fluids  at  rest  or  in  motion,  while  inertia  and friction  are  dynamic  factors  that  apply  only  to fluids   in   motion.   The   mathematical   sum   of gravity, applied force, and atmospheric pressure is  the  static  pressure  obtained  at  any  one  point in a fluid at any given time. Static pressure exists in addition to any dynamic factors that may also be present at the same time. Remember,  Pascal’s  law  states  that  a  pressure set up in a fluid acts equally in all directions and at  right  angles  to  the  containing  surfaces.  This covers  the  situation  only  for  fluids  at  rest  or practically at rest. It is true only for the factors making up static head. Obviously, when velocity becomes  a  factor  it  must  have  a  direction,  and as previously explained, the force related to the velocity  must  also  have  a  direction,  so  that Pascal’s law alone does not apply to the dynamic factors  of  fluid  power. The dynamic factors of inertia and friction are related  to  the  static  factors.  Velocity  head  and friction head are obtained at the expense of static head. However, a portion of the velocity head can always  be  reconverted  to  static  head.  Force,  which can be produced by pressure or head when dealing with  fluids,  is  necessary  to  start  a  body  moving if it is at rest, and is present in some form when the  motion  of  the  body  is  arrested;  therefore, whenever  a  fluid  is  given  velocity,  some  part  of its  original  static  head  is  used  to  impart  this velocity,  which  then  exists  as  velocity  head. BERNOULLI’S  PRINCIPLE Consider  the  system  illustrated  in  figure  2-18. Chamber A is under pressure and is connected by a tube to chamber B, which is also under pressure. The  pressure  in  chamber  A  is  static  pressure  of 100 psi. The pressure at some point (X) along the connecting tube consists of a velocity pressure of Figure 2-18.—Relation of static and dynamic factors— Bernoulli’s   principle. 10 psi exerted in a direction parallel to the line of flow, plus the unused static pressure of 90 psi, which still obeys Pascal’s law and operates equally in  all  directions.  As  the  fluid  enters  chamber  B it  is  slowed  down,  and  its  velocity  is  changed  back to  pressure.  The  force  required  to  absorb  its inertia equals the force required to start the fluid moving originally, so that the static pressure in chamber  B  is  equal  to  that  in  chamber  A. This  situation  (fig.  2-18)  disregards  friction; therefore, it would not be encountered in actual practice.   Force   or   head   is   also   required   to overcome  friction  but,  unlike  inertia  effect,  this force  cannot  be  recovered  again,  although  the energy  represented  still  exists  somewhere  as  heat. Therefore,  in  an  actual  system  the  pressure  in chamber B would be less than in chamber A by the  amount  of  pressure  used  in  overcoming friction  along  the  way. At all points in a system the static pressure is always  the  original  static  pressure,  less  any  velocity head at the point in question and less the friction head consumed in reaching that point. Since both the velocity head and the friction head represent energy  that  came  from  the  original  static  head, and since energy cannot be destroyed, the sum of the static head, the velocity head, and the friction head at any point in the system must add up to the   original   static   head.   This   is   known   as Bernoulli's   principle,   which   states:   For   the horizontal  flow  of  fluid  through  a  tube,  the  sum of  the  pressure  and  the  kinetic  energy  per  unit volume  of  the  fluid  is  constant.  This  principle governs  the  relations  of  the  static  and  dynamic factors  concerning  fluids,  while  Pascal’s  law  states the  manner  in  which  the  static  factors  behave when taken by themselves. MINIMIZING  FRICTION Fluid power equipment is designed to reduce friction to the lowest possible level. Volume and velocity  of  flow  are  made  the  subject  of  careful study. The proper fluid for the system is chosen. Clean, smooth pipe of the best dimensions for the particular  conditions  is  used,  and  it  is  installed along as direct a route as possible. Sharp bends and sudden changes in cross-sectional areas are avoided.  Valves,  gauges,  and  other  components are  designed  to  interrupt  flow  as  little  as  possible. Careful thought is given to the size and shape of the openings. The systems are designed so they 2-14