AIR POWER

Air is one of the most powerful forces of nature.
It can move houses, airplanes, and even oceans under the right conditions

A tornado touches down south of Haviland, Kansas, USA, on Saturday, May 5. — AFP

What is air?

Air is a mixture of gases. The gases are made up of tiny particles moving around randomly and colliding with each other and anything else in their path. The average speed of an air particle at room temperature (20 degrees C) is 500 meters per second. This is faster than the speed of sound.

As you're reading this you are being hit by an unimaginable number of these particles every second. So why don't you feel them? You might think it's because they're so small, but the combined force of all of them is equivalent to a weight of about 10 tons!

The answer lies in the random directions in which they're moving. This means that at any one time we're being hit by roughly the same number from all sides, so the effects cancel each other out. You might ask, quite logically, why don't we get crushed, as we would be if a 10 tonne weight were dropped on us?

Luckily for us, most of our bodies are fairly incompressible, being made of solids and liquids. However, we do have some big spaces inside such as our lungs.

The reason these don't get crushed is because they are full of air too. Just as the outside of your chest is being hit with this incredible force pushing inwards, the air inside your lungs is pushing back with equal force.

Under pressure

The air particles hitting us create air pressure. As long as the air pressure inside and outside our bodies is roughly the same, we are OK.

Above is a picture of an empty aluminium can that has most of the air removed from inside. It was immediately crushed by the air outside pushing in, because there was nothing remaining inside to push back. We call this an implosion. An explosion is the opposite, when the high pressure is on the inside, pushing outwards.

In movies when we see a window on a space ship break we see people and objects being 'sucked' out of the spacecraft. This happens because the pressure inside the space ship is much higher than the pressure outside.

So if you're near the window when it breaks, suddenly you are not being hit equally on all sides, and you actually get pushed out the window by the air particles hitting you from inside the ship.

What could be more natural?

Last time I wrote that a fridge was the most unnatural machine in the world. After reading that my mother said that surely an aeroplane was more unnatural than a fridge. I think she deserves an answer, after all something that heavy being able to fly is pretty amazing.

However, planes don't do anything that nature doesn't do. Birds are denser than air; so are planes. Birds can fly, so can planes. (A fridge can't fly, but it can move heat from a cold place to a hot place, which simply doesn't happen in nature!)

Bernoulli Effect

There are two types of flight - gliding and powered flight. Gliders use the kinetic energy of thermals (hot air rising) to lift them up (giving them gravitational energy).

Powered flight needs engines, and thus requires chemical energy from fuel. The engine's job is to move the plane fast enough to be able to make use of something called the Bernoulli Effect.

Put simply, this means that when the plane is moving fast enough the air pressure below the wings (pushing up) is greater than the air pressure above the wing (pushing down). This produces 'lift', and when the lift is bigger than the weight of the plane, it takes off.

The plane is controlled simply by altering its speed and the shape of the wings to produce more or less lift. When the plane flies through turbulent air things get bumpy because the carefully controlled pressure difference above and below the wing can change very suddenly, causing the plane to move up or down sharply.

To see a video of the can being crushed, and an explanation of why, go to http://corrinscience.spaces.live.com .

Next time: King of the swingers - a brief history of the pendulum.