Fluid Resistance and Relative Motion
Name: Monica W.
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?
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.
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