# Homework Help: Viscosity of liquid for falling sphere viscometer

1. Feb 19, 2015

### Dennydont

1. The problem statement, all variables and given/known data
A falling sphere viscometer measures the viscosity of a liquid from the terminal velocity of a tiny, falling sphere. One such device determines that a tiny sphere of radius 46 μm falls through a liquid with a terminal velocity of 2.5 mm/s. If the density of the sphere is 4171 kg/m3 and the density of the liquid is 718 kg/m3 what is the shear viscosity of the liquid?
r sphere = 46 μm
ρ sphere = 4171 kg/m^3
ρ liquid = 718 kg/m^3
v = 2.5 mm/s

2. Relevant equations
F = 6πηrv
Continuity: ρAv

3. The attempt at a solution
Would I be able to find the Force by using the equation: F = ρAv2 which is just the continuity equation times velocity?

2. Feb 19, 2015

### Staff: Mentor

No. Have you drawn a free body diagram of the sphere, and identified all the forces acting on the sphere? Based on this, have you written out the force balance equation on the sphere?

Chet

3. Feb 19, 2015

### brainpushups

I haven't crunched the numbers but first, unless you are sure which drag term dominates (viscous or inertial), I would first start by evaluating the Reynolds number. My guess is that the viscous term is dominant and you can simply use Newton's second law with the force

set equal to the sphere's weight (the buoyant force is probably negligible).

4. Feb 19, 2015

### Staff: Mentor

It looks like he was instructed to use the creeping flow (viscous flow) equation. That is the equation he has listed under relevant equations.
The buoyant force is small but not negligible. Compare the densities in the problem formulation.

Chet

5. Feb 19, 2015

### brainpushups

Yes I suppose that an order of magnitude difference is certainly not negligible for the buoyant force in this case.

6. Feb 20, 2015

### Dennydont

I used the buoyancy force equation F = ρgV where V is volume, g is gravity and ρ is the density of the sphere. Set it equal to F = 6πηrv and calculated the viscosity there. This is incorrect, and I realise now that I also need to work the density of the liquid into this equation some way. How would I go about doing this?

7. Feb 20, 2015

### Staff: Mentor

See post #2. Please list the forces acting on the sphere.

Chet