Cool air

19th September 2003 at 01:00
Wendy Sadler explores the mysteries of flight and the Bernoulli effect.

While sitting on a plane in those few moments before takeoff you might have wondered how those hundreds of tonnes of metal are ever going to get off the ground and then stay up.

Closer to home, you might also have wondered why your shower curtain is instantly attracted to you when you turn on the shower. You may be surprised to learn that both are caused by the same phenomenon, and there are plenty of ways you can explore it in class.

This phenomenon is the Bernoulli effect, named after the Dutch mathematician Daniel Bernoulli (1700-82). Understanding the Bernoulli effect lies behind explaining key stage 2 physical processes, key stage 3 forces and motion, key stage 4 flight and forces, and A-level physics (some syllabuses).

You may have seen a popular exhibit in science centres, where a beach ball hangs mysteriously suspended in a jet of air. The behaviour of the ball intrigues visitors of all ages; it can be re-created using a hair-dryer and a ping-pong ball.

A ping-pong ball is quite light but gravity is still pulling it to the floor. Make sure your students understand that gravity acts on everything by the same amount, regardless of size. If it weren't for air resistance, all objects would fall at the same speed. If the ball is to defy gravity and become airborne, it needs another force acting in the opposite direction.

It won't surprise too many students to see that if you point the hair-dryer straight up and put the ping-pong ball on the jet you can get the ball to remain stationary in mid-air. The force of the air pushing upwards equals the weight of the ball due to gravity.

Now try something else. Tilt the jet of air very slowly to the side and you should find that the ping-pong ball begins to stick out at an angle as well. This is a little more surprising.

Common sense suggests it should drop to the ground or fly off in the direction of the air-jet, but instead it hovers at an angle. There is now a third force acting on the ball called lift which is partly generated by this thing called the Bernoulli effect. When you tilt the hair-dryer, the ball begins to fall out of the air-stream until it gets to the point when only the top of the ball is sitting in the air-stream. When this happens you get fast-moving air hitting the front of the curved surface and speeding up over the top of the ball.

Bernoulli's law says fast-moving air has lower pressure, but this can be a confusing concept. After all, if you blow straight on to your hand, you've got fast-moving air and you can feel an increased pressure. The key difference is the direction of the pressure. When you had the ball sitting in the jet-stream straight upwards, the fast-moving air was hitting the ball and bouncing off, creating a higher pressure that held the ball in the air against gravity.

But when fast-moving air moves over the top of a surface, it can't push against the sides as much, so the pressure on the top of the ball drops. To visualise what is happening, imagine you are running down a corridor with lots of doors that you need to knock on. If you walk slowly you can knock on all the doors but if you run very fast you would hardly have time to knock (or push against) any of the doors.

This is what's happening with the fast-moving air. When it isn't moving very fast, the air pushes out against things in all directions, but if it is made to move fast in one direction it can't push out to the sides as much, so the pressure drops.

The most simple way of showing this is a famous demonstration where you hold a piece of paper (a page from Yellow Pages works well as it is light) so that it flops over the back of your hands, creating a curve, and then blow over the top of your hands. You should be able to get the paper to lift. The lift is created by the air underneath the paper because that has a certain amount of pressure. The paper lifts when the pressure above it is reduced.

There have been many debates among scientists about how much this Bernoulli effect actually contributes to the lift of aircraft. Some argue that lift is gained just because of the angle of attack of the wing and has nothing (or very little) to do with Bernoulli. Why not ask your students to do a bit of research about the dispute and have a debate in your next lesson? It's good to put across the message that science doesn't always provide one "correct" answer.

As a follow-up to the Bernoulli experiments, you may want to design a paper aeroplane. The design will use a combination of Bernoulli and the angle of attack of the wing.

And the shower curtain? Well, again there are many possible explanations, but most people believe that the fast-moving water creates a draft of fast-moving air which results in a drop in pressure on your side of the curtain, making the curtain move mysteriously towards you. You can find the Bernoulli effect all over the place.

Paper planes: www.josephpalmer.complanesAirplane.shtml

The Bernoulli debate:http:travel.howstuffworks.comairplane.htm

Daniel Bernoulli (1700-82): http:plus.maths.orgissue1bernindex.html

Experiments:education.iop.orgSchoolssupteachdemonstrations.doc

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