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 and balanced.

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.

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.

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 how the steel yard shown in figure 9-2, B, operates.