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Homework Help: Speed of sound, frames of reference

  1. Jun 23, 2011 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://www.antonine-education.co.uk/Physics%20A%20level/Options/Module_8/Topic_6/cars_4.gif [Broken]

    What is the speed of sound, relative to:
    A -
    B -
    C -

    2. Relevant equations

    None.

    3. The attempt at a solution

    For B, since B is a stationary reference point, the speed of sound relative to B should 330ms^-1

    For A, I thought it should be 360, since A is a moving reference in the same direction as the speed of sound. Meaning A = 330+30=360

    For C, I thought it should be 300, since the moving reference is moving in the opposite direction to the speed of sound, making B=330-30=300

    My answers for A and C are wrong, however. Help!
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jun 23, 2011 #2

    gneill

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    Staff: Mentor

    Imagine that the sound is represented by a runner going at some velocity v. From A's point of view, will the runner be moving more quickly or more slowly?
     
  4. Jun 23, 2011 #3
    Slowly, right...?
     
  5. Jun 23, 2011 #4

    gneill

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    Staff: Mentor

    Right... so... what will be the speed of sound as "seen" by car A?
     
  6. Jun 23, 2011 #5
    300?
     
  7. Jun 23, 2011 #6

    gneill

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    Staff: Mentor

    Yup.

    Now figure out what speed car C "sees".
     
  8. Jun 23, 2011 #7
    360...? Because the sound is towards C so the speed will 'seem' faster?
     
  9. Jun 23, 2011 #8

    gneill

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    Staff: Mentor

    I think you understand it well enough now to answer your own question :wink:
     
  10. Jun 23, 2011 #9
    Lol you're right on that :)
     
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