F is the frictional force  
U is the bouyancy force  
W is the weight of the sphere  
r is the radius of the sphere  
ρ_{s} is the density of the sphere  
v_{T} is the terminal speed of the sphere  
ρ_{f} is the density of the fluid  
η is the coefficient of viscosity of the fluid 
in mm  in mm  in mm  in seconds  v_{T} = h/t in m/s  η in Ns/m^{2} 
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