The ping-pong ball sticks in the funnel because, unlike when it's hit from behind by a stream of water, the fluid that's hitting it (air) is on all sides of the ball too. Since it's going very fast in one direction, the static air pressure must be very low there. In the narrow constriction between the ball and the funnel, the air's going very fast - just like water does when you constrict its flow by covering only a bit of the hosepipe end with your thumb. That's what's happening with the ping-pong ball. If you observe a stream of air suddenly speed up, it must be that the static 'air pressure' is going down, from higher at the start to lower where the air's going fast. That means if the dynamic 'forward-streaming' pressure (ie the speed) goes down, the static 'sideways-pushing' pressure must increase to make up for it. In fact, Bernoulli's principle tells us that there's exactly the same total pressure at any point in a flowing stream. Your flowers, after you've removed your thumb from the end of the hosepipe. This pressure is quite different from the directional 'dynamic pressure' with which a stream of water knocks over Pressure pushes equally in all directions, which is why water in the tube will start squirting out from any punctures. That's the pressure you notice building up within a hosepipe when you seal off the end with your thumb. The key is that Bernoulli's principle is referring to 'static pressure'. Hosepipe, and you think of high pressures. Surely it's the opposite: think fast streams, like a powerful garden Similarly, pressure goes up as speed goes down.īut hang on a second, you might say. It says that within a stream ofįluid, pressure goes down at the same time as the speed of flow goes up. Bernoulli's principle is a description of how gases and liquids (fluids) behave.
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