CHAPTER 7WORKCHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you should be able to do the following:lDefine the term “work” when applied to mechanical power.MEASUREMENTYou know that machines help you to do work. Whatis work? Work doesn’t mean simply applying a force. Ifthat were so, you would have to consider that the sailorin figure 7-1 is doing work. He is busy applying his220-pound force on the seabag. However, no work isbeing done!Work in the mechanical sense, is done when aresistance is overcome by a force acting through ameasurable distance. Now, if that sailor were to lift his90-pound bag off the deck and put it on his bunk, hewould be doing work. He would be overcoming aresistance by applying a force through a distance.Notice that work involves two factors-force andmovement through a distance. You measure force inpounds and distance in feet. Therefore, you measurework in units called foot-pounds. You do 1 foot-poundof work when you lift a 1-pound weight through a heightFigure 7-1.—No work is being done.of 1 foot, You also do 1 foot-pound of work when youapply 1 pound of force on any object through a distanceof 1 foot. Writing this as a formula, it becomes—WORKFORCED I S T A N CE(foot-pounds)(pounds)(feet)Thus, if you lift a 90-pound bag through a verticaldistance of 5 feet, you will doWORK = 90 X 5 = 450 ft-lb.You should remember two points about work1. In calculating the work done, you measure theactual resistance being overcome. The resistance is notnecessarily the weight of the object you want to move.To understand this more clearly, look at the job the sailorin figure 7-2 is doing. He is pulling a 900-pound load ofsupplies 200 feet along the dock. Does this mean that heFigure 7-2.—Working against friction.7-1