# Solutions to Problems in Goldstein, Classical Mechanics, Second Edition. Chapter 7

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1 Solutions to Problems in Goldstein, Classical Mechanics, Second Edition Homer Reid April 21, 2002 Chapter 7 Problem 7.2 Obtain the Lorentz transformation in which the velocity is at an infinitesimal angle dθ counterclockwise from the z axis, by means of a similarity transformation applied to Eq. (7-18). Show directly that the resulting matrix is orthogonal and that the inverse matrix is obtained by substituting v for v. We can obtain this transformation by first applying a pure rotation to rotate the z axis into the boost axis, then applying a pure boost along the (new) z axis, and then applying the inverse of the original rotation to bring the z axis back in line with where it was originally. Symbolically we have L = R 1 KR where R is the rotation to achieve the new z axis, and K is the boost along the z axis. Goldstein tells us that the new z axis is to be rotated dθ counterclockise from the original z axis, but he doesn t tell us in which plane, i.e. we know θ but not φ for the new z axis in the unrotated coordinates. We ll assume the z axis is rotated around the x axis, in a sense such that if you re standing on the positive x axis, looking toward the negative x axis, the rotation appears to be counterclockwise, so that the positive z axis is rotated toward the negative y 1

2 Homer Reid s Solutions to Goldstein Problems: Chapter 7 2 axis. Then, using the real metric, L = 0 cos dθ sin dθ sin dθ cos dθ γ βγ βγ γ = = cos dθ sin dθ 0 0 sin dθ cos dθ cos dθ sin dθ 0 0 γ sin dθ γ cos dθ βγ 0 βγ sin dθ βγ cos dθ γ cos 2 dθ + γ sin 2 dθ (γ 1) sin dθ cos dθ βγ sin dθ 0 (γ 1) sin dθ cos dθ sin 2 dθ + γ cos 2 dθ βγ cos dθ 0 βγ sin dθ βγ cos dθ γ cos dθ sin dθ 0 0 sin dθ cos dθ Problem 7.4 A rocket of length l 0 in its rest system is moving with constant speed along the z axis of an inertial system. An observer at the origin observes the apparent length of the rocket at any time by noting the z coordinates that can be seen for the head and tail of the rocket. How does this apparent length vary as the rocket moves from the extreme left of the observer to the extreme right? Let s imagine a coordinate system in which the rocket is at rest and centered at the origin. Then the world lines of the rocket s top and bottom are x t µ = {0, 0, +L 0/2, τ} x b µ = {0, 0, L 0/2, τ}. where we are parameterizing the world lines by the proper time τ. Now, the rest frame of the observer is moving in the negative z direction with speed v = βc relative to the rest frame of the rocket. Transforming the world lines of the rocket s top and bottom to the rest frame of the observer, we have x t µ = {0, 0, γ(l 0/2 + vτ), γ(τ + βl 0 /2c)} (1) x b µ = {0, 0, γ( L 0/2 + vτ), γ(τ βl 0 /2c)}. (2) Now consider the observer. At any time t in his own reference frame, he is receiving light from two events, namely, the top and bottom of the rocket moving past imaginary distance signposts that we pretend to exist up and down the z axis. He sees the top of the rocket lined up with one distance signpost and the bottom of the rocket lined up with another, and from the difference between the two signposts he computes the length of the rocket. Of course, the light that he sees was emitted by the rocket some time in the past, and, moreover, the

3 Homer Reid s Solutions to Goldstein Problems: Chapter 7 3 light signals from the top and bottom of the rocket that the observer receives simultaneously at time t were in fact emitted at different proper times τ in the rocket s rest frame. First consider the light received by the observer at time t 0 coming from the bottom of the rocket. Suppose in the observer s rest frame this light were emitted at time t 0 t, i.e. t seconds before it reaches the observer at the origin; then the rocket bottom was passing through z = c t when it emitted this light. But then the event identified by (z, t) = ( c t, t 0 t ) must lie on the world line of the rocket s bottom, which from (2) determines both t and the proper time τ at which the light was emitted: c t t 0 t = γ( L 0 /2 + vτ) = γ(τ + βl 0 /2c) = τ = ( ) 1/2 1 + β t 0 L 0 1 β 2c τ b (t 0 ). We use the notation τ b (t 0 ) to indicate that this is the proper time at which the bottom of the rocket emits the light that arrives at the observer s origin at the observer s time t 0. At this proper time, from (2), the position of the bottom of the rocket in the observer s reference frame was z b (τ b (t 0 )) = γl 0 /2 + vγτ b (t 0 ) { (1 ) } 1/2 + β = γl 0 /2 + vγ t 0 L 0 1 β 2c (3) Similarly, for the top of the rocket we have τ t (t 0 ) = ( ) 1/2 1 + β t 0 + L 0 1 β 2c and { (1 ) } 1/2 + β z t (τ t (t 0 )) = γl 0 /2 + vγ t 0 + L 0 1 β 2c (4) Subtracting (3) from (4), we have the length for the rocket computed by the observer from his observations at time t 0 in his reference frame: L(t 0 ) = γ(1 + β)l 0 ( ) 1/2 1 + β = L 0. 1 β

