The   mathematical   relationship   for   kinetic energy  is  stated  in  the  rule:  “Kinetic  energy  in foot-pounds is equal to the force in pounds which created  it,  multiplied  by  the  distance  through which  it  was  applied,  or  to  the  weight  of  the moving  object  in  pounds,  multiplied  by  the  square of its velocity in feet per second, and divided by 64.s” The   relationship   between   inertia   forces, velocity, and kinetic energy can be illustrated by analyzing   what   happens   when   a   gun   fires   a projectile against the armor of an enemy ship. (See fig.  2-17.)  The  explosive  force  of  the  powder  in the breach pushes the projectile out of the gun, giving  it  a  high  velocity.  Because  of  its  inertia, the  projectile  offers  opposition  to  this  sudden velocity and a reaction is set up that pushes the gun  backward  (kick  or  recoil).  The  force  of  the explosion  acts  on  the  projectile  throughout  its movement  in  the  gun.  This  is  force  acting  through a distance producing work. This work appears as kinetic  energy  in  the  speeding  projectile.  The resistance of the air produces friction, which uses some of the energy and slows down the projectile. Eventually, however, the projectile hits its target and,  because  of  the  inertia,  tries  to  continue moving.  The  target,  being  relatively  stationary, tends to remain stationary because of its inertia. The result is that a tremendous force is set up that either  leads  to  the  penetration  of  the  armor  or the  shattering  of  the  projectile.  The  projectile is   simply   a   means   of   transferring   energy,   in this  instance  for  destructive  purpose,  from  the gun  to  the  enemy  ship.  This  energy  is  transmitted in   the   form   of   energy   of   motion   or   kinetic energy. A similar action takes place in a fluid power system in which the fluid takes the place of the projectile. For example, the pump in a hydraulic Figure 2-17.—Relationship of inertia, velocity, and kinetic energy. system  imparts  energy  to  the  fluid,  which overcomes  the  inertia  of  the  fluid  at  rest  and causes  it  to  flow  through  the  lines.  The  fluid  flows against some type of actuator that is at rest. The fluid  tends  to  continue  flowing,  overcomes  the inertia  of  the  actuator,  and  moves  the  actuator to  do  work.  Friction  uses  up  a  portion  of  the energy  as  the  fluid  flows  through  the  lines  and components. RELATIONSHIP  OF  FORCE, PRESSURE,  AND  HEAD In   dealing   with   fluids,   forces   are   usually considered in relation to the areas over which they are   applied.   As   previously   discussed,   a   force acting over a unit area is a pressure, and pressure can alternately be stated in pounds per square inch or in terms of head, which is the vertical height of  the  column  of  fluid  whose  weight  would produce  that  pressure. In most of the applications of fluid power in the  Navy,  applied  forces  greatly  outweigh  all  other forces,  and  the  fluid  is  entirely  confined.  Under these circumstances it is customary to think of the forces  involved  in  terms  of  pressures.  Since  the term head is encountered frequently in the study of fluid power, it is necessary to understand what it  means  and  how  it  is  related  to  pressure  and force. All five of the factors that control the actions of  fluids  can,  of  course,  be  expressed  either  as force,  or  in  terms  of  equivalent  pressures  or  head. In  each  situation,  the  different  factors  are  referred to in the same terms, since they can be added and subtracted  to  study  their  relationship  to  each other. At this point you need to review some terms in general use. Gravity head, when it is important enough  to  be  considered,  is  sometimes  referred to as head. The effect of atmospheric pressure is referred  to  as  atmospheric  pressure.  (Atmospheric pressure is frequently and improperly referred to as  suction.)  Inertia  effect,  because  it  is  always directly   related   to   velocity,   is   usually   called velocity head; and friction, because it represents a loss of pressure or head, is usually referred to as  friction  head. STATIC  AND  DYNAMIC  FACTORS Gravity,   applied   forces,   and   atmospheric pressure are static factors that apply equally to 2-13