1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Relativistic Doppler Effect and a Baseball

  1. May 8, 2008 #1


    User Avatar

    [SOLVED] Relativistic Doppler Effect and a Baseball

    1. The problem statement, all variables and given/known data

    A baseball coach uses a radar device to measure the speed of an approaching pitched baseball. This device sends out electromagnetic waves with frequency [tex] f_0 [/tex] and then measures the shift in frequency [tex] \Delta f [/tex] of the waves reflected from the moving baseball.

    If the fractional frequency shift produced by a baseball is [tex]\frac{\Delta f}{f_0}[/tex] 2.88×10−7, what is the baseball's speed? (Hint: Are the waves Doppler-shifted a second time when reflected off the ball?)

    2. Relevant equations

    [tex] u = \frac{c((\frac{\delta f}{f_0})^2) - 1}{\frac{\delta f}{f_0})^2 + 1} [/tex]

    3. The attempt at a solution

    I tried putting the variables in, but becasue there is a double doppler shift, the asnwer was incorrect. Was is the best way to do this question when a Double Shift occurs?

    Any ideas gratly appreciated,


    Edit: in the formulas, that should be a big Delta not a small Delta, Sorry
  2. jcsd
  3. May 8, 2008 #2
    If the DELf given is actually twice the doppler shift we're interested in... then what DELf should we use?
  4. May 8, 2008 #3


    User Avatar

    Would it be half?

  5. May 8, 2008 #4
  6. May 8, 2008 #5


    User Avatar

    When I enter it into the equation, it just seems to spit out the speed of Light...?

  7. May 8, 2008 #6
    You're equation might not be quite right...

    namely: where you have Delf / f; i think it should be (observed f) / (source f).
    Del f / f = (source f - observed f) / (source f)

    .. try it the other?
  8. May 8, 2008 #7


    User Avatar

    I was using the wrong formula :bugeye:

    The equation to use is:

    [tex] \frac{\Delta f}{f_0} = \frac{u}{c} [/tex]

    Thanks for the assistance, lzkelley, :smile:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook