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Name: Monica W.
Status: Student
Grade: 9-12
Location: PA
Country: United States
Date: January 2007

Hold a stick in a stream flowing with speed V. Then you tow that same stick in the same orientation with speed V through still water. If the relative velocity is the same would the water resistance be the same? Why?

Hi Monica,

The general answer to your question is, it makes no difference whether the stick is stationary and the water is moving, with respect to some relatively distant reference, or whether the reverse is the case. All other things being equal, the resistance the stick sees to water passing by it is governed by the geometry of the stick and the water flow displaced by the stick as a result of their relative motion. The water molecules impinging on the stick have no way of "knowing" if it is the stick that is moving, or the body of water generally in motion.

The above assumes one thing: namely that the stream (when the water is moving) and the stationary body of water (when the stick is moving), are sufficiently wide and deep enough that "edge effects" are not significant. In a moving stream of water, for example, the water closest to the edge and bottom, is flowing slower then that in the main section. If the stick were to extend into the zone of slower moving water, then this would result in erroneous measurements.


Bob Wilson

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