# 美国俄亥俄州物理学Essay代写:浮力的起源和规则

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Buoyancy is the force that enables boats and beach balls to float on water. The term buoyant force refers to the upward-directed force that a fluid (either a liquid or a gas) exerts on an object that is partially or completely immersed in the fluid. Buoyant force also explains why we can lift objects underwater more easily than on land. According to the Roman architect Vitruvius, the Greek mathematician and philosopher Archimedes first discovered buoyancy in the 3rd century B.C. while puzzling over a problem posed to him by King Hiero II of Syracuse. King Hiero suspected that his gold crown, made in the shape of a wreath, was not actually made of pure gold, but rather a mixture of gold and silver. Allegedly, while taking a bath, Archimedes noticed that the more he sank into the tub, the more water flowed out of it. He realized this was the answer to his predicament, and rushed home while crying “Eureka!” (“I’ve found it!”) He then made two objects – one gold and one silver – that were the same weight as the crown, and dropped each one into a vessel filled to the brim with water. Thus, Archimedes demonstrated that his crown indeed contained silver. Though this tale illustrates the principle of buoyancy, it may be a legend.Archimedes observed that the silver mass caused more water to flow out of the vessel than the gold one. Next, he observed that his "gold" crown caused more water to flow out of the vessel than the pure gold object he had created, even though the two crowns were of the same weight. Archimedes never wrote down the story himself. Furthermore, in practice, if a tiny amount of silver were indeed swapped for the gold, the amount of water displaced would be too small to reliably measure. Prior to the discovery of buoyancy, it was believed that an object’s shape determined whether or not it would float. The buoyant force arises from differences in hydrostatic pressure – the pressure exerted by a static fluid. A ball that is placed higher up in a fluid will experience less pressure than the same ball placed further down. This net force – which points upwards regardless of the object’s shape – is the buoyancy force. The hydrostatic pressure is given by P = rgh, where r is the density of the fluid, g is acceleration due to gravity, and h is the depth inside the fluid. The hydrostatic pressure does not depend on the shape of the fluid. This is because there is more fluid, and therefore more weight, acting on the ball when it is deeper in the fluid. Thus, the pressure at the top of an object is weaker than the pressure at the bottom. Pressure can be converted to force using the formula Force = Pressure x Area. There is a net force pointing upward.

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