# Where does this equation come from?

I am investigating about viscosity of fluids and I found the following equation to calculate the viscosity: $$\nu = \frac{2(\rho_s-\rho_l)\cdot g \cdot r^{2}}{9\cdot\Delta v}$$

The experiment consists of having a tube with whatever liquid you would like to measure and drop a ball and see at which speed it reaches the bottom.

$$\rho_s$$ is the density of the sphere, $$\rho_l$$ the density of the liquid, $$g$$ the acceleration of gravity, $$v$$ the final velocity and $$r$$ is the radius of the ball.

The problem is, I really do not know where this equation comes from nor do I know if it is correct.

• Sounds like a job for Stokes' law. – Qmechanic Dec 27 '18 at 13:19
• This comes from applying Stokes law and Archemides principal to determine the terminal velocity of a sphere falling through a fluid. – Chet Miller Dec 27 '18 at 14:36