MDCAT Physics · Physics · Fluid dynamics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Viscosity is the friction between the layers of a fluid as they slide over one another. A body moving through a fluid feels a resistive force called drag, which grows as the speed grows. For a small sphere moving slowly, Stokes' law gives the drag.
A body falling through a fluid speeds up at first. The drag rises until it balances the weight. The net force is then zero and the body falls at a constant speed, the terminal velocity.
In laminar (streamline) flow every particle passing a point follows the same smooth path, and streamlines never cross. Above a certain speed the flow becomes turbulent: irregular, with eddies, and with much more drag.
Students say no forces act at terminal velocity, or that the body stops. The forces are balanced, so the body keeps moving at its greatest, constant speed.
Answer: 4.8 cm s−1.
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