For a 'street science' experiment I approached holidaymakers with a ping-pong ball and a funnel, to show them a classic and counterintuitive science demonstration. It involves something known as 'Bernoulli's principle', which is worth further discussion.
Bernoulli's principle is a description of how gases and liquids (fluids) behave. It says that within a stream of fluid, pressure goes down at the same time as the speed of flow goes up. Similarly, pressure goes up as speed goes down.
But hang on a second, you might say. Surely it's the opposite: think fast streams, like a powerful garden hosepipe, and you think of high pressures.
The key is that Bernoulli's principle is referring to 'static pressure'. That's the pressure you notice building up within a hosepipe when you seal off the end with your thumb. Static pressure pushes equally in all directions, which is why water in the tube will start squirting out from any punctures.
This pressure is quite different from the directional 'dynamic pressure' with which a stream of water knocks over your flowers, after you've removed your thumb from the end of the hosepipe [see Note 1, below].
In fact, Bernoulli's principle tells us that there's exactly the same total pressure at any point in a flowing stream. 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 .
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's what's happening with the ping-pong ball. 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. Since it's going very fast in one direction, the static air pressure must be very low there.
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. That means air pressure is pushing on the ball from all directions, including from the side opposite to where you’re blowing.
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. 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.
What I find particularly counterintuitive is that the ball isn't blown out of the funnel by the stream of air from your mouth. It goes to show that the dynamic pressure of the airstream hitting the ball isn't really that strong, compared to the atmospheric pressure all around us. Maybe the strength of atmospheric pressure shouldn't surprise regular viewers of Bang Goes The Theory: that's what clamps Jem to walls as he climbs tall buildings with his vacuum-gloves.
You sometimes see it said that Bernoulli's principle also explains how wings work. That's actually a much trickier issue, and probably best avoided.
For a start, there's a strong argument that you're better off thinking in terms of the mass of air thrown downwards. Newton's laws of motion describe how you're forced backwards when you throw something heavy forwards. In exactly the same way, aeroplanes are forced up by the vast amount of air that the wings throw down .
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 wing than on the bottom. That pressure difference is due to the way that streams of air are forced to curve around the wing.
Contrary to a common but wrong explanation of aeroplane wings , Bernoulli's principle doesn't explain why there's this difference in pressure - that's down to the shape and orientation of the wing .
Nevertheless, it can be used to visualise what's going on, because it predicts this pressure difference will be associated with air moving faster over the top of the wing than underneath. That difference in speed is relatively easy to see, for example by injecting smoke into the airstream.
However, as one of the experts who sometimes helps us here at Bang says, if you want to explain how wings work "There is no need even to introduce Bernoulli's equation" .
1. I've found Mark Mitchell's animated demonstration of Bernoulli's principle rather useful in picturing what’s going on.
2. There's actually an extra pressure we need to account for once we move away from horizontally flowing streams. Imagine a vertical tube of water: near the bottom there's a greater pressure of water, because of the weight of water above. In liquids, that's called 'hydrostatic pressure'. Gases, however, are so light that this makes very little difference, even when they aren't flowing horizontally. So it's usually safe to ignore this additional effect when discussing streams of air.
3. A Physical Description of Flight; Revisited (PDF), David Anderson and Scott Eberhardt (2009).
4. Theories of lift: Bernoulli and Newton, NASA (Glenn Research Centre) (2008).
5. Theories of lift: Incorrect Theory #1, NASA (Glenn Research Centre) (2008).
6. Physics of Flight - reviewed, Klaus Weltner and Martin Ingelman-Sundberg (1999).
7. How do wings work? (PDF), Holger Babinsky (2003) - Physics Education Vol 38, pages 497-503.
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