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 layers of a fluid as they slide over each other. Honey has a high viscosity and water a low one. The viscosity of a liquid falls when it is heated. A body moving through a fluid feels a retarding force called viscous drag, which grows with speed.
When a small sphere is dropped in a fluid it speeds up at first. The drag increases until it equals the weight. The net force is then zero, and the sphere falls at a constant speed called the terminal velocity.
In streamline (laminar) flow every particle passing a point follows the same path with the same velocity. Above a certain speed the flow becomes turbulent, with irregular whirls and eddies.
Drag depends on r but terminal velocity depends on r2. Doubling the radius doubles the drag at the same speed, but makes the terminal velocity four times as large.
Answer: About 1.9 × 10−3 N
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