4 Homer Reid s Solutions to Goldstein Problems: Chapter 7 4 Problem 7.17 Two particles with rest masses m 1 and m 2 are observed to move along the observer s z axis toward each other with speeds v 1 and v 2, respectively. Upon collision they are observed to coalesce into one particle of rest mass m 3 moving with speed v 3 relative to the observer. Find m 3 and v 3 in terms of m 1, m 2, v 1, and v 2. Would it be possible for the resultant particle to be a photon, that is m 3 = 0, if neither m 1 nor m 2 are zero? Equating the 3rd and 4th components of the initial and final 4-momentum of the system yields Solving the second for m 3 yields γ 1 m 1 v 1 γ 2 m 2 v 2 = γ 3 m 3 v 3 γ 1 m 1 c + γ 2 m 2 c = γ 3 m 3 c m 3 = γ 1 γ 3 m 1 + γ 2 γ 3 m 2 (5) and plugging this into the first yields v 3 in terms of the properties of particles 1 and 2: v 3 = γ 1m 1 v 1 γ 2 m 2 v 2 γ 1 m 1 + γ 2 m 2 Then β 3 = v 3 c = γ 1m 1 β 1 γ 2 m 2 β 2 γ 1 m 1 + γ 2 m 2 1 β 2 3 = γ2 1 m γ 1γ 2 m 1 m 2 + γ 2 2 m2 2 [γ2 1 m2 1 β2 1 + γ2 2 m2 2 β2 2 2γ 1γ 2 m 1 m 2 β 1 β 2 ] (γ 1 m 1 + γ 2 m 2 ) 2 and hence = γ2 1m 2 1(1 β 2 1) + γ 2 2m 2 2(1 β 2 2) + 2γ 1 γ 2 m 1 m 2 (1 β 1 β 2 ) (γ 1 m 1 + γ 2 m 2 ) 2 = m2 1 + m γ 1γ 2 m 1 m 2 (1 β 1 β 2 ) (γ 1 m 1 + γ 2 m 2 ) 2 γ 2 3 = 1 1 β 2 3 = (γ 1 m 1 + γ 2 m 2 ) 2 m m γ 1γ 2 m 1 m 2 (1 β 1 β 2 ). (6) Now, (5) shows that, for m 3 to be zero when either m 1 or m 2 is zero, we must have γ 3 =. That this condition cannot be met for nonzero m 1, m 2 is evident from the denominator of (6), in which all terms are positive (since β 1 β 2 < 1 if m 1 or m 2 is nonzero).

5 Homer Reid s Solutions to Goldstein Problems: Chapter 7 5 Problem 7.19 A meson of mass π comes to rest and disintegrates into a meson of mass µ and a neutrino of zero mass. Show that the kinetic energy of motion of the µ meson (i.e. without the rest mass energy) is (π µ) 2 c 2. 2π Working in the rest frame of the pion, the conservation relations are πc 2 = (µ 2 c 4 + p 2 µc 2 ) 1/2 + p ν c (energy conservation) 0 = p µ + p ν (momentum conservation). From the second of these it follows that the muon and neutrino must have the same momentum, whose magnitude we ll call p. Then the energy conservation relation becomes πc 2 = (µ 2 c 4 + p 2 c 2 ) 1/2 + pc (πc p) 2 = µ 2 c 2 + p 2 Then the total energy of the muon is p = π2 µ 2 c. 2π E µ = (µ 2 c 4 + p 2 c 2 ) 1/2 = c 2 ( µ 2 + (π2 µ 2 ) 2 4π 2 ) 1/2 = c2 ( 4π 2 µ 2 + (π 2 µ 2 ) 2) 1/2 2π = c2 2π (π2 + µ 2 ) Then subtracting out the rest energy to get the kinetic energy, we obtain as advertised. K = E µ µc 2 = c2 2π (π2 + µ 2 ) µc 2 = c2 2π (π2 + µ 2 2πµ) = c2 (π µ)2 2π

