Physics Help Forum Strange question on length contraction

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Jul 30th 2009, 07:42 PM   #1
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Strange question on length contraction

I'm reading a book on relativity and working through the exercises so I don't get too rusty. The following question is rather odd.
 The measuring rod employed by S will appear from S' to be shortened by a factor $\displaystyle (1-u^2/c^2)^{1/2}$. Hence, when S measures the length of the bar fixed in S' he might be expected to obtain the result $\displaystyle l = l'/(1-u^2/c^2)^{1/2}$ This contradicts equation (6.3). Resolve the contradiction (Hint: It will be observed from S' that S fixes the position of the forward end of the bar first and the position of the rear end a time $\displaystyle ul'/c^2$ later.)
where 6.3 is the normal length contraction relation. I don't see why he might be expected to obtain $\displaystyle l = l'/(1-u^2/c^2)^{1/2}$.

Jul 30th 2009, 07:55 PM   #2
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 S will appear from S' to be shortened by a factor (1-u^2/c^2)^{1/2}
$\displaystyle S \sqrt {1 - \frac{u^2}{c^2}} = S'$

$\displaystyle S = \frac{S'}{\sqrt {1 - \frac {u^2}{c^2}}}$

???

I'm not understanding how this is a contradiction?

I haven't taken relativity yet, know some background knowledge, but I don't see any difference between the equation derived and:

Length contraction - Wikipedia, the free encyclopedia
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Jul 30th 2009, 09:09 PM   #3
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 Originally Posted by Deco I'm not understanding how this is a contradiction?
It isn't. I think the author of the book is suggesting a way someone may misinterpret Lorentz contraction. I don't follow what he's saying though. Perhaps because I know relativity.
 Originally Posted by Deco I haven't taken relativity yet, know some background knowledge, but I don't see any difference between the equation derived and: Length contraction - Wikipedia, the free encyclopedia
When talking about Lorentz contraction you have to be very careful to specify in what frame the object is at rest and in what frame the object is moving. I guess the problem is inquiring about a particular misinterpretation.

If you're interested in the relativity I've created my own tutorials on these derivations. You might enjoy working through them.

Try following the proof of the expression for time dilation here
Light Clock

After you have the expression for time dilation worked out you can follow the proof for Lorentz contraction. There are two that you might want to follow.
Lorentz Contraction
Lorentz Contraction – Version 2

I'm always looking for feedback on my web pages so if you read them let me know what you think. Thanks.

Pete

Jul 30th 2009, 09:15 PM   #4
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 When talking about Lorentz contraction you have to be very careful to specify in what frame the object is at rest and in what frame the object is moving.
That was my solution to the contradiction before I re-evaluated it.

 If you're interested in the relativity I've created my own tutorials on these derivations. You might enjoy working through them. Try following the proof of the expression for time dilation here Light Clock After you have the expression for time dilation worked out you can follow the proof for Lorentz contraction. There are two that you might want to follow. Lorentz Contraction Lorentz Contraction – Version 2 I'm always looking for feedback on my web pages so if you read them let me know what you think. Thanks.
I've read a derivation of Lorentz Contraction in a textbook I was look at recently.

So far, it has shown me the simplest (I think) derivation. These websites seem alright though.
__________________
"Dissent is the highest form of patriotism." - Thomas Jefferson.

"Give me control of a nation's money and I care not who makes her laws." -Mayer Amschel Rothschild

I study Mathematical Physics at the
University of Waterloo.

-DC

Jul 30th 2009, 09:26 PM   #5
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 Originally Posted by Deco So far, it has shown me the simplest (I think) derivation.
Interesting. What was the approach used? I'm always looking for new approaches.

 Jul 30th 2009, 09:35 PM #6 Physics Team     Join Date: Jul 2009 Posts: 310 McGraw-Hill Ryerson Canada - School - Physics 12 That's the book that contains (what I think to be) a simple proof. Bad overall textbook though. __________________ "Dissent is the highest form of patriotism." - Thomas Jefferson. "Give me control of a nation's money and I care not who makes her laws." -Mayer Amschel Rothschild I study Mathematical Physics at the University of Waterloo. -DC

 Tags contraction, length, question, strange

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