The Wavewatcher's Companion by Gavin Pretor-Pinney

Author:Gavin Pretor-Pinney [Pretor-Pinney, Gavin]
Language: eng
Format: epub
Published: 2010-12-14T16:00:00+00:00

Fortunately, you don’t need to experience a bomb blast or a lightning strike to feel a shock wave at first hand. In fact, you cause one whenever you tuck your knees to your chest and dive-bomb into a swimming pool. The turbulent wall of water thrown out as you hit the surface is the aquatic equivalent of the crash of thunder near a lightning bolt.

Feel free to add the sound of the shock waves yourself.

Competitive swimmers are not big fans of shock waves generally. They try to avoid them as much as possible, since they’re a waste of energy. This is one reason why you don’t see them doing whopping great dive-bombs when the gun goes off for the 200m front crawl. (Although I rather wish they did.) Instead, they aim to produce the smallest possible splash as they dive into the water, pointed fingers first. This way, they minimise the water resistance and thus the shock wave produced by pushing the water out of the way. A reduced shock wave means less energy lost to the water. Once they are off, however, another equally unwelcome aquatic shock wave rears up. This is the ‘bow wave’, just in front of the swimmer’s head, as he or she speeds through the water.

Wait a minute. A bow wave? Where’s the crash and boom – or even the splash – that we’ve come to expect of a shock wave? Actually, although explosions are the most obvious ways shock waves are created, they are not the only ones. They can also result from the bow waves that spread from the front of a moving object – be it a swimmer, or a boat on the water, or some object travelling through the air – so long as it is travelling fast enough through the medium. A bow wave only develops into a shock wave when the object is going at least as fast as waves naturally travel through the medium so that they can’t get out of the way fast enough and end up bunching up on top of each other.

If the bow wave of a swimmer sounds rather mild for a shock wave, how about another shock wave produced in this way: the sonic boom created as a jet flies at supersonic speeds.

This is another example of a shock wave of air pressure – one that we can hear as sound – rather than a shock wave on the surface of the water. But the principle is just the same. As it flies through the sky, a plane will produce pressure waves at its front and back, since it displaces the air, which has to move aside rapidly at the front and rush back in behind, as it once more fills the space left in the plane’s wake. These sudden movements of air cause jumps and dips in the air pressure, which spread out from the plane as pressure waves. These pressure waves expand as spheres, some emanating from the nose, some from the tail.

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