Since area equals force divided by pressure, the
formula for area is written as follows: A = F divided
Figure 3-1 shows a memory device for recalling
the different variations of the formula. Any letter in the
triangle may be expressed as the product or quotient of
the other two, depending on its position within the
Incompressibility and Expansion
For all practical purposes, fluids are incom-
pressible. Under extremely high pressures: the volume
of a fluid can be decreased somewhat, though the
decrease is so slight that it is considered to be
negligible except by design engineers.
Liquids expand and contract because of
temperature changes. When liquid in a closed
container is subjected to high temperatures, it expands
and this exerts pressure on the walls of the container;
therefore, it is necessary that pressure-relief
mechanisms and expansion chambers be incorporated
into hydraulic systems. Without these precautionary
measures, the expanding fluid could exert enough
pressure to rupture the system.
Transmission of Forces through Liquids
When the end of a solid bar is struck, the main
force of the blow is carried straight through the bar to
the other end (fig. 3-2, view A). This happens because
the bar is rigid. The direction of the blow almost
entirely determines the direction of the transmitted
force. The more rigid the bar, the less force is lost
inside the bar or transmitted outward at right angles to
the direction of the blow.
Figure 3-1.Device for determining the arrangement of the
force, pressure, and area formula.
Figure 3-2.Transmission of force: (A) Solid; (B) Fluid.
When a force is applied to the end of a column of
confined liquid (fig. 3-2, view B), it is transmitted
straight through the other end and also undiminished in
every direction throughout the columnforward,
backward, and sidewaysso that the containing
vessel is literally tilled with pressure.
An example of this distribution of force is shown in
figure 3-3. The flat hose takes on a circular cross
section when it is filled with water under pressure. The
outward push of the water is equal in every direction.
The foundation of modern hydraulics was
established when Blaise Pascal, a French scientist,
Figure 3-3.Distribution of force.