6 Homer Reid s Solutions to Goldstein Problems: Chapter 7 6 Problem 7.20 A π + meson of rest mass MeV collides with a neutron (rest mass MeV) stationary in the laboratory system to produce a K + meson (rest mass 494 MeV) and a Λ hyperon (rest mass 1115 MeV). What is the threshold energy for this reaction in the laboratory system? We ll put c = 1 for this problem. The four-momenta of the pion and neutron before the collision are p µ,π = (p π, γ π m π ), p µ,n = (0, m n ) and the squared magnitude of the initial four-momentum is thus p µ,t p µ T = p π 2 + (γ π m π + m n ) 2 = p π 2 + γ 2 π m2 π + m2 n + 2γ πm π m n = m 2 π + m2 n + 2γ πm π m n = (m π + m n ) 2 + 2(γ π 1)m π m n (7) The threshold energy is the energy needed to produce the K and Λ particles at rest in the COM system. In this case the squared magnitude of the fourmomentum of the final system is just (m K + m Λ ) 2, and, by conservation of momentum, this must be equal to the magnitude of the four-momentum of the initial system (7): (m K + m Λ ) 2 = (m π + m n ) 2 + 2(γ π 1)m π m n = γ π = 1 + (m K + m Λ ) 2 (m π + m n ) 2 2m π m n = 6.43 Then the total energy of the pion is T = γ π m π = ( MeV) = 898 MeV, while its kinetic energy is K = T m = 758 MeV. The above appears to be the correct solution to this problem. On the other hand, I first tried to do it a different way, as below. This way yields a different and hence presumably incorrect answer, but I can t figure out why. Can anyone find the mistake? The K and Λ particles must have, between them, the same total momentum in the direction of the original pion s momentum as the original pion had. Of course, the K and Λ may also have momentum in directions transverse to the original pion momentum (if so, their transverse momenta must be equal and opposite). But any transverse momentum just increases the energy of the final system, which increases the energy the initial system must have had to produce the final system. Hence the minimum energy situation is that in which the K and Λ both travel in the direction of the original pion s motion. (This is equivalent to Goldstein s conclusion that, just at threshold, the produced particles are at

7 Homer Reid s Solutions to Goldstein Problems: Chapter 7 7 rest in the COM system). Then the momentum conservation relation becomes simply and the energy conservation relation is (with c = 1) p π = p K + p λ (8) (m 2 π + p 2 π) 1/2 + m n = (m 2 K + p 2 K) 1/2 + (m 2 Λ + p 2 Λ) 1/2. (9) The problem is to find the minimum value of p π that satisfies (9) subject to the constraint (8). To solve this we must first resolve a subquestion: for a given p π, what is the relative allocation of momentum to p K and p Λ that minimizes (9)? Minimizing E f = (m 2 K + p 2 K) 1/2 + (m 2 Λ + p 2 Λ) 1/2. subject to p K + p Λ = p π, we obtain the condition p K (m 2 K + p2 K )1/2 = p Λ (m 2 Λ + p2 Λ )1/2 = p K = m K m Λ p Λ (10) Combining this with (8) yields p Λ = m Λ m K p π p K = p π. (11) m K + m Λ m K + m Λ For a given total momentum p π, the minimum possible energy the final system can have is realized when p π is partitioned between p K and p Λ according to (11). Plugging into (8), the relation defining the threshold momentum is (m 2 π + p2 π )1/2 + m n = (m 2K + ( m K m K + m Λ ) 2 p 2 π) 1/2 + (m 2Λ + ( m Λ m K + m Λ Solving numerically yields p π 655 MeV/c, for a total pion energy of about 670 MeV. ) 2 p 2 π ) 1/2

