PRINCIPLES OF NAVAL ENGINEERING 



as the result of the process. It is also necessary 

 to consider the energy exchanges that occur 

 between the system and its surroundings during 

 the process, since such energy exchanges will 

 have an effect on the final state of the system. 



The lifting of an object— as, for example, the 

 lifting of a rock from the base of a cliff to the 

 top of the cliff— is a simple example of a process 

 involving work against gravity. Before the proc- 

 ess begins, the energy which will be required to 

 lift the rock is stored in some form in some other 

 energy system. While the process is occurring, 

 energy in the form of work flows from the ex- 

 ternal system to the earth-rock system. At the 

 end of the process, the energy is stored in the 

 earth-rock system in the form of mechanical 

 potential energy. The change which has been 

 brought about by this process is manifested by 

 the separation of the rock and the earth. 



Now suppose we push the rock off the top of 

 the cliff and allow it to fall freely toward the 

 base of the cliff. Disregarding the push (which 

 is actually an inputof energy from some external 

 system), the process which now takes place is an 

 example of work done by gravity. The work done 

 by gravity converts the mechanical potential 

 energy of the system into mechanical kinetic 

 energy. Thus it is clear that energy in transi- 

 tion—work, in this case— begins and ends as 

 stored energy. 



When the rock hits the earth, other processes 

 occur. Some work will be expended in compress- 

 ing the earth upon which the rock falls, and some 

 energy will then be stored as internal kinetic 

 energy in the rock and in the earth. The increase 

 in internal kinetic energy will be manifested by 

 a rise in the temperature of the rock and of the 

 earth, and still another process will then take 

 place as heat flows from the rock and from the 

 earth. Some energy may also be stored as in- 

 ternal potential energy because of molecular 

 displacements in the rock and the earth. 



The compression of a spring provides an 

 example of a process involving elastic defor- 

 mation. As force is applied to compress the 

 spring, work is done. The major effect of the 

 energy thus supplied as work is to decrease the 

 distance between molecules in the spring, thus 

 increasing the amount of internal potential 

 energy stored in the spring. If we suddenly 

 release the spring, the stored internal potential 

 energy is suddenly released and the spring shoots 

 away. 



The turning of a shaft— as, for example, a 

 propeller shaft of a ship— is another example of 



a process involving elastic deformation. Suppose 

 that a strong twisting force is applied to a shaft 

 at rest. The first part of this process will cause 

 an elastic deformation of the shaft. The distance 

 between molecules in the shaft is changed, and 

 there is a storage of internal potential energy 

 before the shaft begins to turn. When the applied 

 force becomes great enough to turn the shaft, 

 there will also be a storage of mechanical kinetic 

 energy. As long as the applied force remains 

 constant and the shaft continues to turn, these 

 stored forms of energy will remain stored in 

 unchanging amount. Meanwhile, a great deal of 

 mechanical energy in transition (work) will 

 continuously flow through the shaft to some 

 other system. 



When a solid body is dragged across a rough 

 horizontal surface, the process is one of work 

 against friction. The work done in moving the 

 object will be equal to the force required to 

 overcome the friction multiplied by the distance 

 through which the object is moved. In this proc- 

 ess, the energy supplied as work is transformed 

 very largely into internal kinetic energy, as evi- 

 denced by an increase in temperature. Some of 

 the energy may be transformed into internal 

 potential energy because of molecular displace- 

 ments in the object and in the surface over which 

 it is being moved. 



A propeller rotating in water is an example 

 of a process in which work causes fluid turbu- 

 lence. The first effect of the movement of the 

 propeller is to impart various motions to the 

 water, thus causing turbulence. For a short 

 time this movement of the water represents 

 mechanical kinetic energy, but the energy is 

 rapidly transformed into internal kinetic energy, 

 as evidenced by a rise in the temperature of the 

 water. 



The addition of thermal energy to a piece of 

 metal is a simple example of a process involv- 

 ing heat. As the metal is heated, the temperature 

 rises, indicating a storage within the metal of 

 internal kinetic energy. Also, the metal expands; 

 thus we know that some part of the energy de- 

 livered as heat is transformed into work as the 

 metal expands against the resistance of its sur- 

 roundings. If we continue heating the metal to its 

 melting point, we will note a process in which 

 the flow of heat results in a change in the physi- 

 cal state of the substance but does not, at this 

 point, result in a further rise in temperatm-e. 



Because of the enormous number and variety 

 of processes that may occur, some basic clas- 

 sification of processes involving heat and work is 



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