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Homework Help: What will speed of sound for sourse and observer if both are moving wrt air?

  1. Jan 21, 2012 #1
    wrt means with respect to
    1. The problem statement, all variables and given/known data
    see attachment that contains full question.
    firstly read paragraph then see on question.

    3. The attempt at a solution
    velocity of sound will be 340ms-1 wrt air or ground(since air i snot blowng).
    Now speed of soud wrt train A= speed of sound wrt ground - speed of train A=340-20=320.
    speed of soud wrt trains B= speed of sound wrt ground - speed of train B=340-30=310.
    there is no such option. ????
    where am i doin it wrong.
    waiting for ur valuable reply..

    Attached Files:

  2. jcsd
  3. Jan 21, 2012 #2

    Doc Al

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    Staff: Mentor

    The question seems ambiguous to me. The speed of the sound with respect to what? The air? (I suspect that that's what they want.)
  4. Jan 21, 2012 #3


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    Homework Helper

    When I have been in my car and sounded the horn, the pitch sounded the same whether I was stationary or driving down the highway, so I suspect the pitch and speed of the wave to the passengers on train A will be the same as when stationary, so 340.

    Train B is moving away from train A so the pitch will sound lower. If the sound is travelling slower, relative to a passenger, the pitch might do that.
    I must say I am most uncomfortable with this question referring to the speed of the sound, rather than the pitch of the sound - [though that is varying]

    EDIT: I agree with Doc Al; this is a very poorly worded question.
  5. Jan 21, 2012 #4
    sometimes personal experinces are not as they are in ideal physics. Similar is here.
    After all it's answer is B.
    Now think once again on it.
  6. Jan 21, 2012 #5
    yes that is with respect to air.;
  7. Jan 21, 2012 #6


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    Is that with respect to the air inside the train - the air that is moving along with the passengers - or with respect to the air that is outside the train, seemingly moving past the passengers because it is actually still?
  8. Jan 21, 2012 #7
    sirens of train are usually not placed inside the train. they are placed on the roof of engine.
    When it comes to speed of soud wave it doesn't depend on the frame. It speed is always 340 ms-1 with repect to air. no matter how air is flowing.
  9. Jan 22, 2012 #8


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    Right you are then. So you now realise the answer is D then don't you?

  10. Jan 25, 2012 #9
    Last day i ask this question to my techer and he told me the correct answer. And that is as below.

    since passenger in train A must seated behind the sren so sound of siren will reach till him with velocity -340.(negative for direction opposite to that of train).So relative velocity becomes 340+20=360;;;
    that's answer.
    Good thing about the answer is that, that the concept i apply was correct but i did a mistake.
    I think both of u agree with my answer.
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