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Methods of Proofs

  1. May 4, 2010 #1
    Hi there,

    I have some exams later this month, and some of the previous exam questions are to prove a formula given another formula fx here with EM doppler shift:

    define ratio: r= f/ f0
    using relativistic doppler frequency for EM: f = square root of: ((c+v) / (c-v)) * f0


    v/c = (r^2 - 1) / (r^2 +1)

    Are there any general methods or ways to go about such a question as there are quite a few of them and i find it hard to know where to start, i usually try and rearrange and substitute into each other using the equations given, but never seem to get them right... Please help me!
  2. jcsd
  3. May 4, 2010 #2


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    So if you plug the equation for f into the equation for r, you will get a direct relation between r and f.

    To show the identity, it is probably easiest to substitute for the variable which you have isolated, i.e. calculate (r2 - 1) / (r2 + 1) and show that you get v/c.
    That is usually easier than trying to rework the equation for r to an equation for v.
  4. May 4, 2010 #3


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    You wont be able to show that equation because it's wrong. In general though the answer to proving something like that is just algebra, algebra and practice.

    You've got ((c+v) / (c-v)) = r and you want to find v/c, so start by dividing num and denom on the LHS by c. This gives you,

    [tex] \frac{1+v/c}{1-v/c} = r [/tex]

    Straight away it looks much easier to handle, you've now got an equation with just got one variable (v/c) to isolate. From this point onward we will keep all "v/c" terms together as if they were just one variable.

    So now just mulitply by (1-v/c) and collect the v/c terms.

    [tex] 1+v/c =r - r v/c [/tex]

    [tex] (1+r) v/c =r - 1 [/tex]

    [tex] v/c = \frac{r-1}{r+1}[/tex]
    Last edited: May 4, 2010
  5. May 4, 2010 #4


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    BTW. I should add. This is a maths question pure and simple. The equation chosen was motivated by physics but this is not really a physics question.
    Last edited: May 4, 2010
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