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A relativistic fly flies at 0.7c in the same direction as a car traveling at 0.8c. According to the driver of the car, how quickly will the fly approach the car?

To solve this problem, I applied the relativistic velocity addition formula: $$ u' = \frac{u - v}{1 - \frac{uv}{c^2}} $$

where u = 0.7c and v = 0.8c. The answer that I get is 0.23c. however, the answer in my book is 0.96c. I was wondering which answer is correct, as I think the book's answer could be wrong.

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    $\begingroup$ Can you edit the name of the textbook into your question? $\endgroup$
    – PM 2Ring
    Commented Oct 11 at 11:04
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    $\begingroup$ The question seems a bit ambiguous, since the fly (travelling at 0.7 c), won't approach the car (travelling at 0.8 c), indeed don't you get -0.23 c? $\endgroup$
    – jim
    Commented Oct 11 at 12:38

1 Answer 1

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The book's answer is wrong. This is the relative speed for when they are flying in opposite directions.

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