Examples of Pressure Measurement. A table with a 10-inch by 10-
inch flat top contains 100 square inches of surface. If a 100-pound
slab of exactly the same dimensions is placed on the table top, one
pound per square inch pressure is exerted over the entire table
surface.
Now, think of the same table (100 square inches) with a 100-pound
block instead of the slab resting on its top. Assume this block has
a face of only 50 square inches contacting the table.
Because the
area of contact has been cut in half and the weight of the block
remains the same, the pressure exerted on the table doubles to 2 psi.
As a final example, suppose a long rod weighing 100 pounds with a
face of 1 square inch is balanced upright on the table top.
The
pressure now being exerted on the table is increased to 100 psi,
since the entire load is being supported on a single square inch of
the table surface. These examples are illustrated in Figure 1-1.
Force-Area-Pressure Formulas. From the preceding discussion, you
can see that the formula to find the pressure acting on a surface is
"pressure equals force divided by area."
If "P" is the symbol for
pressure, "A" the symbol for area, and "F" the symbol for force, the
formula can be expressed as follows:
By transposing the symbols in this formula, two other important
formulas are derived: one for area; one for force.
Respectively,
they are--
However, when using any of these formulas, two of the factors must
be known to be able to determine the third unknown factor.
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