Here is a sign of a direct relationship between the lengthof the lever arm and the force acting on that arm.You can state this relationship in general terms bysaying: the length of the effort arm is the same numberof times greater than the length of the resistance arm asthe resistance to be overcome is greater than the effortyou must apply. Writing these words as a mathematicalequation, we havewhereL = length of effort arm,l = length of resistance arm,R = resistance weight or force, andE = effort force.Remember that all distances must be in the sameunits, such as feet, and that all forces must be in the sameunits, such as pounds.Now let’s take another problem and see how itworks out. Suppose you want to pry up the lid of a paintcan (fig. 1-8) with a 6-inch file scraper, and you knowthat the average force holding the lid is 50 pounds. If thedistance from the edge of the paint can to the edge of thecover is 1 inch, what force will you have to apply on theend of the file scraper?According to the formula,here,L = 5 inchesl = 1 inchR = 50 pounds, andE is unknown.Then, substituting the numbers in their proper places,we haveandE=soxl5= 10 poundsYou will need to apply a force of only 10 pounds.Figure 1-8.-A first-class job.The same general formula applies for the secondclass of levers; but you must be careful to measure theproper lengths of the effort arm and the resistance arm.Looking back at the wheelbarrow problem, assume thatthe length of the handles from the axle of thewheel—which is the fulcrum-to the grip is 4 feet. Howlong is the effort arm? You’re right, it’s 4 feet. If thecenter of the load of sand is 1 foot from the axle, thenthe length of the resistance arm is 1 foot.By substituting in the formula,andE = 50 pounds.Now for the third-class lever. With one hand, youlift a projectile weighing approximately 10 pounds. Ifyour biceps muscle attaches to your forearm 1 inchbelow your elbow and the distance from the elbow tothe palm of your hand is 18 inches, what pull must yourmuscle exert to hold the projectile and flex your arm atthe elbow?By substituting in the formula,1-4
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