Poiseuille's Law

[rddimtbo]

Trouble with a problem. What is the flow rate if a tube consists of two sections, the first with a length of 20cm and radius 0.15cm and the second part with length 1.0cm radius 0.05? with a pressure difference across the entire tube of 3cmHg (p1-p3). (viscosity = 0.801 cP)

I've tried this and end up with two many unknowns and not enough equations. Can anybody help?

[upsidedown]

\dot V = \frac{\pi \, r_1^4}{8\,\eta}\frac{p_3-p_2}{l_{32}}
\dot V = \frac{\pi \, r_2^4}{8\,\eta}\frac{p_2-p_1}{l_{21}}
Well, just solve this for p2 und dV/dt and you have your result :)