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An Explorit "Science Bytes" article by Anne Hance (1994)

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FLOAT, SINK OR SWIM - Beliefs and Theories

by Anne Hance (1994)

The ancient Chinese propounded the notion during the first century AD, that the sun, moon and stars float freely in space. This notion, while an improvement over earlier ideas, is not acceptable in light of our modern understanding of bodies in space. But they correctly recognized that water particles float in the air and, as part of a water cycle, sink down to earth as rain to replenish lakes, rivers and oceans.

Since a fluid is any substance that takes the shape of any vessel containing it, air and water are both 'fluids'. The physical and chemical properties of air and water have enabled the development of many forms of life ever since life began on this planet. But the purposeful study of these properties had to wait until sentient beings paid deliberate attention to them.

Of course, prehistoric peoples recognized that some things would float on water and, without the need for any understanding of science to explain this phenomenon, they built rafts and boats. Experience, rather than a formalized knowledge of 'science', taught them how to take advantage of the physical properties of water and the materials with which they built their craft.

Learning to take advantage of the physical properties of air was historically a much slower process. From earliest times, people watched birds and insects use the fluid nature of air to enable them to fly. People could not fly and so they invented things that could. Kites, which take advantage of air currents and wind to enable them to swim in the air, are thought to have been invented by the Greek scientist Archytas of Tarentum in the fourth century BC.

Leonardo da Vinci's fifteenth-century helicopter was designed to 'swim' through the air and was probably derived from stories of Chinese helicopter tops or 'bamboo dragonflies'. It relied more on imagination and intelligent intuition than on formalized scientific knowledge and was not actually tested.

Functioning air-craft which floated in air were successfully developed and tested in 1783 when two French paper-makers, after experimenting with hot air and paper bags, were responsible for the development of the Montgolfier hot air balloon which floated over Paris in November of that year. Although their invention was a success, they did not understand the principle of the difference in density of the hot and cold air upon which it depended.

Just over a hundred years later, in 1903, the Wright brothers made the first flight in a heavier than air machine which 'swam' through the air, powered by an internal combustion engine. By observing bird flight and by conducting their own experimental investigations, they formulated new understandings of the principles of aerodynamics - lift, drag and turbulence - that enabled their vehicle to remain airborne. Now, at the end of the twentieth century, we can travel (swim) through the air at three times the speed of sound.

Understanding and making use of the physics of floating, swimming and sinking in water has been less of a problem. Two hundred years BC in Greece, history tells us that Archimedes was faced with a task set by King Heiron who suspected that his crown was not pure gold but had been adulterated with silver by the goldsmith who made it. He asked Archimedes to determine if the crown contained silver.

In this story Archimedes, stepping into his bath one day, suddenly realized the useful significance of the fact that when he sat down, the water flowed over the sides of the tub. The goldsmith had made the King's crown to be the same weight as the gold he had been given. But, if it was adulterated with silver (which is less dense than gold), Archimedes knew that, to make up the weight difference, the crown would have to have a greater volume than the original gold. So, he sank a piece of gold the same weight as the crown and noted how much water it displaced. Then he sank the crown and found that it displaced more water and so was of greater volume than should have been the case if it were pure gold.

This was certainly not the first time anyone had noticed the physical phenomenon of displacement of water but Archimedes was able to interpret it usefully and scientifically.

From earliest times people floated things on water and swam on and under water. Of course, from earliest times people and their inventions sank in water. Cornelius van Drebbel in 1620 was not the first to design an underwater craft but his invention was effective and 'swam' (powered by either eight or twelve oars - reports differ) twelve feet below the surface of London's River Thames carrying paying customers for ten years. Drebble was a physicist and the construction of his submarine required knowledge of flotation, mechanics, and the physical properties of the materials that were known at the time.

Floating, swimming and sinking clearly are natural as well as technologically-devised activities. They are studied by biologists as well as by physicists. The water vapor known by the ancient Chinese to be in the air is now known to be accompanied by minute pollen grains, encysted spores, and dust particles which float or sink depending upon the physical circumstances.

Birds and insects fly (swim) and soar (float) in the air and the structural adaptations that enable them to take advantage of the laws of aerodynamics have been thoroughly studied. Locally, in the Sacramento Valley at certain times of the year, these are joined by floating, filmy, clustered threads of spider silk which, with their creators, are carried by air currents to festoon trees, car windows and the helmets of bicyclists.

In water, icebergs float and boats of all kinds float, swim and sink in company with flotsam and jetsam, phyto- and zoo- plankton, fish, seaweed, marine mammals, and on occasion, humans. The salinity and temperature of the water affect its density and thus affect the behavior of all floating, swimming and sinking bodies. Flat bottomed boats behave differently from those with an angular hull and a keel although both float. Phytoplankton and zooplankton float in the ocean and are food for other sea dwellers which swim. Fish have swim (air) bladders in their body cavities so that they can adjust their densities to swim even in the deeper parts of the ocean. And, humans too can float, swim and sink both with and without mechanical aids.

We are now quite advanced in our understanding of floating swimming and sinking in fluids but there is still much to learn and there always exists the probability that some of our modern understandings will be explained differently in the future!

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