Chapter 12: Q49P (page 564)
Work out the remaining five parts to Eq. 12.118.
Short Answer
All the remaining five parts to equation 12.118 are proved.
Chapter 12: Q49P (page 564)
Work out the remaining five parts to Eq. 12.118.
All the remaining five parts to equation 12.118 are proved.
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Get started for freeShow that the Liénard-Wiechert potentials (Eqs. 10.46 and 10.47) can be expressed in relativistic notation as
Every years, more or less, The New York Times publishes an article in which some astronomer claims to have found an object traveling faster than the speed of light. Many of these reports result from a failure to distinguish what is seen from what is observed—that is, from a failure to account for light travel time. Here’s an example: A star is traveling with speed v at an angleto the line of sight (Fig. 12.6). What is its apparent speed across the sky? (Suppose the light signal fromb reaches the earth at a timelocalid="1656138453956" after the signal from a, and the star has meanwhile advanced a distancelocalid="1656138461523" across the celestial sphere; by “apparent speed,” I meanlocalid="1656138468709" . What anglelocalid="1656140989446" gives the maximum apparent speed? Show that the apparent speed can be much greater than c, even if v itself is less than c.
Consider a particle in hyperbolic motion,
(a) Find the proper time role="math" localid="1654682576730" as a function of , assuming the clocks are set so that when . [Hint: Integrate Eq. 12.37.]
(b) Find x and v (ordinary velocity) as functions of .
(c) Find (proper velocity) as a function of .
The twin paradox revisited. On their birthday, one twin gets on a moving sidewalk, which carries her out to star X at speed ; her twin brother stays home. When the traveling twin gets to star X, she immediately jumps onto the returning moving sidewalk and comes back to earth, again at speed . She arrives on her birthday (as determined by her watch).
(a) How old is her twin brother?
(b) How far away is star X? (Give your answer in light years.) Call the outbound sidewalk system and the inbound one (the earth system is S). All three systems choose their coordinates and set their master clocks such that at the moment of departure.
(c) What are the coordinates of the jump (from outbound to inbound sidewalk) in S?
(d) What are the coordinates of the jump in ?
(e) What are the coordinates of the jump in ?
(f) If the traveling twin wants her watch to agree with the clock in , how must she reset it immediately after the jump? What does her watch then read when she gets home? (This wouldn’t change her age, of course—she’s still —it would just make her watch agree with the standard synchronization in .)
(g) If the traveling twin is asked the question, “How old is your brother right now?”, what is the correct reply (i) just before she makes the jump, (ii) just after she makes the jump? (Nothing dramatic happens to her brother during the split second between (i) and (ii), of course; what does change abruptly is his sister’s notion of what “right now, back home” means.)
(h) How many earth years does the return trip take? Add this to (ii) from (g) to determine how old she expects him to be at their reunion. Compare your answer to (a).
As an illustration of the principle of relativity in classical mechanics, consider the following generic collision: In inertial frame S, particle A (mass, velocity ) hits particle B (mass, velocity ). In the course of the collision some mass rubs off A and onto B, and we are left with particles C (mass, velocity ) and D (mass , velocity ). Assume that momentum is conserved in S.
(a) Prove that momentum is also conserved in inertial frame, which moves with velocity relative to S. [Use Galileo’s velocity addition rule—this is an entirely classical calculation. What must you assume about mass?]
(b) Suppose the collision is elastic in S; show that it is also elastic in .
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