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Homework Help: How long should she wait?

  1. Jun 16, 2010 #1
    1. The problem statement, all variables and given/known data
    An orchestra wants to have a listener in front of a conductor hear notes at the same time, played by the violinist located next to the conductor and from a drummer located 33m further back.

    a) How long should the violinist wait before playing her note? (assuming drummer plays as soon as conductor gives command)

    b) Why may one assume that the two players see the conductor command at the same time?

    2. Relevant equations

    3. The attempt at a solution

    I really have no clue on how to attempt this, I don't know where to start. I really need your guys help please.
  2. jcsd
  3. Jun 16, 2010 #2
    sound and light both travel at certain speeds. How long does it take for the light to travel to the drummer from the conductor and how long does it take sound to travel to the conductor from the drummer?
  4. Jun 16, 2010 #3
    Would I use the equation speed of light=(frequency)(wavelength) and then solve for frequency? Speed of light = 3x108 and wavelength = 33m?
  5. Jun 16, 2010 #4
    No. What is the basic relationship between time spent traveling, speed, and distance traveled?
  6. Jun 16, 2010 #5
    speed = distance / time
    normal / average speed of sound in air is taken as 330 m / s

    330 = 33 / t

    t = 33 / 330 = 0.1 seconds.

    Did you mean anything like this ???
  7. Jun 16, 2010 #6
  8. Jun 16, 2010 #7
    No. Even though it's against the rules to do peoples' homework, look at the post above you. This problem has nothing to do with frequency.

    You can think of light and sound both traveling in a straight beam at a constant speed. The problem basically asks you to calculate how long it takes that beam of sound to travel from the drummer to the listener. If there is some delay T between the drummer playing and the listener hearing whereas the delay is 0 for the violinist playing and the listener hearing, to synchronize the music, you'd make the violinist wait time T and then start playing. Does that make sense?

    Also, when you do the same calculation for T but with light (the time it takes for the beam of light to travel from the conductor to the drummer, representing the delay between the "go" signal and when the drummer sees it), you will find it is extraordinarily tiny and basically rounds to zero when added to the delay caused by sound. Therefore, its contribution is irrelevant compared to the delay caused by sound.
  9. Jun 17, 2010 #8
    So basically it has nothing to do with any wave equations? It`s more of a kinematics question?

    So light would be:




    So you would just add the two and it would basically be 0.1sec that the violinist should wait since t is so small for light?

    b) They see the same players at the same time because the speed of light is so fast compared with sound?
  10. Jun 17, 2010 #9
    Yes. For b, I wouldn't say they see the signal at the same time. I'd say the order of magnitude in the visual delay is far less than order of magnitude of the delay caused by sound, so you can ignore it without affecting your answer.
  11. Jun 17, 2010 #10
    Alright, thanks alot :D
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