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Exercise combining Wave and Hydrostatic: Pulley and Submerged Ball

  1. Feb 28, 2013 #1
    1. The problem statement, all variables and given/known data
    One end of a horizontal string is attached to the wall, and the other end passes over a pulley. A sphere of an unknown material hangs on the end of the string. The string is vibrating with a frequency of 392 cycles per second. A container of water is raised under the sphere so that the sphere is completely submerged. In this configuration, the string vibrates with a frequency of 343 cycles per second.
    What is the density of the sphere?
    2. Relevant equations
    [itex]v=\lambda * f[/itex]
    [itex]F=W-B=\rho*V*g - \rho_{H20}V_{i}g[/itex] (assuming W>B)
    3. The attempt at a solution
    Wave Perspective:
    [itex]T_{1} = \rho * \lambda_{1}^{2} * f_{1}^{2}[/itex]
    [itex]T_{2} = \rho * \lambda_{2}^{2} * f_{2}^{2}[/itex]
    Mechanical Perspective:
    [itex]T_{1} = W = \rho*Vg[/itex]
    [itex]T_{2} = W - B = \rho*Vg - \rho_{H20}V_{i}g[/itex]
    [itex]T_{1} = W[/itex] then, assuming displaced volume = volume of the sphere,
    [itex] V_{i}g = \lambda_{1}^{2} * f_{1}^{2} [/itex]
    [itex]T_{2} = W - B = \rho*\lambda_{1}^{2} * f_{1}^{2} - \rho_{H20}*\lambda_{1}^{2} * f_{1}^{2}[/itex]

    Now, I have arrived to the answer in the solutions (it's the right answer, but negative, I can't explain that either), but I had to assume that [itex]\lambda_{1}=\lambda_{2}[/itex]. Can someone explain me , if frequencies are different, why should I assume the wavelenght is the same before and after submerging the ball?

    For those who want the answer:
    4267 Kg m-3, which i obtained from [itex]T_{2}=T_{2} \Leftrightarrow \rho * \lambda^{2} * f_{2}^{2} = \rho\lambda^{2} * f_{1}^{2} - \rho_{H20}\lambda^{2} * f_{1}^{2}[/itex]
    thus [itex]\rho = (-\rho_{H20}*f_{1}^{2})/ (f_{2}^{2} - f_{1}^{2})[/itex]
    (The formula gives me a negative result though).
    Last edited: Feb 28, 2013
  2. jcsd
  3. Feb 28, 2013 #2


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    Hello, ricmacas. Welcome to PF!

    The equation for the speed of a wave on a string is ##v = \sqrt{T/\mu}## where ##\mu## is the mass per unit length of the string. So, you'll need to rethink the mathematical steps.

    In order to see why the wavelength should be the same, keep in mind that the standing waves must have a node at the wall and a node at the pulley.
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