Figure 9-2.—Balances.
Unfortunately, the more springs are used, the more they
lose their ability to snap back to their original position.
Hence, an old spring or an overloaded spring will give
inaccurate readings.
Balanced Scale
The problem with the spring-type scale eventually
led to the invention of the balanced scale, shown in
figure 9-2. This type of scale is an application of first-
class levers. The one shown in figure 9-2, A, is the
simplest type. Since the distance from the fulcrum to the
center of each platform is equal, the scales balance when
equal weights are placed on the platforms. With your
knowledge of levers, you can figure out
yard shown in figure 9-2, B, operates.
PRESSURE
how the steel
Pressure is the amount of force within a specific
area. You measure air, steam, and gas pressure and the
fluid pressure in hydraulic systems in pounds per square
inch (psi). However, you measure water pressure in
pounds per square foot. You’ll find more about pressure
measurements in chapter 10. To help you better
understand pressure, let’s look at how pressure affects
your ability to walk across snow.
Have you ever tried to walk on freshly fallen snow
to have your feet break through the crust when you put
your weight on it? If you had worn snowshoes, you
could have walked across the snow without sinking; but
do you know why? Snowshoes do not reduce your
weight, or the amount of force, exerted on the snow; they
merely distribute it over a larger area. In doing that, the
snowshoes reduce the pressure per square inch of the
force you exert.
Let’s figure out how that works. If a man weighs
160 pounds, that weight, or force, is more or less evenly
distributed by the soles of his shoes. The area of the soles
of an average man’s shoes is roughly 60 square inches.
Each of those square inches has to carry 160 ÷ 60= 2.6
pounds of that man’s weight. Since 2 to 6 pounds per
square inch is too much weight for the snow crest to
support, his feet break through.
When the man puts on snowshoes, he distributes his
weight over an area of about 900 square inches,
depending on the size of the snowshoes. The force
on each of those square inches is equal to only
160 ÷ 900 = 0.18 pounds. Therefore, with snowshoes
on, he exerts a pressure of 0.18 psi. With this decreased
pressure, the snow can easily support him.
9-2
