CHAPTER 9FORCE AND PRESSURECHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you should be able to do the following:lExplain the difference in force and pressure.lDiscuss the operation of force- and pressure-measuring devices.By this time you should have a pretty good idea ofwhat force is. Now you will learn the difference betweenforce and pressure and how force affects pressure.FORCEForce is the pull of gravity exerted on an object oran object’s thrust of energy against friction. You applya force on a machine; the machine, in turn, transmits aforce to the load. However, other elements besides menand machines can also exert a force. For example, ifyou’ve been out in a sailboat, you know that the windcan exert a force. Further, after the waves have knockedyou on your ear a couple of times, you have grasped theidea that water, too, can exert a force. Aboard ship, fromreveille to taps you are almost constantly either exertingforces or resisting them.MEASURING FORCEWeight is a measurement of the force, or pull ofgravity, on an object. You’ve had a lot of experience inmeasuring forces. At times, you have estimated or“guessed’ the weight of a package you were going tomail by “hefting” it. However, to find its accurateweight, you would have put it on a force-measuringdevice known as a scale. Scales are of two types: springand balanced.Spring ScaleYou 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 aspring caused the spring to stretch a certain distance andthat hanging a 2-pound weight on the spring caused it tostretch twice as far. By attaching a pointer to the springand inserting the pointer through a face, he could markpoints on the face to indicate various measurements inpounds and ounces.We use this type of scale to measure the pullof gravity-the weight-of an object or the force of apull exerted against friction, as shown in figure 9-1.Figure 9-1.—You can measure force with a scale.9-1
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