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.
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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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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鈥攖hat is, from a failure to account for light travel time. Here鈥檚 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 鈥渁pparent speed,鈥 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.
Question: Two charges approach the origin at constant velocity from opposite directions along the axis. They collide and stick together, forming a neutral particle at rest. Sketch the electric field before and shortly after the collision (remember that electromagnetic "news" travels at the speed of light). How would you interpret the field after the collision, physically?
A car is traveling along the line in S (Fig. 12.25), at (ordinary) speedc .
(a) Find the components Ux and Uyof the (ordinary) velocity.
(b) Find the components andof the proper velocity.
(c) Find the zeroth component of the 4-velocity, .
System is moving in the x direction with (ordinary) speed , relative to S. By using the appropriate transformation laws:
(d) Find the (ordinary) velocity components andin .
(e) Find the proper velocity components in .
(f) As a consistency check, verify that
You may have noticed that the four-dimensional gradient operator functions like a covariant 4-vector鈥攊n fact, it is often written , for short. For instance, the continuity equation, , has the form of an invariant product of two vectors. The corresponding contravariant gradient would be . Prove that is a (contravariant) 4-vector, if is a scalar function, by working out its transformation law, using the chain rule.
Solve Eqs. 12.18 forin terms of and check that you recover Eqs. 12.19.
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