A law that predicts the frictional force F on a spherical ball moving through a viscous medium. According to this law F=6πrηv, where r is the radius of the ball, v is its velocity, and η is the viscosity of the medium. The sphere accelerates until it reaches a steady terminal speed. For a falling ball, F is equal to the gravitational force on the sphere, less any upthrust. The law was discovered by Sir George Gabriel Stokes in 1851.