding rmin and rmax from Eq. (8.51) and equating the result to that obtained by adding rmin and rmax from Eq. (8.50). The result is
e = 1+ 2cEα
= 1+ 2El2
µα 2 . (8.52)
In-‐class Problem: From Eq. (8.51), show that the radius of the circular orbit for a given l, α , and µ is r = c.
The Orbits
The general equation for a conic section is
r(φ) = λ1+ ecosφ
, (8.53)
The parameter e is called the eccentricity and it determines the particular conic section. The parameter λ is called the semi-‐latus rectum. We see that equation for a conic section has exactly the same form as the equation of the orbit for an inverse-‐square force, with λ = c. It follows that the orbits are conic sections. The table below gives the conic section for a given value of e and the corresponding sign of the total energy E.
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Eccentricity Energy Conic section 0 <0, minimum circle
0 < e <1 <0 ellipse 1 0 parabola >1 >0 hyperbola
Elliptical Orbits
To show that e < 1 in Eq. (8.48) corresponds to an ellipse, one can convert from polar coordinates to Cartesian coordinates. The result is
(x + d)2
a2+ y
2
b2= 1, (8.54)
where
a = c1− e2 , b = c
1− e2, and d = ea. (8.55)
Eqn. (8.54) is the equation of an ellipse with semimajor axis a and semiminor axis b. The origin is at O, the position of the body being orbited (e.g., the sun), which is also the
position of one focus of the ellipse. The position of the orbiting body (e.g., a planet) when it is closest to the body being orbited is called the perigee. If the body being orbited is the sun, it is called the perihelion. At this point, the distance between the two bodies is rmin. The position at which the orbiting body is farthest away is called the apogee (or aphelion in the case of the sun). The distance between the bodies is then rmax. It is easy to see that
rmin = a(1− e) and rmax = a(1+ e). (8.56)
The significance of the eccentricity e can be seen by computing the ratio of the semimajor and semiminor axes using Eq. (8.55):
ba= 1− e2 , or, e = 1− b
2
a2 . (8.57)
b
a
c
d O
rmin
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Clearly, as e→1, b becomes very small compared to a and the ellipse becomes very elongated. As e approaches zero, the ellipse becomes more circular and is precisely a circle when e = 0. The fact that the planets describe elliptical orbits about the sun is, of course, the content of Kepler’s first law.
Kepler’s second law, which states that the line joining a planet and the sun sweeps out equal areas in equal times, is the result of the conservation of angular momentum (you proved it in chapter 3). The rate of sweeping out area is dA / dt = l / 2µ, where l is the constant angular momentum and µ is the reduced mass. One can use this result and the fact that the area of an ellipse is πab to derive Kepler’s third law, which relates the orbital period to the semimajor axis length:
τ 2 = 4π2µ
αa3. (8.58)
The quantity α is the proportionality constant in the generic attractive inverse-‐square force law F(r) = −α r2 . For the gravitational force, α = Gm1m2 , so that
τ 2 = 4π 2µGm1m2
a3. (8.59)
If the mass of the sun is m1 = Ms and the mass of a planet is m2, then the equation above becomes, to an excellent approximation,
τ 2 = 4π 2
GMs
a3. (8.60)
Finally, one can calculate the total energy of an object in an elliptical orbit in terms of the orbit parameters. Using Eq. (8.51), we find that
E = − α2a, (8.61)
where 2a is the length of the major axis. For the gravitational force, the energy of an object in an elliptical orbit is therefore
E = −Gm1m2
2a. (8.62)
In-‐class Problem: Taylor, problem 8.18.
Unbounded Orbits: Parabolas and Hyperbolas
As seen in the table above, when e ≥1 , the orbit is unbounded. This condition corresponds to E ≥ 0 , i.e., if the total energy is zero or positive, the orbit is unbounded. For e = 1 and E = 0, the orbit is a parabola. Note that in the orbit equation Eq. (8.48), the denominator approaches zero and r approaches infinity when φ → ±π . One can easily show that when e
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= 1 is inserted into the orbit equation and the equation is converted to Cartesian coordinates, the result is y2 = c2 − 2cx , which is the equation of a parabola.
If e > 1 (and E > 0), we see from the orbit equation that r→∞ as φ → ±φmax , where φmax satisfies
ecosφmax = −1. (hyperbolic orbit) (8.63)
Thus, the orbit is confined to the range of φ values −φmax ≤φ ≤φmax. Converting the orbit equation to Cartesian coordinates yields the more readily recognized equation for a hyperbola:
(x −δ )2
γ 2 − y2
β 2 = 1, (8.64)
where the parameters δ , β, and γ depend on semi-‐latus rectum c and the eccentricity e.
6. Orbital Transfers
To send probes to other planets and to asteroids, one needs to be able to change orbits. The probe, while on Earth, is in a nearly circular orbit about the sun. To send it to Mars, say, you will need to transfer the probe to an elliptical orbit (with the sun at one focus) that intersects Mars. Before the probe crashes into Mars, you will have to transfer it to an orbit around Mars. How are such transfers done? The spacecraft fires rockets over a time interval to change its speed and trajectory. We will concentrate on the simplest case, which is when a tangential boost is delivered at the perigee or the apogee.
When a tangential boost is given at the perigee or the apogee, since there is no radial component, the radial speed r will remain zero. Therefore, the new orbit will have its perigee or apogee at that same point. In other words, the orientation of the major axis does not change.
Let us assume the space probe is at the perigee. Let the speed just before firing be v1 and that just after firing be v2 . The thrust factor is defined as
λ = v2v1. (8.65)
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Since the radial distance does not change (assuming the firing is instantaneous), then the angular momenta before and after have the same relationship as the speeds, i.e., l2 = λl1 . It follows that
c2 = λ 2c1. (8.66)
Since rmin is the same before and after, we have
c11+ e1
= c21+ e2
. (8.67)
The new eccentricity can now be calculated using Eq. (8.67). One finds
e2 = λ 2e1 + (λ2 −1). (8.68)
If λ >1 (forward thrust), we see that e2 > e1 . The new orbit is more elongated, i.e., it lies entirely outside the old one (see diagram on previous page). The major axis will be longer and hence the probe has greater total energy in the new orbit (but it is still negative!). If λ <1 (backward thrust), e2 < e1. The orbit is less elongated and lies entirely inside the old one. If λ is smaller still, e2 becomes negative. This means the new orbit has its apogee at the firing point instead of the perigee. Of course, the eccentricity is not really negative; φ = 0 now corresponds to the apogee and φ = π corresponds to the perigee, which is the reverse of the usual scenario and so e2 becomes “negative.” In the typical transfer orbit problem, one is asked to solve for λ . You can do this by determining e1 and e2 and then using Eq. (8.68) to solve for λ . It is useful to note that transfer orbit problems can also be solved by using the conservation of mechanical energy and Eq. (8.62) for elliptical orbits.
In-‐class Problem: Taylor, problem 8.33.