fluids at rest or in motion, while inertia andfriction are dynamic factors that apply only tofluids in motion. The mathematical sum ofgravity, applied force, and atmospheric pressureis the static pressure obtained at any one pointin a fluid at any given time. Static pressure existsin addition to any dynamic factors that may alsobe present at the same time.Remember, Pascal’s law states that a pressureset up in a fluid acts equally in all directions andat right angles to the containing surfaces. Thiscovers the situation only for fluids at rest orpractically at rest. It is true only for the factorsmaking up static head. Obviously, when velocitybecomes a factor it must have a direction, andas previously explained, the force related to thevelocity must also have a direction, so thatPascal’s law alone does not apply to the dynamicfactors of fluid power.The dynamic factors of inertia and friction arerelated to the static factors. Velocity head andfriction head are obtained at the expense of statichead. However, a portion of the velocity head canalways be reconverted to static head. Force, whichcan be produced by pressure or head when dealingwith fluids, is necessary to start a body movingif it is at rest, and is present in some form whenthe motion of the body is arrested; therefore,whenever a fluid is given velocity, some part ofits original static head is used to impart thisvelocity, which then exists as velocity head.BERNOULLI’S PRINCIPLEConsider the system illustrated in figure 2-18.Chamber A is under pressure and is connected bya tube to chamber B, which is also under pressure.The pressure in chamber A is static pressure of100 psi. The pressure at some point (X) along theconnecting tube consists of a velocity pressure ofFigure 2-18.—Relation of static and dynamic factors—Bernoulli’s principle.10 psi exerted in a direction parallel to the lineof flow, plus the unused static pressure of 90 psi,which still obeys Pascal’s law and operates equallyin all directions. As the fluid enters chamber Bit is slowed down, and its velocity is changed backto pressure. The force required to absorb itsinertia equals the force required to start the fluidmoving originally, so that the static pressure inchamber B is equal to that in chamber A.This situation (fig. 2-18) disregards friction;therefore, it would not be encountered in actualpractice. Force or head is also required toovercome friction but, unlike inertia effect, thisforce cannot be recovered again, although theenergy represented still exists somewhere as heat.Therefore, in an actual system the pressure inchamber B would be less than in chamber A bythe amount of pressure used in overcomingfriction along the way.At all points in a system the static pressure isalways the original static pressure, less any velocityhead at the point in question and less the frictionhead consumed in reaching that point. Since boththe velocity head and the friction head representenergy that came from the original static head,and since energy cannot be destroyed, the sum ofthe static head, the velocity head, and the frictionhead at any point in the system must add up tothe original static head. This is known asBernoulli's principle, which states: For thehorizontal flow of fluid through a tube, the sumof the pressure and the kinetic energy per unitvolume of the fluid is constant. This principlegoverns the relations of the static and dynamicfactors concerning fluids, while Pascal’s law statesthe manner in which the static factors behavewhen taken by themselves.MINIMIZING FRICTIONFluid power equipment is designed to reducefriction to the lowest possible level. Volume andvelocity of flow are made the subject of carefulstudy. The proper fluid for the system is chosen.Clean, smooth pipe of the best dimensions for theparticular conditions is used, and it is installedalong as direct a route as possible. Sharp bendsand sudden changes in cross-sectional areas areavoided. Valves, gauges, and other componentsare designed to interrupt flow as little as possible.Careful thought is given to the size and shape ofthe openings. The systems are designed so they2-14