<text><span class="style10">orces affecting Solids and Fluids (9 of 9)</span><span class="style7">The </span><span class="style26">buoyancy force</span><span class="style7"> was described by the Greek mathematician and physicist Archimedes (287-212 BC). Archimedes' principle states that an object placed in a fluid is buoyed up by a force equal to the weight of fluid displaced by the body. It was the realization of this principle that - according to legend - prompted Archimedes to leap out of his bath with shouts of </span><span class="style26">'Eeureka! Eureaka!' ('I have found it! I have found it!').</span><span class="style7">A body with density greater than that of the fluid will sink, because the fluid it displaces weighs less than it does itself. A body with density less than that of the fluid will float.A submarine varies its density by flooding ballast tanks with sea water or emptying them; this enables it to dive or rise to the surface.</span><span class="style10">Surface tension</span><span class="style7">Surface tension occurs at an interface between a liquid and either a gas or a solid. Molecules in a liquid exert forces on other molecules. At the surface there is asymmetry in these forces, resulting in surface tension. Thus falling rainwater coalesces into spherical drops.Surface tension may be reduced by the presence of a detergent, which acts as a wetting agent, spreading out the liquid over the solid surface. In the human lung, fluid on the alveoli surfaces contains a detergent to prevent collapse of the lung during breathing; its absence, particularly in babies, is fatal.AS </span><span class="style10">VISCOSITY AND TURBULENCE</span><span class="style26">Viscosity</span><span class="style7"> relates to the internal friction in the flow of a fluid - how adjacent layers in the fluid exert retarding forces on each other. This arises from cohesion of the molecules in the fluid. In a solid the deformation of adjacent layers is usually elastic. In a fluid, however, there is no permanent resistance to change of shape; the layers can slide past each other, with continuous displacement of these layers. Fluids are described as </span><span class="style26">newtonian</span><span class="style7"> if they obey Newton's law that the ratio of the applied stress to the rate of shearing has a constant value. This is not true for many substances. Some paints, for example, do not have constant values for the coefficient of viscosity; as the paint is stirred it flows more easily and the coefficient is diminished. Molten lava is another non-newtonian fluid.If adjacent layers flow smoothly past each other the steady flow is described as </span><span class="style26">laminar flow</span><span class="style7">. If the flow velocity is increased the flow may become disordered with irregular and random motions called </span><span class="style26">turbulence</span><span class="style7">. Smoke rising from a cigarette starts with smooth laminar flow but soon breaks into turbulent flow with the formation of eddies. Reynold's number is used to predict the onset of turbulence. It is defined as: Re = speed x density x dimension viscosity or, alternatively, as the ratio of the inertial force to the viscous force: Re = inertial force viscous force This is a pure ratio so has no units. It is a characteristic of the system and the dimension may be the diameter of a pipe or the radius of a ballbearing. Viscosity is relevant for small values of Reynold's number. Above a certain value, turbulence is likely to break out. Thus, for the fall of a very small raindrop, resistance is viscous and is proportional to the product of the viscosity of air, the radius of the raindrop and its speed. For a large raindrop, the resistance is proportional to the product of the density of air, the square of the radius of the raindrop and the square of its speed. Sometimes when special smooth surfaces are involved the onset of turbulence is delayed.The study of viscosity and turbulence is important in understanding problems such as the flow of arterial blood around the body. For larger arteries, turbulence will be a major consideration.</span></text>
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<text><span class="style10">. Archimedes' principle</span><span class="style7">. Total upward force is equal to the weight of fluid displaced.</span></text>
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<text>ΓÇó MOTION AND FORCEΓÇó THERMODYNAMICSΓÇó HOW CARS WORK</text>
