Figure 7-3.—No motion, no work.

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 of gravity-or the total weight—of the load. He’s pulling only against the rolling friction of the truck and that may be as little as 90 pounds. That is the resistance that is being overcome. Always be sure you know what resistance is being overcome by the effort, as well as the distance through which it is moved. The resistance in one case may be the weight of the object; in another it may be the frictional resistance of the object as it is dragged or rolled along the deck. 2. You have to move the resistance to do any work on it. Look at the sailor in figure 7-3. The poor guy has been holding that suitcase for 15 minutes waiting for the bus. His arm is getting tired; but according to the definition of work, he isn’t doing any because he isn’t moving the suitcase. He is merely exerting a force against the pull of gravity on the bag. You already know about the mechanical advantage of a lever. Now consider how it can be used to get work done easily. Look at figure 7-4. The load weighs 300 pounds, and the sailor wants to lift it up onto a platform a foot above the deck. How much work must he do? Since he must raise 300 pounds 1 foot, he must do 300 x 1, or 300 foot-pounds of work. Figure 7-4.—Push’em up. He can’t make this weight any smaller with any machine. If he uses the 8-foot plank as shown, he can do the amount of work by applying a smaller force through a longer distance. Notice that he has a mechanical advantage of 3, so a 100-pound push down on the end of the plank will raise the 300-pound crate. Through how long a distance will he have to exert that 100-pound push? If he neglects friction, the work he exerts on the machine will be equal to the work done by the machine. In other words, work put in = work put out. Since Work = Force x Distance, you can substitute Force x Distance on each side of the work equation. Thus: in which FI = s, = Fz = S2 = effort applied, in pounds distance through which effort moves, in feet resistance overcome, in pounds distance resistance is moved, in feet Now substitute the known values, and you get: IOOXSI=300XI S1 = 3 feet The advantage of using the lever is not that it makes any less work for you, but it allows you to do the job with the force at your command. You’d probably have some difficulty lifting 300 pounds directly upward without a machine to help you! 7-2