THERMODYNAMIC PROPERTIES ThermodynamicsAccording to Newton’s Second Law of Motion, force (F) = ma, where a is acceleration. Forexample, on earth an object has a certain mass and a certain weight. When the same object isplaced in outer space, away from the earth’s gravitational field, its mass is the same, but it isnow 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 accelerationhas the units of ft/sec2 and using Newton’s second law, we can determine that the units of massare lbf-sec2/ft. For simplification, 1 lbf-sec2/ft is called a slug. The basic unit of mass in theEnglish system is the slug. However, the slug is an almost meaningless unit for the averageindividual. The unit of mass generally used is the pound-mass (lbm). In order to allow lbm tobe used as a unit of mass, we must divide Newton’s second law by the gravitational constant (gc).32.17lbmftlbfsec2gcNewton’s second law can be expressed by Equation 1-2.(1-2)FmagcUse of the gravitational constant, gc, adapts Newton’s second law such that 1 lbf = 1 lbm at thesurface of the earth. It is important to note that this relationship is only true at the surface of theearth, where the acceleration due to gravity is 32.17 ft/sec2. However, because all of ourdiscussions will be based upon experiences and observations on earth, we will use the lbm as theunit of mass.NOTE: In Equation 1-2, acceleration "a" is often written as "g" because, in this case, theacceleration 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:Fmggc1 lbf(1 lbm) (32.17 ft/sec2)32.17(lbmft)(lbfsec2)1 lbf1 lbf (an equality)HT-01 Page 2 Rev. 0
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