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Homework Help: Advanced Doppler effect questions

  1. Nov 26, 2009 #1
    1. The problem statement
    1) A train whistle has a frequency of 1000 Hz. If the train is speeding
    at a velocity of 60 km/h past a stationary railroad crossing
    attendant, what is the apparent frequency a) as the train approaches him, b) as it moves away from him?
    Assume speed of sound to be 350 m/s.

    2) A second train is approaching the first train (above) with a speed of
    60 km/h. Obtain the frequency of the whistle of the first as heard by
    the engineer of the second train.

    3) Sound is reflected by a screen moving with a speed 20 km/h toward
    the source and observer. If the frequency of the source is 450 khz,
    what is the apparent frequency of the reflected sound?

    2. Relevant equations

    f2 = f1Vs/(Vs + or - Vo), f2 = f1(1 + or - Vo/Vs)

    meh I'm really not that quite sure :S

    3. The attempt at a solution

    I could calculate number 1 using the doppler effect equation f2 = f1Vs/(Vs + or - Vo), but I'm not quite sure how to solve for number 2 and 3... help please ?
  2. jcsd
  3. Nov 27, 2009 #2
    i think (not 100% sure) for part 2 and 3 you can use the same method as part 1 except the velocity will be the sum of the 2. so for part 2 the total velocity will be 60 + 60 = 120 km/h
  4. Nov 27, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    What do Vs and Vo mean?

    Look up the Doppler formula for sound. There's a formula for when the source is moving and another for when the observer is moving. Of course you can combine them into a single formula when both are moving.

    No, that's not the right way to solve part 2. Instead you must combine the effect of a moving source (Vs) and a moving observer (Vo) using the complete Doppler formula.

    For part 3, the reflecting screen becomes the source. Apply the Doppler formula twice.
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