Principles of naval engineering - Historic Naval Ships Association

Chapter 8 -INTRODUCTION TO THERMODYNAMICSto affect other energy systems, since no onesystem can be completely isolated as far asenergy is concerned. However, it is easier tounderstand the basic energy concepts if we disregardall the other energy systems that miglitbe involved in or affected by each energy process.Hence we will generally consider one system ata time, disregarding energy boosts that may bereceived from an outside source and disregardingthe energy transfers that may take placebetween the system we are considering and anyother system.It should be emphasized that mechanicalpotential energy and mechanical kinetic energyare both stored forms of energy. Some confusionarises because mechanical kinetic energy isoften referred to as the "energy of motion,"thus leading to the false conclusion that "energyin transition" is somehow involved. This is notthe case, however. Work— mechanical work— isthe only form of mechanical energy which canproperly be considered as energy in transition.Mechanical potential energy and mechanicalkinetic energy are mutually convertible. To takethe example of the rock resting on the edge ofthe cliff, let us suppose that some external forcepushes the rock over the edge so that it falls.As the rock falls, the system loses potentialenergy but gains kinetic energy. By the time therock reaches the ground at the base of the cliff,all the potential energy of the system has beenconverted into kinetic energy. The sum of thepotential energy and the kinetic energy is identicalat each point along the line of fall, but theproportions of potential energy and kineticenergy are constantly changing as the rock falls.To take another example, consider a baseballthat is thrown straight up into the air. The ballhas kinetic energy while it is in upward motion,but the amount of kinetic energy is decreasingand the amount of potential energy is increasingas the ball travels upward. When the ball hasjust reached its uppermost position, before itstarts to fall back toward the earth, it has onlypotential energy. Then, as the ball falls backtoward the earth, the potential energy is convertedinto kinetic energy again.The magnitude of the mechanical potentialenergy stored in a system by virtue of the relativepositions of the bodies that make up thesystem is proportional to (1) the force of attractionbetween the bodies, and (2) the distanceIn the case of the rock whichbetween the bodies.is ready to fall from the edge of the cliff, we areconcerned with (1) the force of attractionbetween the earth and the rock— tliat is, the forceof gravity acting upon the rock, or the weight ofthe rock, and (2) the linear separation betweenthe two objects. If we measure the weight inpounds and the distance in feet, the amount ofmechanical potential energy stored in the systemby virtue of the elevation of the rock is measuredin the unit called the foot-pound. Specifically,whereE =PWx DE = mechanical potential energy, in foot-^ poundsW =D -weight of body, in poundsdistance between earth and body, in feetThe magnitude of mechanical kinetic energyis proportional to the mass and to the square ofthe velocity of an object which has velocity withrespect to another object, orwhereE = mechanicalpoundsM =MVmass of body, in poundskinetic energy, in foot-V = velocity of body relative to the earth,in feet per secondWhere it is more convenient to use the weightof the body, rather than the mass, the equationbecomeskwhere W^ is the weight of the body, in pounds,and_g^is the acceleration due to gravity, generallytaken as 32.2 feet per second per second.Work, as we have seen, is mechanical energyin transition— that is, it is a transitory form ofmechanical energy which occurs only betweentwo or more other forms of energy. Work isdone when a tangible body or substance is movedthrough a tangible distance by the action of atangible force. Thus we may define work as the2g159