Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Doppler Effect Stationary Source/Observer on a Spring

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data
    A microphone is attached to a spring that is suspended from the ceiling. Directly below on the floor is a stationary 540-Hz source of sound. The microphone vibrates up and down in simple harmonic motion with a period of 2.20 s. The difference between the maximum and minimum sound frequencies detected by the microphone is 1.83 Hz. Ignoring any reflections of sound in the room and using 343 m/s for the speed of sound, determine the amplitude (in m) of the simple harmonic motion.

    2. Relevant equations
    f(obs)= f(source) (1-(v(obs)/v))

    3. The attempt at a solution
    When I inquired about help I was told that I need to combine the equations for the max and min frequencies to get the maximum velocity, but I can't figure out how to do that. I am also not sure what to do with the difference of the max and min frequencies that was given in the problem. I have figured the value of ω to be 2.86 rad/s.

    I think I would use the equation v(max)=Aω once I had the max velocity to get the amplitude.

    I feel like this problem should be easier but I just can't figure it out!
  2. jcsd
  3. Jan 29, 2012 #2


    User Avatar
    Homework Helper

    Write your dopplar equation twice, once with positive Vo and again with negative Vo. One gives the maximum frequency observed, the other the minimum, so the difference between the two is your 1.83 Hz. That's your clue to subtract the two equations. I think you will be able to get Vo out of that.

    Regarding the amplitude, I wonder if you have an equation something like
    x = A*sin(ωt) for the position as a function of time. And can differentiate it with respect to time to get a similar equation for the velocity. The two of them would constitute a relationship between the amplitude and the maximum velocity.
  4. Jan 29, 2012 #3
    Thank you so much! I just subtracted the min frequency from the max and set it equal to 1.83 Hz.

    I did, however, use the v(max)=Aω to find the amplitude, but versus time the other equation would have worked better.

  5. Jan 29, 2012 #4


    User Avatar
    Homework Helper

    Most welcome!
    v(max)=Aω comes from differentiating x = A*sin(ωt).
  6. Jan 29, 2012 #5
    Didn't even notice that, I haven't thought about differentiation (or calculus) in a few semesters :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook