Figure 7-3.—No motion, no work.is doing 900 x 200, or 180,000 foot-pounds of work?Of course not. He isn’t working against the pull ofgravity-or the total weight—of the load. He’s pullingonly against the rolling friction of the truck and that maybe as little as 90 pounds. That is the resistance that isbeing overcome. Always be sure you know whatresistance is being overcome by the effort, as well as thedistance through which it is moved. The resistance inone case may be the weight of the object; in another itmay be the frictional resistance of the object as it isdragged or rolled along the deck.2. You have to move the resistance to do any workon it. Look at the sailor in figure 7-3. The poor guy hasbeen holding that suitcase for 15 minutes waiting for thebus. His arm is getting tired; but according to thedefinition of work, he isn’t doing any because he isn’tmoving the suitcase. He is merely exerting a forceagainst the pull of gravity on the bag.You already know about the mechanical advantageof a lever. Now consider how it can be used to get workdone easily. Look at figure 7-4. The load weighs 300pounds, and the sailor wants to lift it up onto a platforma foot above the deck. How much work must he do?Since he must raise 300 pounds 1 foot, he must do300 x 1, or 300 foot-pounds of work.Figure 7-4.—Push’em up.He can’t make this weight any smaller with anymachine. If he uses the 8-foot plank as shown, he cando the amount of work by applying a smaller forcethrough a longer distance. Notice that he has amechanical advantage of 3, so a 100-pound push downon the end of the plank will raise the 300-pound crate.Through how long a distance will he have to exert that100-pound push? If he neglects friction, the work heexerts on the machine will be equal to the work done bythe machine. In other words,work put in = work put out.Since Work = Force x Distance, you can substituteForce x Distance on each side of the work equation.Thus:in whichFI =s, =Fz=S2 =effort applied, in poundsdistance through which effort moves, in feetresistance overcome, in poundsdistance resistance is moved, in feetNow substitute the known values, and you get:IOOXSI=300XIS1 = 3 feetThe advantage of using the lever is not that it makesany less work for you, but it allows you to do the jobwith the force at your command. You’d probably havesome difficulty lifting 300 pounds directly upwardwithout a machine to help you!7-2