Figure 8-2.-One horsepower.HORSEPOWERYou measure force in pounds, distance in feet, andwork in foot-pounds. What is the common unit used formeasuring power? It is called horsepower (hp). If youwant to tell someone how powerful an engine is, youcould say that it is many times more powerful than a manor an ox or a horse. But what man? and whose ox orhorse? James Watt, the man who invented the steamengine, compared his early models with the horse. Byexperiment, he found that an average horse, hitched toa rig as shown in figure 8-2, could lift a 330-pound loadstraight up a distance of 100 feet in 1 minute. Scientistsagree that 1 horsepower equals 33,000 foot-pounds ofwork done in 1 minute.Since 60 seconds equals a minute, 1 horsepower isequal to 3S’W!W = 550 foot-pounds per second. Usethe following formula to figure horespower:CALCULATING POWERIt isn’t difficult to figure how much power you needto do a certain job in a given length of time. Nor is itdifficult to predict what size engine or motor you needto do it. Suppose an anchor winch must raise a6,600-pound anchor through 120 feet in 2 minutes. Whatmust be the theoretical horsepower rating of the motoron the winch?The first step is to find the rate at which the workmust be done using the formula:Substitute the known values in the formula, and youget:So far, you know that the winch must work at a rateof 396,000 ft-lb/min. To change this rate to horsepower,you divide by the rate at which the average horse canwork—33,000 ft-lb/min.8-2

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