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SPEEDING TRAIN w/2 frequencies given while person is standing at rest. Please help.

  1. Dec 2, 2011 #1
    1. A person standing close to a railroad crossing hears the whistle of an approaching train. He notes that the pitch of the whistle drops as the train passes by and moves away from the crossing. The frequency of the distant approaching whistle is 555 Hz; it drops to 472 Hz after the train is well past the crossing. What is the speed of the train? Use 340 m/s for the speed of sound in air.

    HINT: Calculate the ratio of frequency of the whistle before and after the crossing. That ratio does not include the frequency of the train at rest.



    2. i end up with this 472(340/340-Vs)=555(340/340+Vs) but do not know how to solve for Vs. i do not even know if this is set up correct i've tried numerous ways already and this seems most logical.


    3.
     
  2. jcsd
  3. Dec 2, 2011 #2
    Re: SPEEDING TRAIN w/2 frequencies given while person is standing at rest. Please hel

    Can you show the equation(s) you are starting from in order to get that expression?
    You need the basic Doppler formula that includes the frequency of the whistle at rest, together with the speed of sound and the speed of the train. How did you then rearrange and manipulate?
     
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