CHAPTER 3THE WHEEL AND AXLECHAPTER LEARNING OBJECTIVESUpon completion of this chapter, you should be able to do the following:lExplain the advantage of the wheel and axle.Have you ever tried to open a door when the knobwas missing? If you have, you know that trying to twistthat small four-sided shaft with your fingers is toughwork. That gives you some appreciation of theadvantage you get by using a knob. The doorknob is anexample of a simple machine called a wheel and axle.The steering wheel on an automobile, the handle ofan ice cream freezer, and a brace and bit are all examplesof a simple machine. All of these devices use the wheeland axle to multiply the force you exert. If you try toturn a screw with a screwdriver and it doesn’t turn, sticka screwdriver bit in the chuck of a brace. The screw willprobably go in with little difficulty.There’s something you’ll want to get straight rightat the beginning. The wheel-and-axle machine consistsof a wheel or crank rigidly attached to the axle, whichturns with the wheel. Thus, the front wheel of anautomobile is not a wheel-and-axle machine because theaxle does not turn with the wheel.MECHANICAL ADVANTAGEHow does the wheel-and-axle arrangement help tomagnify the force you exert? Suppose you use ascrewdriver bit in a brace to drive a stubborn screw.Look at figure 3-1, view A. You apply effort on thehandle that moves in a circular path, the radius of whichis 5 inches. If you apply a 10-pound force on the handle,how much force will you exert against the resistance atthe screw? Assume the radius of the screwdriver bladeis 1/4 inch. You are really using the brace as asecond-class lever—see figure 3-1, view B. You can findthe size of the resistance by using the formulaIn thatL=1=R=E=radius of the circle through which thehandle turns,one-half the width of the edge of thescrewdriver blade,force of the resistance offered by thescrew,force of effort applied on the handle.Figure 3-1.-It magnifies your effort.3-1
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