Posted by **anna** on Wednesday, November 2, 2011 at 11:16pm.

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 -
**drwls**, Thursday, November 3, 2011 at 12:37am
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 -
**drwls**, Thursday, November 3, 2011 at 12:38am
Make that last term V, not Vc

- physics -
**Mustafa Qader**, Tuesday, February 28, 2012 at 4:42pm
a) Vterm = (mg)/(6pi N R)

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

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