Figure 1-4.-This makes it easier.Figure 1-5.-A third-class lever.weight or resistance to be overcome is at the other end,with the effort applied at some point between. You canalways spot the third-class levers because you will findthe effort applied betweenthe fulcrum and theresistance. Look at figure 1-5. It is easy to see that whileE moved the short distance (e), the resistance (R) wasmoved a greater distance (r). The speed of R must havebeen greater than that of E, since R covered a greaterdistance in the same length of time.Your arm (fig. 1-6) is a third-class lever. It is thislever action that makes it possible for you to flex yourarms so quickly. Your elbow is the fulcrum. Your bicepsmuscle, which ties onto your forearm about an inchbelow the elbow, applies the effort; your hand is theresistance, located about 18 inches from the fulcrum. Inthe split second it takes your biceps muscle to contractan inch, your hand has moved through an 18-inch arc.You know from experience that it takes a big pull at Eto overcome a relatively small resistance at R. Just toexperience this principle, try closing a door by pushingon it about 3 or 4 inches from the hinges (fulcrum). Themoral is, you don’t use third-class levers to do heavyjobs; you use them to gain speed.Figure 1-6.-Your arm is a lever.Figure 1-7.-Easy does it.One convenience of machines is that you candetermine in advance the forces required for theiroperation, as well as the forces they will exert. Considerfor a moment the first class of levers. Suppose you havean iron bar, like the one shown in figure 1-7. This bar is9 feet long, and you want to use it to raise a 300-poundcrate off the deck while you slide a dolly under the crate;but you can exert only 100 pounds to lift the crate. So,you place the fulcrum-a wooden block-beneath oneend of the bar and force that end of the bar under thecrate. Then, you push down on the other end of the bar.After a few adjustments of the position of the fulcrum,you will find that your 100-pound force will just fit thecrate when the fulcrum is 2 feet from the center of thecrate. That leaves a 6-foot length of bar from the fulcrumto the point where you pushdown. The 6-foot portion isthree times as long as the distance from the fulcrum tothe center of the crate. And you lifted a load three timesas great as the force you applied (3 x 100 = 300 pounds).1-3
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