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*From*: "Paul Camp" <pjcamp@coastal.edu>*Date*: Tue, 2 Apr 1996 10:51:37 EST

A spaceship has a length of 200m in its own reference

frame. It is traveling at 0.95c relative to Earth.

Suppose that the tail of the spaceship emits a flash

of light. (a)In the reference frame of the spaceship,

how long does the light take to reach the nose?

(b)In the reference frame of the Earth, how long does

this take? Calculate the time directly from the motions of

the spaceship and the flash of light, and explain

why you cannot obtain the answer by applying the

time-dilation factor to the result from Part (a).

(from Ohanian's Principles of Physics)

I would rather have the

understanding of the problem and work from there.

The crazy thing is, although the book says "explain why you cannot obtain

the answer by applying the time-dilation factor to the result of Part

(a)", that is exactly what you have to do to get the answer they list!?

What's going on here? Am I missing some basic step or do I understand

even less than I previously thought!? Any insight or advice would be

greatly appreciated!

Thanks in advance!

Jonathan Gillis

In any relativity problem, a lot of confusion can be alleviated by

looking at the sequence of events. Clearly identify each emission and

reception event and put them in time order and your problem should

then be trivial. That is also how you go about clearing up most

relativity paradoxes. You can't use time dilation because you aren't

comparing clock rates.

An excellent reference on how to do this sort of thing is Taylor and

Wheeler's Spacetime Physics. If you are going to teach introductory

level relativity, I would highly recommend picking up a copy.

Paul J. Camp "The Beauty of the Universe

Assistant Professor of Physics consists not only of unity

Coastal Carolina University in variety but also of

Conway, SC 29526 variety in unity.

pjcamp@csd1.coastal.edu --Umberto Eco

(803)349-2227 The Name of the Rose

fax: (803)349-2926

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