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B Cox and Forshaw: Why E=mc^2 - a proper newbie question

  1. Sep 6, 2016 #1
    Hi All

    My first post here and I'm sure it's going to be one of the easiest for you to answer.

    I've just read Einstein's book on the theory of relativity and now I'm currently reading 'Why does e=mc2 (and why should we care)' by Brian Cox and Jeff Forshaw.

    My Math isn't up to much but I have a inquisitive mind... I'm stumped by something rather basic and early on in this book.

    On page 51/52(ish) - not 100% sure as I'm reading the Kindle version - there's the first bit of maths which seems to be getting to the Lorentz transformations starting from good old Pythagoras.

    The authors get us to T2 = 1/ (c2—υ2) which is all good.

    But then we get this:

    Taking the square root of our equation above for T2, and multiplying by 2, we find that 2T= 2/√c2υv2

    I'm baffled by where the 'v' has come from. I thought it was a problem with the Kindle fonts first of all - the 'u' is printed in italics and the 'v' not, wasn't sure if they were the same thing - but I found a PDF of the book and it's the same in there.

    Can someone walk me through this?


    Last edited: Sep 6, 2016
  2. jcsd
  3. Sep 6, 2016 #2


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    Of what? :smile:
    [added] Oh, never mind... it's the book named in the title of your post. :oops: Carry on.... Unfortunately I've never seen that book. However, I don't recognize the equation you ended up with.
  4. Sep 6, 2016 #3
    Sorry, should have been clearer. I put the title of the book as the thread name and then referred to it in the body as 'this book'.

    The book is: Cox and Forshaw: Why E=mc^2 (and why we should care)

    Thanks for pointing this out - just edited the original post to be clearer.

  5. Sep 6, 2016 #4
    Could you post verbatim what they said? I'd like to be able to derive it myself but I don't know the context.
  6. Sep 6, 2016 #5


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  7. Sep 6, 2016 #6
    I'll try copying and pasting from the PDF. It's a discussion about the light-clock thought experiment...

  8. Sep 6, 2016 #7


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    I can't make any sense of that. But what the answer is supposed to be is:

    [itex]\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}[/itex] where [itex]v[/itex] is the speed of the clock, and [itex]c[/itex] is the speed of light.
  9. Sep 6, 2016 #8


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    Looks like a simple misprint.
  10. Sep 6, 2016 #9


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    It is a misprint. It should look like
  11. Sep 6, 2016 #10
    That's great. Thanks everyone. I got to the same result with my remembered algebra from eons ago but didn't have enough confidence in that to assume it was a misprint. It's been bothering me too much to go further into the book until I'd got to the bottom of it. I can now carry on :)

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