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Need help on some Questions!

  1. Apr 25, 2007 #1
    1.A string of length L is fixed at the ends with a tension S=300N. Find the string length if the fundamental oscillation freqnency (lowest harmonics) is 784Hz and the mass per unit length is 0.5g/m.

    a. 0.1m
    b. 0.3m
    c. 0.5m
    d. 0.8m

    2.An ambulance car appears to have a frequency of the siren of 600Hz, when approaching an observer. As the ambulance moves away from the observer, the sound appears to have a frequency of 500Hz. FInd the speed of the car. Assume the speed of sound to be 330m/s.

    a. 88km/hour
    b. 98km/hour
    c. 108km/hour
    d. 118km/hour

    i have no idea of what the answers might be and have no equations to work with :(
  2. jcsd
  3. Apr 25, 2007 #2
    the string theory is explained on scienceworld:


    (if you look on wiki you get the correct formula for the frequency, but the derivation looks svery strange)

    You get there the speed of the wave.
    Combined with the frequency you can obtain the wavelength and solve your problem.
    Last edited: Apr 25, 2007
  4. Apr 25, 2007 #3
    well thanks for the link but i dont really understand it at all. i have a cwk with a timelimit on there, and now i only have 20 minutes left :(

    Can anybody provide me with an answer to these 2 questions please
  5. Apr 25, 2007 #4


    User Avatar

    Staff: Mentor

    Thread moved from Advanced Physics to Introductory Physics.

    avfcavfc, we do not provide answers here on the PF. We can provide hints and other tutorial help, but one of the PF rules is that you must show us your work in order for us to help you.

    -1- What is the basic equation relating oscillation frequency, tension and mass density for a string?

    -2- What is the basic equation for the Doppler effect?

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