Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGproduced by energy. For example, heat can flowfrom one body to another without doing any workat all, but the heat must still be considered asenergy and the process of heat transfer must berecognized as a process that has produced aneffect. A broader definition, then, and one whichsatisfies more of the conditions under which weknow energy to exist, is "the capacity for producingan effect."Energy exists in many forms. For convenience,we usually classify energy according tothe size and nature of the bodies or particleswith which the energy is associated. Thus wesay that mechanical energy is the energy associatedwith large bodies or objects— usually,things that are big enough to see. Thermalenergy is energy associated with molecules.Chemical energy is energy that arises from theforces that bind the atoms together in a molecule.Chemical energy is demonstrated whenevercombustion or any other chemical reaction takesplace. Electrical energy, light, X-rays, andradio waves are examples of energy associatedwith particles that are even smaller than atoms.Each of these types of energy must be furtherclassified as (1) stored energy, or (2) energy intransition. Stored energy can be thought of asenergy that is actually "contained in" or "storedin" a substance or system. There are two kindsof stored energy: (1) potential energy, and (2)kinetic energy. When energy is stored in a systembecause of the relative positions of two ormore objects or particles, we call it potentialenergy . When energy is stored in a system becauseof the relative velocities of two or moreobjects or particles, we call it kinetic energy .It should be emphasized that all stored energyis either potential energy or kinetic energy.Energy in transition is, as the name implies,energy that is intheprocess of being transferredfrom one object or system to another. All energyin transition begins and ends as stored energy.In order to understand any form of energy,then, we need to know the relative size of thebodies or particles in the energy system and weneed to know whether the energy is stored or intransition. Bearing in mind these two modes ofclassification, let us now examine mechanicalenergy and thermal energy— the two forms ofenergy which are of particular interest in practicallyall aspects of shipboard engineering.MECHANICAL ENERGYEnergy associated with a system composedof relatively large bodies is called mechanicalenergy. The two forms of stored mechanicalenergy are (1) mechanical potential energy,and (2) mechanical kinetic energy. 2 Mechanicalenergy in transition is manifested by work.Mechanical potential energy is stored in asystem by virtue of the relative positions ofthe bodies that make up the system. The mechanicalpotential energy associated with thegravitational attraction between the earth andanother body provides us with many everydayexamples. A rock resting on the edge of a cliffin such a position that it will fall freely ifpushed has mechanical potential energy. Waterat the top of a dam has mechanical potentialenergy. A sled that is being held at the top of anicy hill has mechanical potential energy. Notethat in each of these examples the energy residesneither in the earth alone nor in the other objectalone but rather in an energy system of whichthe earth is merely one component.Mechanical kinetic energy is stored in asystem by virtue of the relative velocities ofthe component parts of the system. Push thatrock over the edge of the cliff, open the gate ofthe dam, or let go of the sled— and somethingwill move. The rock will fall, the water willflow, the sled will slide down the hill. In eachcase the mechanical potential energy will bechanged to mechanical kinetic energy. Since itis customary toascribezero velocity to an objectwhich is at rest with respect to the earth, it isalso customary to think of kinetic energy asthough it pertained only to the object which isin motion with respect to the earth. It should beremembered, however, that kinetic energy, likepotential energy, is properly assigned to thesystem rather than to any one component of thesystem.In these examples of mechanical potentialenergy and mechanical kinetic energy, we haveused an external source of energy to get thingsstarted. Energy from some outside source isrequired to push the rock, open the gate of thedam, or let go of the sled. All real machinesand processes require this kind of a boost froman energy source outside of the system; similarly,the energy from any one system is bound2 Although all forms of energy may be stored as potentialenergy or as kinetic energy, these terms refer, incommon usage, to mechanical potential energy andmechanical kinetic energy, unless some other form ofenergy (thermal, chemical, etc.) is specified.158