Question

When an object moves through a fluid, the fluid exerts a viscous force F on the object that tends to slow it down. For a small sphere of radius R, moving slowly with a speed v, the magnitude of the viscous force is given by Stokes, law, F = 6πηRv, where η is the viscosity of the fluid. (a) What is the viscous force on a sphere of radius R = 8.9 x 10-4 m falling through water (η = 1.00 x 10-3 Pa·s) when the sphere has a speed of 3.9 m/s? (b) The speed of the falling sphere increases until the viscous force balances the weight of the sphere. Thereafter, no net force acts on the sphere, and it falls with a constant speed called the "terminal speed." If the sphere has a mass of 1.5 x 10-5 kg, what is its terminal speed?

Answer 1