PRINCIPLES OF NAVAL ENGINEERING 



produced by energy. For example, heat can flow 

 from one body to another without doing any work 

 at all, but the heat must still be considered as 

 energy and the process of heat transfer must be 

 recognized as a process that has produced an 

 effect. A broader definition, then, and one which 

 satisfies more of the conditions under which we 

 know energy to exist, is "the capacity for pro- 

 ducing an effect." 



Energy exists in many forms. For conveni- 

 ence, we usually classify energy according to 

 the size and nature of the bodies or particles 

 with which the energy is associated. Thus we 

 say that mechanical energy is the energy asso- 

 ciated with large bodies or objects— usually, 

 things that are big enough to see. Thermal 

 energy is energy associated with molecules. 

 Chemical energy is energy that arises from the 

 forces that bind the atoms together in a molecule. 

 Chemical energy is demonstrated whenever 

 combustion or any other chemical reaction takes 

 place. Electrical energy, light, X-rays, and 

 radio waves are examples of energy associated 

 with particles that are even smaller than atoms. 



Each of these types of energy must be further 

 classified as (1) stored energy, or (2) energy in 

 transition. Stored energy can be thought of as 

 energy that is actually "contained in" or "stored 

 in" a substance or system. There are two kinds 

 of stored energy: (1) potential energy, and (2) 

 kinetic energy. When energy is stored in a sys- 

 tem because of the relative positions of two or 

 more objects or particles, we call it potential 

 energy . When energy is stored in a system be- 

 cause of the relative velocities of two or more 

 objects or particles, we call it kinetic energy . 

 It should be emphasized that all stored energy 

 is either potential energy or kinetic energy. 



Energy in transition is, as the name implies, 

 energy that is intheprocess of being transferred 

 from one object or system to another. All energy 

 in transition begins and ends as stored energy. 

 In order to understand any form of energy, 

 then, we need to know the relative size of the 

 bodies or particles in the energy system and we 

 need to know whether the energy is stored or in 

 transition. Bearing in mind these two modes of 

 classification, let us now examine mechanical 

 energy and thermal energy— the two forms of 

 energy which are of particular interest in prac- 

 tically all aspects of shipboard engineering. 



MECHANICAL ENERGY 



Energy associated with a system composed 

 of relatively large bodies is called mechanical 



energy. The two forms of stored mechanical 

 energy are (1) mechanical potential energy, 

 and (2) mechanical kinetic energy. 2 Mechanical 

 energy in transition is manifested by work. 



Mechanical potential energy is stored in a 

 system by virtue of the relative positions of 

 the bodies that make up the system. The me- 

 chanical potential energy associated with the 

 gravitational attraction between the earth and 

 another body provides us with many everyday 

 examples. A rock resting on the edge of a cliff 

 in such a position that it will fall freely if 

 pushed has mechanical potential energy. Water 

 at the top of a dam has mechanical potential 

 energy. A sled that is being held at the top of an 

 icy hill has mechanical potential energy. Note 

 that in each of these examples the energy resides 

 neither in the earth alone nor in the other object 

 alone but rather in an energy system of which 

 the earth is merely one component. 



Mechanical kinetic energy is stored in a 

 system by virtue of the relative velocities of 

 the component parts of the system. Push that 

 rock over the edge of the cliff, open the gate of 

 the dam, or let go of the sled— and something 

 will move. The rock will fall, the water will 

 flow, the sled will slide down the hill. In each 

 case the mechanical potential energy will be 

 changed to mechanical kinetic energy. Since it 

 is customary toascribezero velocity to an object 

 which is at rest with respect to the earth, it is 

 also customary to think of kinetic energy as 

 though it pertained only to the object which is 

 in motion with respect to the earth. It should be 

 remembered, however, that kinetic energy, like 

 potential energy, is properly assigned to the 

 system rather than to any one component of the 

 system. 



In these examples of mechanical potential 

 energy and mechanical kinetic energy, we have 

 used an external source of energy to get things 

 started. Energy from some outside source is 

 required to push the rock, open the gate of the 

 dam, or let go of the sled. All real machines 

 and processes require this kind of a boost from 

 an energy source outside of the system; simi- 

 larly, the energy from any one system is bound 



2 



Although all forms of energy may be stored as poten- 

 tial energy or as kinetic energy, these terms refer, in 

 common usage, to mechanical potential energy and 

 mechanical kinetic energy, unless some other form of 

 energy (thermal, chemical, etc.) is specified. 



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