According to Stokes's law, the drag force on a small sphere moving slowly through a viscous fluid is proportional to
- A. the square of the speed
- B. the speed
- C. the cube of the radius
- D. the inverse of the radius
Explanation
Stokes's law gives the drag as 6 pi eta r v, so it rises in direct proportion to both the radius and the speed, provided the flow stays laminar. At higher speeds the flow becomes turbulent and drag grows roughly with the square of the speed instead. The law is what allows viscosity to be measured by timing a falling ball.