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THERMODYNAMIC PROPERTIES
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Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3
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THERMODYNAMIC PROPERTIES Thermodynamics According  to  Newton’s  Second  Law  of  Motion,  force  (F)  =  ma,  where  a  is  acceleration.   For example, on earth an object has a certain mass and a certain weight.   When the same object is placed in  outer space, away from  the earth’s gravitational field, its  mass is the same,  but it is now in a "weightless" condition (that is, gravitational acceleration and, thus, force equal zero). The English system uses the pound-force (lbf) as the unit of weight.   Knowing that acceleration has the units of ft/sec2 and using Newton’s second law, we can determine that the units of mass are lbf-sec2/ft.   For simplification, 1 lbf-sec2/ft is called a slug.   The basic unit of mass in the English  system  is  the  slug.    However,  the  slug  is  an  almost  meaningless  unit  for  the  average individual.   The unit of mass generally used is the pound-mass (lbm).   In order to allow lbm to be used as a unit of mass, we must divide Newton’s second law by the gravitational constant (gc). 32.17 lbm ft lbf sec2 gc Newton’s second law can be expressed by Equation 1-2. (1-2) F ma gc Use of the gravitational constant, gc, adapts Newton’s second law such that 1 lbf = 1 lbm at the surface of the earth.  It is important to note that this relationship is only true at the surface of the earth,  where  the  acceleration  due  to  gravity  is  32.17  ft/sec2.    However,  because  all  of  our discussions will be based upon experiences and observations on earth, we will use the lbm as the unit of mass. NOTE: In Equation 1-2, acceleration "a" is often written as "g" because, in this case, the acceleration is the gravitational acceleration due to the earth’s gravitational field (g = 32.17 ft/sec2). Example: Using Equation 1-2, prove that 1 lbf = l lbm on earth. Solution: F mg gc 1  lbf (1  lbm)  (32.17  ft/sec2) 32.17 (lbm ft) (lbf sec2) 1  lbf 1  lbf  (an  equality) HT-01 Page 2 Rev. 0







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