Viscosity Woes :( Hi, I'm conducting a Physics Investigation where I'm changing the temperature of Motor Oil and seeing how the change in temperature affects viscosity of the oil. Im measuring the terminal velocity of the sphere in the oil (V). I'm Using Stokes' Law Equation: F=6piRNV. I've re-arranged this equation to: n = F/6piRV. I want to calculate the coefficient of viscosity directly, however I can't understand how to calculate the frictional force (F). Can anybody shed some light as to how I calculate this?