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A Desk Trick

All you need for this trick is a twenty-five-cent piece and a small postage stamp. Put the stamp on the desk and hold the quarter horizontally about half an inch above the stamp. Now blow hard onto the quarter. The stamp immediately rises from the desk and seems to stick to the back of the quarter until you stop blowing.

This is a quick and easy demonstration of the same principle that holds airplanes up in flight. A stream of air — or any fluid — has lower pressure than the atmosphere around it. The faster the stream, the lower the pressure. This is generally referred to as Bernoulli’s principle, after the eighteenth-century Swiss physicist Daniel Bernoulli.

The fast-moving air flowing past the edges of the quarter makes relatively low air pressure around the edge of the coin. Atmospheric pressure beneath the stamp then pushes the stamp against the quarter.

The wings of birds and airplanes exploit the same effect. The top surface of a wing is curved, the bottom relatively flat. As the wing moves through the air, that special shape forces air to move faster over the top of the wing than along the bottom. The faster-moving air on top of the wing has lower pressure than the slower-moving air on the bottom. The result is a force pushing up on the wing. If the wing is big enough and the airspeed fast enough, that force can help carry a bird or an airplane through the air.

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