Which principle explains the phrase "deep water runs still" in fluid dynamics?

Answer: Continuity equation
Explanation

 The continuity equation in fluid dynamics states that the product of cross-sectional area and velocity remains constant for incompressible fluids.

So, in deeper (wider) parts of a river or pipe, the velocity of water is lower — hence the phrase:

"Deep water runs still."

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EVERYDAY SCIENCE (2 times)

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