Figure 2-16 illustrates a possible relationshipof these factors with respect to a particle of fluid(P) in a system. The different forces are shownin terms of head, or in other words, in terms ofvertical columns of fluid required to providethe forces. At the particular moment underconsideration, a particle of water (P) is being actedon by applied force (A), by atmospheric pressure(B), and by gravity (C) produced by the weightof the fluid standing over it. The particle possessessufficient inertia or velocity head to rise to levelP1, since head equivalent to F was lost in frictionas P passed through the system. Since atmosphericpressure (B) acts downward on both sides of thesystem, what is gained on one side is lost on theother.If all the pressure acting on P to force itthrough the nozzle could be recovered in the formof elevation head, it would rise to level Y. Ifaccount is taken of the balance in atmosphericpressure, in a frictionless system, P would rise tolevel X, or precisely as high as the sum of thegravity head and the head equivalent to theapplied force.Kinetic EnergyIt was previously pointed out that a force mustbe applied to an object in order to give it a velocityor to increase the velocity it already has. Whetherthe force begins or changes velocity, it acts overa certain distance. A force acting over a certaindistance is work. Work and all forms into whichit can be changed are classified as energy.Obviously then, energy is required to give anobject velocity. The greater the energy used, thegreater the velocity will be.Disregarding friction, for an object to bebrought to rest or for its motion to be sloweddown, a force opposed to its motion must beapplied to it. This force also acts over somedistance. In this way energy is given up by theobject and delivered in some form to whateveropposes its continuous motion. The moving objectis therefore a means of receiving energy at oneplace (where its motion is increased) and deliveringit to another point (where it is stopped orretarded). While it is in motion, it is said tocontain this energy as energy of motion or kineticenergy.Since energy can never be destroyed, it followsthat if friction is disregarded the energy deliveredto stop the object will exactly equal the energythat was required to increase its speed. At all timesthe amount of kinetic energy possessed by anobject depends on its weight and the velocity atwhich it is moving.Figure 2-16.—Physical factors governing fluid flow.2-12
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