Chapter 8 Machines
251
Calculating Pressure
To calculate pressure, divide effort by area. The formula is:
Pressure =
Effort
Area
For example, if a 120-lb. woman rests her mass on a 4-in2 heel, the
pressure is 30 psi (pounds per square inch). On the other hand, if she rests
her mass on a 1/4-in2 heel, the pressure increases to 480 psi.
In the metric system, effort is measured in newtons. Area is measured
in meters squared (m2). Pressure is calculated in newtons per meter
squared (N/m2). The metric unit of measure for pressure is the pascal (Pa).
Since a pascal is small, kilopascals are generally used (1 kPa = 1000 Pa).
Consider another example of how area affects pressure. A knife has a
sharp edge. Pressed against a surface, it takes up a very small area. That
is why it cuts: the material offers little resistance to such a tiny surface.
A dinner fork works in a similar way. The narrow prongs place enough
pressure on the food to pierce it easily.
Hydraulics and Pneumatics
The study and technology of the characteristics of liquids at rest and
in motion is called hydraulics. Pneumatics is the study and technology of
the characteristics of gases. Unlike solids, liquids and gases flow freely in
all directions. Pressure, therefore, can be transmitted in all directions. For
example, water will flow in a garden hose even when the hose is curved
in many directions.
Figure 8-54 shows a model of a hydraulic lift. The effort is being
applied to a piston. Pressure produced by the effort is being transmitted
by the liquid to a second piston. This piston moves the resistance. The
second piston has a larger area and so the pressure presses on a larger
area. This produces a larger effort. If the resistance piston has four times
the area of the effort piston, the effort on it will be four times greater.
gure 8-53. Imagine someone
stepping on your foot. Which will hurt
more, the pointed heel or the flat heel?