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Homework Help: Trajectory of a particle in a spinning fluid

  1. Feb 24, 2017 #1
    1. The problem statement, all variables and given/known data
    I need to show that a particle suspended on a spinning liquid (which is spinning with constant angular velocity) describes a spiral .

    (I need to solve this without using Lagrangian-Hamiltonian formalism)

    2. Relevant equations

    Weight and Bouyant force

    3. The attempt at a solution

    I have tried solving this problem using a rotating frame, and subsequently I've obtained the following equation:

    $$\vec a^{(eff)}= -g(\frac{ \rho_2 - \rho_1}{\rho_1} )\hat k +\omega^2r\hat r -2\omega\dot r{ \hat{ \theta}}$$
    where $\rho_2$ is the liquid's density, $\rho_1$ the particle's density and $\omega$ the constant angular velocity of the liquid.

    Hence I solved the differential equation for the particle's movement along the z-axis, but now I'm stuck I cannot solve the other two equations, I just keep getting complicated expressions and nothing resembling circular motion over the r-$\theta$ plane.

    Any help will be appreciated.
  2. jcsd
  3. Feb 24, 2017 #2


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    Science Advisor
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    Not sure what that means. Is it floating on the surface? Sinking?
    Any particular kind of spiral?
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