That pressure difference is due to the way that streams of air are forced to curve aroundĬontrary to a common but wrong explanation of aeroplane wings, Bernoulli's principle doesn't explain It's true that this can also be seen in terms of air pressure : there's much lower air pressure on the top of a In exactly the same way, aeroplanes are forced up by the vast amount of air that the wings throw down. Newton's laws of motion describe how you're forced backwards when you throw something heavy forwards. That's actually a much trickier issue, and probably best avoided.įor a start, there's a strong argument that you're better off thinking in terms of the mass of air thrown downwards. You sometimes see it said that Bernoulli's principle also explains how wings work. Regular viewers of Bang Goes The Theory: that's what clamps Jem to walls as he climbs tall buildings with his Maybe the strength of atmospheric pressure shouldn't surprise It goes to show that the dynamic pressure of the airstream hitting the ball isn't really that strong,Ĭompared to the atmospheric pressure all around us.
What I find particularly counterintuitive is that the ball isn't blown out of the funnel by the stream of air from In fact, as you turn the funnel upside-down, you'll find that pressure difference is even strong enough to hold the ball against gravity. It's this strong outside air pressure, compared to the lower pressure around the constriction, which is holding the ball in place as you blow. That means air pressure is pushing on the ball from all directions, including from the side opposite to where you’re blowing. 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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