Figure 2-16 illustrates a possible relationship
of these factors with respect to a particle of fluid
(P) in a system. The different forces are shown
in terms of head, or in other words, in terms of
vertical columns of fluid required to provide
the forces. At the particular moment under
consideration, a particle of water (P) is being acted
on by applied force (A), by atmospheric pressure
(B), and by gravity (C) produced by the weight
of the fluid standing over it. The particle possesses
sufficient inertia or velocity head to rise to level
P1, since head equivalent to F was lost in friction
as P passed through the system. Since atmospheric
pressure (B) acts downward on both sides of the
system, what is gained on one side is lost on the
other.
If all the pressure acting on P to force it
through the nozzle could be recovered in the form
of elevation head, it would rise to level Y. If
account is taken of the balance in atmospheric
pressure, in a frictionless system, P would rise to
level X, or precisely as high as the sum of the
gravity head and the head equivalent to the
applied force.
Kinetic Energy
It was previously pointed out that a force must
be applied to an object in order to give it a velocity
or to increase the velocity it already has. Whether
the force begins or changes velocity, it acts over
a certain distance. A force acting over a certain
distance is work. Work and all forms into which
it can be changed are classified as energy.
Obviously then, energy is required to give an
object velocity. The greater the energy used, the
greater the velocity will be.
Disregarding friction, for an object to be
brought to rest or for its motion to be slowed
down, a force opposed to its motion must be
applied to it. This force also acts over some
distance. In this way energy is given up by the
object and delivered in some form to whatever
opposes its continuous motion. The moving object
is therefore a means of receiving energy at one
place (where its motion is increased) and delivering
it to another point (where it is stopped or
retarded). While it is in motion, it is said to
contain this energy as energy of motion or kinetic
energy.
Since energy can never be destroyed, it follows
that if friction is disregarded the energy delivered
to stop the object will exactly equal the energy
that was required to increase its speed. At all times
the amount of kinetic energy possessed by an
object depends on its weight and the velocity at
which it is moving.
Figure 2-16.—Physical factors governing fluid flow.
2-12
