one-forty-eighth of a foot. You gain force at theexpense of distance.FRICTIONSuppose you are going to push a 400-pound crateup a 12-foot plank; the upper end is 3 feet higherthan the lower end. You decide that a 100-poundpush will do the job. The height you will raise thecrate is one-fourth of the distance through which youwill exert your push. The theoretical mechanicaladvantage is 4. Then you push on the crate, applying100 pounds of force; but nothing happens! You’veforgotten about the friction between the surface of thecrate and the surface of the plank. This friction actsas a resistance to the movement of the crate; youmust overcome this resistance to move the crate. Infact, you might have to push as much as 150 poundsto move it. You would use 50 pounds to overcome thefrictional resistance, and the remaining 100 poundswould be the useful push that would move the crateup the plank.Friction is the resistance that one surface offers toits movement over another surface. The amount offriction depends upon the nature of the two surfacesand the forces that hold them together.In many instances fiction is useful to you. Frictionhelps you hold back the crate from sliding down theinclined ramp. The cinders you throw under thewheels of your car when it’s slipping on an icypavement increase the friction. You wear rubber-soled shoes in the gym to keep from slipping.Locomotives carry a supply of sand to drop on thetracks in front of the driving wheels to increase thefriction between the wheels and the track. Nails holdstructures together because of the friction betweenthe nails and the lumber.You make friction work for you when you slow upan object in motion, when you want traction, andwhen you prevent motion from taking place. Whenyou want a machine to run smoothly and at highefficiency, you eliminate as much friction as possibleby oiling and greasing bearings and honing andsmoothing rubbing surfaces.Where you apply force to cause motion, frictionmakes the actual mechanical advantage fall short ofthe theoretical mechanical advantage. Because offriction, you have to make a greater effort toovercome the resistance that you want to move. If youplace a marble and a lump of sugar on a table andgive each an equal push, the marble will movefarther. That is because rolling friction is always lessthan sliding friction. You take advantage of this factwhenever you use ball bearings or roller bearings.See figure 7-7.Figure 7-7.—These reduce friction.Figure 7-8.—It saves wear and tear.The Navy takes advantage of that fact that rollingfriction is always less than sliding friction. Look atfigure 7-8. This roller chock cuts down the wear andtear on lines and cables that are run through it. Italso reduces friction and reduces the load the winchhas to work against.7-4
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