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Homework Help: Doppler effect and frequency of the horn

  1. Nov 30, 2008 #1
    John is listening to a horn. He knows the frequency of the horn is 300 Hz when both he and the horn are at rest. If he hears a pitch of 330 Hz, there are clearly several possibilities. (Give ALL correct answers, i.e., B, AC, BCD...)

    A) Both can be moving and have the same speed.
    B) Both can be moving and have different speeds.
    C) John is moving towards the horn at rest.
    D) The distance between John and the horn is increasing with time.
    E) Both cannot be moving in the same direction.
    F) Both can be moving, in opposite directions.

    I select B,C,F and get the wrong answer.... why?... can anyone help me?...

    is the pitch inverse to the frecuecy or proportional?...
  2. jcsd
  3. Nov 30, 2008 #2
    The pitch is proportional to frequency, so we want to have the cases where John hears a higher frequency than the actual frequency. When does that happen?
  4. Nov 30, 2008 #3
    Imagine yourself standing right next to a moving train, does the horn sound high pitched when you are close to train or away from train. Just turn on some music and move your head towards and away from the source of music. You'll get your answer then.
    And btw, frequency and wavelength are inversely proportional.
  5. Nov 30, 2008 #4
    ok, when both are moving at the same speed, it can't be, but when moving with diferrent speed iti coulb be, when john is moving towards the horn the pitch is going to be going up so it could be, when the distance betwenn them is increasing the frequency is going down so it is not an option, when both are moving in the same or oposite direction it depends on their velocities right?...

    so i know that the answer B and C are correct but i dont know if it is also the E or the F
  6. Nov 30, 2008 #5
    Right then, notice that F implies D... if two bodies are moving in opposite directions, their distance must increase. As for E, chew on this... exactly what determines the frequency in the Doppler effect, relative velocity or direction of travel?
  7. Nov 30, 2008 #6
    so the correect answer is BCE.... right?..those are the three posibilities?...
  8. Nov 30, 2008 #7
    May I ask for your justification for option E?
  9. Nov 30, 2008 #8
    direction on travel.... if the source and observer moves towards the frequency goes up but if the source and the observer go away the frequency decrease... right?...
  10. Nov 30, 2008 #9
    Ah, I see where your confusion lies... actually it's not as simple as saying that it is determine by them moving towards each other or otherwise.

    Doppler's effect is actually determined by the relative speeds of the source and listener. If the listener is approaching the source with respect to the frame of the source, then the frequency heard by the listener increases. This could happen in two cases... can you identify them?
  11. Nov 30, 2008 #10
    i really dont understand... too much but my answer B and C are correct, but there is any other option that is correct too?... which is?...
  12. Nov 30, 2008 #11
    B and C are definitely correct. If this is a web assignment, check the formatting for your initial answer B, C and F, which should be inputted as BCF. F can hold true, if the source and the listener are approaching each other (I made a mistake earlier regarding the F implying D, I'm so sorry). My answer would be BCF, then , your original answer, but if the formatting was correct, then try BC only.
  13. Nov 30, 2008 #12
    i have tried BCF, BC, and give me the wrong answers... i also tried the BCEF and is still wrong.. it shuold have another posibility that we are not thinking about...
  14. Nov 30, 2008 #13
    Oh dear, I think I finally got it. Option A says same SPEED. Yes, they can have the same speed but different velocities, i.e they can be moving towards each other at the same speed. I completely missed that point. ABCF... that should be it. I'm so sorry I missed that out completely.
  15. Nov 30, 2008 #14
    ok thanks that was the correct answer but what is the diference betwenn speed and velocity?..
  16. Nov 30, 2008 #15
    Speed is a scalar, meaning that it is only the magnitude, regardless of direction. Two objects may have the same speed but may move in two completely different directions, i.e. possibly towards each other or away from each other or any other combination.

    Velocity is a vector, meaning that it has both magnitude and direction... Two objects of the same velocity must have the same speed and same direction, and their seperation must be constant.
  17. Nov 30, 2008 #16
    oh ok thanks...a lot
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