Chapter 8 -INTRODUCTION TO THERMODYNAMICS 



to affect other energy systems, since no one 

 system can be completely isolated as far as 

 energy is concerned. However, it is easier to 

 understand the basic energy concepts if we dis- 

 regard all the other energy systems that miglit 

 be involved in or affected by each energy process. 

 Hence we will generally consider one system at 

 a time, disregarding energy boosts that may be 

 received from an outside source and disregard- 

 ing the energy transfers that may take place 

 between the system we are considering and any 

 other system. 



It should be emphasized that mechanical 

 potential energy and mechanical kinetic energy 

 are both stored forms of energy. Some confusion 

 arises because mechanical kinetic energy is 

 often referred to as the "energy of motion," 

 thus leading to the false conclusion that "energy 

 in transition" is somehow involved. This is not 

 the case, however. Work— mechanical work— is 

 the only form of mechanical energy which can 

 properly be considered as energy in transition. 



Mechanical potential energy and mechanical 

 kinetic energy are mutually convertible. To take 

 the example of the rock resting on the edge of 

 the cliff, let us suppose that some external force 

 pushes the rock over the edge so that it falls. 

 As the rock falls, the system loses potential 

 energy but gains kinetic energy. By the time the 

 rock reaches the ground at the base of the cliff, 

 all the potential energy of the system has been 

 converted into kinetic energy. The sum of the 

 potential energy and the kinetic energy is iden- 

 tical at each point along the line of fall, but the 

 proportions of potential energy and kinetic 

 energy are constantly changing as the rock falls. 



To take another example, consider a baseball 

 that is thrown straight up into the air. The ball 

 has kinetic energy while it is in upward motion, 

 but the amount of kinetic energy is decreasing 

 and the amount of potential energy is increasing 

 as the ball travels upward. When the ball has 

 just reached its uppermost position, before it 

 starts to fall back toward the earth, it has only 

 potential energy. Then, as the ball falls back 

 toward the earth, the potential energy is con- 

 verted into kinetic energy again. 



The magnitude of the mechanical potential 

 energy stored in a system by virtue of the rela- 

 tive positions of the bodies that make up the 

 system is proportional to (1) the force of at- 

 traction between the bodies, and (2) the distance 

 between the bodies. In the case of the rock which 

 is ready to fall from the edge of the cliff, we are 

 concerned with (1) the force of attraction 



between the earth and the rock— tliat is, the force 

 of gravity acting upon the rock, or the weight of 

 the rock, and (2) the linear separation between 

 the two objects. If we measure the weight in 

 pounds and the distance in feet, the amount of 

 mechanical potential energy stored in the system 

 by virtue of the elevation of the rock is meas- 

 ured in the unit called the foot-pound. Specifi- 

 cally, 



E = 

 P 



Wx D 



where 



E = mechanical potential energy, in foot- 

 ^ pounds 



W = weight of body, in pounds 



D - distance between earth and body, in feet 



The magnitude of mechanical kinetic energy 

 is proportional to the mass and to the square of 

 the velocity of an object which has velocity with 

 respect to another object, or 



where 



E = mechanical 

 pounds 



MV 



kinetic energy, in foot- 



M = mass of body, in pounds 



V = velocity of body relative to the earth, 

 in feet per second 



Where it is more convenient to use the weight 

 of the body, rather than the mass, the equation 

 becomes 



k 2g 



where W^ is the weight of the body, in pounds, 

 and_g^is the acceleration due to gravity, gen- 

 erally taken as 32.2 feet per second per second. 

 Work, as we have seen, is mechanical energy 

 in transition— that is, it is a transitory form of 

 mechanical energy which occurs only between 

 two or more other forms of energy. Work is 

 done when a tangible body or substance is moved 

 through a tangible distance by the action of a 

 tangible force. Thus we may define work as the 



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