According to Stokes's law, the drag force on a small sphere moving slowly through a viscous fluid is proportional to
Correct answer: B. the speed
- 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.
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About Fluid Drag
Fluid drag is the resistive force exerted by a liquid or gas on an object moving through it, and its value depends on speed, fluid density, shape and exposed area. Questions examine drag at low and high speeds, terminal velocity when drag balances weight, and the difference between drag and ordinary solid friction.
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