The mathematical relationship for kineticenergy is stated in the rule: “Kinetic energy infoot-pounds is equal to the force in pounds whichcreated it, multiplied by the distance throughwhich it was applied, or to the weight of themoving object in pounds, multiplied by the squareof its velocity in feet per second, and divided by64.s”The relationship between inertia forces,velocity, and kinetic energy can be illustrated byanalyzing what happens when a gun fires aprojectile against the armor of an enemy ship. (Seefig. 2-17.) The explosive force of the powder inthe breach pushes the projectile out of the gun,giving it a high velocity. Because of its inertia,the projectile offers opposition to this suddenvelocity and a reaction is set up that pushes thegun backward (kick or recoil). The force of theexplosion acts on the projectile throughout itsmovement in the gun. This is force acting througha distance producing work. This work appears askinetic energy in the speeding projectile. Theresistance of the air produces friction, which usessome of the energy and slows down the projectile.Eventually, however, the projectile hits its targetand, because of the inertia, tries to continuemoving. The target, being relatively stationary,tends to remain stationary because of its inertia.The result is that a tremendous force is set up thateither leads to the penetration of the armor orthe shattering of the projectile. The projectileis simply a means of transferring energy, inthis instance for destructive purpose, from thegun to the enemy ship. This energy is transmittedin the form of energy of motion or kineticenergy.A similar action takes place in a fluid powersystem in which the fluid takes the place of theprojectile. For example, the pump in a hydraulicFigure 2-17.—Relationship of inertia, velocity, and kineticenergy.system imparts energy to the fluid, whichovercomes the inertia of the fluid at rest andcauses it to flow through the lines. The fluid flowsagainst some type of actuator that is at rest. Thefluid tends to continue flowing, overcomes theinertia of the actuator, and moves the actuatorto do work. Friction uses up a portion of theenergy as the fluid flows through the lines andcomponents.RELATIONSHIP OF FORCE,PRESSURE, AND HEADIn dealing with fluids, forces are usuallyconsidered in relation to the areas over which theyare applied. As previously discussed, a forceacting over a unit area is a pressure, and pressurecan alternately be stated in pounds per square inchor in terms of head, which is the vertical heightof the column of fluid whose weight wouldproduce that pressure.In most of the applications of fluid power inthe Navy, applied forces greatly outweigh all otherforces, and the fluid is entirely confined. Underthese circumstances it is customary to think of theforces involved in terms of pressures. Since theterm head is encountered frequently in the studyof fluid power, it is necessary to understand whatit means and how it is related to pressure andforce.All five of the factors that control the actionsof fluids can, of course, be expressed either asforce, or in terms of equivalent pressures or head.In each situation, the different factors are referredto in the same terms, since they can be added andsubtracted to study their relationship to eachother.At this point you need to review some termsin general use. Gravity head, when it is importantenough to be considered, is sometimes referredto as head. The effect of atmospheric pressure isreferred to as atmospheric pressure. (Atmosphericpressure is frequently and improperly referred toas suction.) Inertia effect, because it is alwaysdirectly related to velocity, is usually calledvelocity head; and friction, because it representsa loss of pressure or head, is usually referred toas friction head.STATIC AND DYNAMIC FACTORSGravity, applied forces, and atmosphericpressure are static factors that apply equally to2-13