cept came to him in a flash of insight when he noticed the water rise as he stepped into his tub at the public baths, and that he leaped from the bath and ran naked through the streets yelling “I’ve found it!” (“Eureka!”). There is probably more truth to the former than the latter. But, in any event, his discovery bears his name, and Archimedes’ Principle states: “An object wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced” (figure 3-4). In other words, an object that is denser than water will sink, whether in Archimedes’ bathtub or your swimming pool, because the buoyant force of the water is not enough to support it or push it to the surface. But also any object will weigh less in the water than out of the water because of the upward force of buoyancy provided by the water. Useful terms are “in-water weight” or “buoyed weight” of a submerged object as distinguished from the “weight” of the object in air (air weight or dry weight). An object that is less dense than water will sink only until it has displaced a weight of water equal to the weight of the object. Then it will sink no further. If it is very much less dense, it will float high in the water. If it is near to the same density as water, it will float low in the water. Any object that has the same density as the water will neither float nor sink, but will remain wherever it is placed in the water column. Such an object weighs exactly the same as the water it displaces, and the forces are exactly balanced. Density is weight (mass) per unit volume, so in buoyancy problems both the weight and the volume of the immersed object, as well as the weight and volume of the displaced liquid, are factors. Buoyancy is always an upward force. We speak of a diver as being positively buoyant, negatively buoyant, or neutrally buoyant, but this is a loose use of the term. Whether a diver floats, sinks, or hovers is a result of the combined forces of the weight of the diver (downward) and the buoyant force (upward) of the displaced water–the density of the object and the density of the water. It is the difference between the weight of the diver with all equipment and the weight of an equal volume of water that determines whether the diver will float or Chapter 3- Diving Physics sink. When you adjust your buoyancy underwater by adding some air to your BC, you have increased your total volume so that you will displace more water and increase the buoyant force. The buoyant force depends on the density of the fluid in which the object is immersed. Ocean water is about 2.5% denser than fresh water, so it exerts a greater buoyant force than fresh water (by 2.5%). Most people are very close to neutrally buoyant in fresh water, but they will float easily when swimming in the ocean. The greater density of the ocean water is also why divers require additional ballast weight when moving from a fresh water environment to an ocean environment. The density of air is very low, but a hot-air balloon or a helium filled balloon will float in the air. You actually weigh slightly less buoyed up by the air than you would in a vacuum, but only by perhaps 100 grams/4 ounces, hardly enough for you to float in the air–except in dreams. Any fluid will provide buoyancy. Mercury is a very high density liquid, and a silver coin will actually float on mercury, although a gold object is more dense than mercury and will sink. In most buoyancy problems, the air weight and the volume of the immersed (or floating) object are known, and the density of the liquid is either given or implied by a knowledge of the liquid (for example, “at the bottom of the ocean”). The air weight of the object is the downward force. The upward buoyant force is Diving Physics 77 A B FIGURE 3-4. A) A BUOYANT OBJECT DISPLACES WATER EQUAL TO ITS WEIGHT B) MORE WATER IS DISPLACED WHEN THE OBJECT IS SUBMERGED
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