physics

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water at 20°C has viscosity n=1.0*10^-3 Ns/m^2. sand grains have density 2400kg/m^3. suppose a 1.1 mm diameter sand grain is dropped into a 51m deep lake whose water is a constant 20°C. If the sand grain reaches terminal speed almost instantly (a quite good approximation), how long will it take the sand grain to settle to the bottom of the lake?

  • physics -

    Use Stokes' drag equation for the terminal velocity.

    Stoke's equation says, for a sphere in a viscous fluid,
    F=6(pi)RnVc,
    where F is the force, R is the radius of the sphere, n is the viscosity, and V is the velocity through the fluid.
    For the force F, use the weight minus the buoyancy force.

    Divide the lake depth by that velocity.

  • physics -

    Make that last term V, not Vc

  • physics -

    a) Vterm = (mg)/(6pi N R)

    b)V term = ((4/3)pi R^3)pg))/(6pi N R)

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