CLASS16 - CHAPTER 16 HYDROSTATICS 1. Introduction This...

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CHAPTER 16 HYDROSTATICS 1. Introduction This relatively short chapter deals with the pressure under the surface of an incompressible fluid, which in practice means a liquid, which, compared with a gas, is nearly, if not quite, incompressible. It also deals with Archimedes’ principle and the equilibrium of floating bodies. The chapter is perhaps a little less demanding than some of the other chapters, though it will assume a familiarity with the concepts of centroids and radius of gyration, which are dealt with in chapters 1 and 2. 2. Density There is little to be said about density other than to define it as mass per unit volume. However, this expression does not literally mean the mass of a cubic metre, for after all a cubic metre is a large volume, and the density may well vary from point to point throughout the volume. Density is an intensive quantity in the thermodynamical sense, and is defined at every point . A more exact definition of density, for which I shall usually use the symbol ρ, is . 0 V m Lim V δ δ = ρ δ 16.2.1 The awful term “specific gravity” was formerly used, and is still regrettably often heard, as either a synonym for density, or the dimensionless ratio of the density of a substance to the density of water. It should be avoided. The only concession I shall make is that I shall use the symbol s to mean the ratio of the density of a body to the density of a fluid in which is may be immersed on or which it may be floating, The density of water varies with temperature, but is approximately 1 g cm 3 or 1000 kg m 3 . 3. Pressure Pressure is force per unit area, or, more precisely, . 0 A F Lim P A δ δ = δ 16.3.1 There is no particular direction associated with pressure – it acts in all directions – and it is a scalar quantity. The SI unit is the pascal (Pa), which is a pressure of one newton per square metre (N m 2 ). Blaise Pascal (1623-1662) was a French mathematician and philosopher who contributed greatly to the theory of conic sections and to hydrostatics.
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He showed that the barometric pressure decreases with height – hence the famous examination question: “Explain how you would use a barometer to measure the height of a tall building” – to which the most accurate answer is said to be: “I would drop it out of the window and time how long it took to reach the ground.” The CGS unit of pressure is dyne cm 2 , and 1 Pa = 10 dyne cm 2 . Some other silly units for pressure are often seen, such as psi, bar, Torr or mm Hg, and atm. A psi or “pound per square inch” is all right for those who define a “pound” as a unit of force (US usage) but is less so for those who define a pound as a unit of mass (UK usage). A psi is about 6894.76 Pa. [The “British Engineering System”, as far as I know, is used exclusively in the U.S. and is not and never
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This note was uploaded on 10/25/2010 for the course MECHANIC Mechanic taught by Professor Monfered during the Fall '10 term at MIT.

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CLASS16 - CHAPTER 16 HYDROSTATICS 1. Introduction This...

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