it becomes110—=—18 EandE = 18 x 10 = 180 pounds.Your muscle must exert a 180-pound pull to hold upa 10-pound projectile. Our muscles are poorly arrangedfor lifting or pulling-and that’s why some work seemspretty tough. But remember, third-class levers are usedprimarily to speed up the motion of the resistance.Curved Lever ArmsUp to this point, you have been looking at leverswith straight arms. In every case, the direction in whichthe resistance acts is parallel to the direction in whichthe effort is exerted. However, not all levers are straight.You ’ll need to learn to recognize all types of levers andto understand their operation.Look at figure 1-9. You may wonder how to measurethe length of the effort arm, which is represented by thecurved pump handle. You do not measure around thecurve; you still use a straight-line distance. To determinethe length of the effort arm, draw a straight line (AB)through the point where the effort is applied and in thedirection that it is applied. From point E on this line,draw a second line (EF) that passes through the fulcrumand is perpendicular to line AB. The length of line EF isthe actual length (L) of the effort arm.To find the length of the resistance arm, use the samemethod. Draw a line (MN) in the direction that theresistance is operating and through the point where theresistance is attached to the other end of the handle.From point R on this line, draw a line (RF) perpendicularto MN so that it passes through the fulcrum. The lengthof RF is the length (l) of the resistance arm.Regardless of the curvature of the handle, thismethod can be used to find lengths L and l. Then, curvedlevers are solved just like straight levers.MECHANICAL ADVANTAGEThere is another thing about the first and secondclasses of levers that you have probably noticed by now.Since they can be used to magnify the applied force, theyprovide positive mechanical advantages. The third-classlever provides what is called a fractional mechanicaladvantage, which is really a mechanical disadvantage—you use more force than the force of the load you lift.In the wheelbarrow problem, you saw that a50-pound pull actually overcame the 200-pound weightFigure 1-9.-A curved lever arm.of the sand. The sailor’s effort was magnified four times,so you may say that the mechanical advantage of thewheelbarrow is 4. Expressing the same idea inmathematical terms,MECHANICAL ADVANTAGE =RESISTANCEEFFORTorThus, in the case of the wheelbarrow,This rule—mechanical advantage equals resistancedivided by effort —applies to all machines.The mechanical advantage of a lever may also befound by dividing the length of effort arm A by the lengthof resistance arm a. Stated as a formula, this reads:orHow does this apply to third-class levers? Yourmuscle pulls with a force of 1,800 pounds to lift a100-pound projectile. So you have a mechanicaladvantage ofwhich is fractional-less than 1.1-5