FORCE AND PRESSURE
CHAPTER LEARNING OBJECTIVES
Upon completion of this chapter, you should be able to do the following:
l Explain the difference in force and pressure.
l Discuss the operation of force- and pressure-measuring devices.
By this time you should have a pretty good idea of
what force is. Now you will learn the difference between
force and pressure and how force affects pressure.
Force is the pull of gravity exerted on an object or
an objects thrust of energy against friction. You apply
a force on a machine; the machine, in turn, transmits a
force to the load. However, other elements besides men
and machines can also exert a force. For example, if
youve been out in a sailboat, you know that the wind
can exert a force. Further, after the waves have knocked
you on your ear a couple of times, you have grasped the
idea that water, too, can exert a force. Aboard ship, from
reveille to taps you are almost constantly either exerting
forces or resisting them.
Weight is a measurement of the force, or pull of
gravity, on an object. Youve had a lot of experience in
measuring forces. At times, you have estimated or
guessed the weight of a package you were going to
mail by hefting it. However, to find its accurate
weight, you would have put it on a force-measuring
device known as a scale. Scales are of two types: spring
You can readily measure force with a spring scale.
An Englishman named Hooke invented the spring scale.
He discovered that hanging a 1-pound weight on a
spring caused the spring to stretch a certain distance and
that hanging a 2-pound weight on the spring caused it to
stretch twice as far. By attaching a pointer to the spring
and inserting the pointer through a face, he could mark
points on the face to indicate various measurements in
pounds and ounces.
We use this type of scale to measure the pull
of gravity-the weight-of an object or the force of a
pull exerted against friction, as shown in figure 9-1.
Figure 9-1.You can measure force with a scale.