# I Stoke's equation -- particle size

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1. Jun 29, 2017

### Kbaba

I want to solve stokes eq. for particle diameter in um size. Do i have to change the units to make them uniform. please help in solving the eq. by plugging these numbers.
velocity= 50 mm/h
Particle density= 2650 kg/m^3
Medium density= 1000 kg/m^3
viscosity = 1.002 mPa
Gravity= 9.8 m/s^2

2. Jun 29, 2017

### Staff: Mentor

As long as you work with units consistently, you cannot make anything wrong.
Some unit conversions can be necessary to get a nice unit for the result.

3. Jun 30, 2017

### Andy Resnick

Your units of viscosity are incorrect, could/should be mPa*s.

4. Jul 1, 2017

### Kbaba

Thanks for advise. Also, I want to solve the following equation for particle size (d) using stokes eq
V= g (Ps – Pw) d2/18 µ

for unknown variable (d) i rearranged the eq. as follow and took the square root of my answer. Is this the correct way to rearrange and solve this eq for particle size determination. Please, advise.

d= 18 µ V/ g (Ps – Pw)

5. Jul 1, 2017

### Staff: Mentor

Something went wrong with the formatting. I guess you mean this:$$V= \frac {g(P_s-P_w)d^2}{18 \mu}$$
Solved for d:$$d= \sqrt \frac{18 \mu V} {g(P_s-P_w)}$$
Sure.

6. Jul 3, 2017

### NFuller

I would recommend that you use dimensionless variables. It will greatly simplify your calculations if you are planning to find the flow profile.