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Homework Help: Still have some problems

  1. Oct 10, 2005 #1
    Went back and tried these again, still no luck.

    A sound wave travels at a frequency 2.25 MHZ(10^6) through a pregnant woman's abdomen and is reflected from the fetal heart wall of her unborn baby. The heart wall is moving toward the sound receiver as the heart beats. The reflected sound is then mixed with the transmitted sound, and 87.0 beats per second are detected. The speed of sound in body tissue is 1510 m/s. Calculate the speed of the fetal heart wall at the instant this measurement is made.

    I come up with:
    [1510(((2.25*10^6)+87)/(2.25*10^6))]-1510 m/s
    Does anyone see an error in this?

    A certain pipe produces a fundamental frequency of f in air.
    If the pipe is filled with helium at the same temperature, what fundamental frequency does it produce? (Take the molar mass of air to be M_air, and the molar mass of helium to be M_He.)

    I get:
    This is off by a multiplicative factor...
    Does anyone see my error?
  2. jcsd
  3. Oct 11, 2005 #2


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    It seems right to me, but I am always forgetting factors of [itex] 2 \pi[/itex] when dealing with frequencies.

    Gosh, it sure looks right to me. The frequency depends on the speed of sound in the gas, which is proportional to 1/sqrt(mass). Do you suppose that they are looking for an equivalent form like (f*sqrt(M_air/M_He)).

  4. Jan 23, 2008 #3
    Mastering Physics? i understand
    Multiply your final equation by 2
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