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 Question 119

 Why is high pressure air/fluids slow moving, while low-pressure fluids/air are fast moving?
First, I should point out that high pressure air/fluids can move either fast or slow, depending on the situation. The same holds for low-pressure air/fluids. What Bernoulli's equation tells us is that when air/fluids slows down, its pressure rises (assuming that it isn't moving up or down so that gravity is out of the picture) and when air/fluids speed up, its pressure drops. Here are two common examples.

First, when you spray water from a garden hose against your hand, the water goes from moving quickly through the air at atmospheric pressure to moving slowly on your hand at more than atmospheric pressure. You know that this pressure increase has occurred because you feel the water pushing hard on your hand. The water is exchanging kinetic energy for pressure potential energy and its pressure is rising.

Second, when you put your thumb over the end of the garden hose and allow only a fine spray to emerge, the water goes from slow moving water at high pressure inside the hose to fast moving water at atmospheric pressure in the air. You know that this pressure drop has occurred because you feel the water in the hose pushing hard against your thumb. The water is exchanging pressure potential energy for kinetic energy and its pressure is dropping.