8 Homer Reid s Solutions to Goldstein Problems: Chapter 7 8 Problem 7.21 A photon may be described classically as a particle of zero mass possessing nevertheless a momentum h/λ = hν/c, and therefore a kinetic energy hν. If the photon collides with an electron of mass m at rest it will be scattered at some angle θ with a new energy hν. Show that the change in energy is related to the scattering angle by the formula λ λ = 2λ c sin 2 θ 2, where λ c = h/mc, known as the Compton wavelength. Show also that the kinetic energy of the recoil motion of the electron is T = hν 2 ( λ c ) λ sin 2 θ ( λ c ) λ sin 2 θ/2. Let s assume the photon is initially travelling along the z axis. Then the sum of the initial photon and electron four-momenta is p µ,i = p µ,γ + p µ,e = 0 h/λ = 0 h/λ. (12) h/λ mc mc + h/λ Without loss of generality we may assume that the photon and electron move in the xz plane after the scatter. If the photon s velocity makes an angle θ with the z axis, while the electron s velocity makes an angle φ, the four-momentum after the collision is p µ,f = p µ,γ + p µ,e = (h/λ ) sin θ 0 (h/λ ) cos θ h/λ + p e sin φ 0 p e cos φ m2 c 2 + p 2 e Equating (12) and (13) yields three separate equations: = (h/λ ) sin θ + p e sin φ 0 (h/λ ) cos θ + p e cos φ (h/λ ) + m 2 c 2 + p 2 e (13) (h/λ ) sin θ + p e sin φ = 0 (14) (h/λ ) cos θ + p e cos φ = h/λ (15) h/λ + m 2 c 2 + p 2 e = mc + h/λ (16) From the first of these we find [ sin φ = h ( ) 2 1/2 h λ sin θ = cos φ = 1 + p e λ sin θ] 2 p e.

9 Homer Reid s Solutions to Goldstein Problems: Chapter 7 9 and plugging this into (15) we find On the other hand, we can solve (16) to obtain p 2 e = h2 λ 2 + h2 h2 2 cos θ. (17) λ 2 λλ ( 1 p 2 e = h2 λ 1 ) 2 ( 1 λ + 2mch λ 1 ) λ. Comparing these two determinations of p e yields or cos θ = 1 mc h (λ λ) sin 2 θ 2 = mc 2h (λ λ) = 1 2λ c (λ λ) so this is advertised result number 1. Next, to find the kinetic energy of the electron after the collision, we can write the conservation of energy equation in a slightly different form: where we put χ = λ c /λ. mc + h λ = γmc + h ( λ 1 = (γ 1)mc = K = h λ 1 ) λ ( λ ) λ = h λλ ( 2λ c sin 2 ) (θ/2) = h λ[λ + 2λ c sin 2 (θ/2)] = h ( 2χ sin 2 ) (θ/2) λ 1 + 2χ sin 2 (θ/2) Problem 7.22 A photon of energy E collides at angle θ with another photon of energy E. Prove that the minimum value of E permitting formation of a pair of particles of mass m is 2m 2 c 4 E th = E(1 cos θ). We ll suppose the photon of energy E is traveling along the positive z axis, while that with energy E is traveling in the xz plane (i.e., its velocity has

10 Homer Reid s Solutions to Goldstein Problems: Chapter 7 10 spherical polar angles θ and φ = 0). Then the 4-momenta are ( p 1 = 0, 0, E c, E ) c ( E p 2 = c sin θ, 0, E c cos θ, E ) c ( E E + E cos θ p t = p 1 + p 2 = sin θ, 0,, E + E ) c c c It s convenient to rotate our reference frame to one in which the space portion of the composite four-momentum of the two photons is all along the z direction. In this frame the total four-momentum is p t = ( 0, 0, 1 c E2 + E 2 + 2EE cos θ, E + E c ). (18) At threshold energy, the two produced particles have the same four-momenta: ( p 3 = p 4 = 0, 0, p, (m 2 c 2 + p 2 ) 1/2) (19) and 4-momentum conservation requires that twice (19) add up to (18), which yields two conditions: 2p = 1 c E2 + E 2 + 2EE cos θ p 2 c 2 = 1 4 (E2 + E 2 + 2EE cos θ) 2 m 2 c 2 + p 2 = E+E c m 2 c 4 + p 2 c 2 = 1 4 (E2 + E 2 + 2EE) Subtracting the first of these from the second, we obtain or m 2 c 4 = EE (1 cos θ) 2 E = 2m 2 c 4 E(1 cos θ) as advertised.

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