it  becomes 1 10 —=— 18 E and E =  18 x  10  =  180  pounds. Your muscle must exert a 180-pound pull to hold up a 10-pound projectile. Our muscles are poorly arranged for  lifting  or  pulling-and  that’s  why  some  work  seems pretty  tough.  But  remember,  third-class  levers  are  used primarily  to  speed  up  the  motion  of  the  resistance. Curved Lever Arms Up to this point, you have been looking at levers with straight arms. In every case, the direction in which the resistance acts is parallel to the direction in which the effort is exerted. However, not all levers are straight. You ’ll need to learn to recognize all types of levers and to understand their operation. Look at figure 1-9. You may wonder how to measure the length of the effort arm, which is represented by the curved  pump  handle.  You  do  not  measure  around  the curve; you still use a straight-line distance. To determine the length of the effort arm, draw a straight line (AB) through the point where the effort is applied and in the direction that it is applied. From point E on this line, draw a second line (EF) that passes through the fulcrum and is perpendicular to line AB. The length of line EF is the actual length (L) of the effort arm. To find the length of the resistance arm, use the same method. Draw a line (MN) in the direction that the resistance is operating and through the point where the resistance is attached to the other end of the handle. From point R on this line, draw a line (RF) perpendicular to MN so that it passes through the fulcrum. The length of RF is the length (l) of the resistance arm. Regardless  of  the  curvature  of  the  handle,  this method can be used to find lengths L and l. Then, curved levers are solved just like straight levers. MECHANICAL  ADVANTAGE There is another thing about the first and second classes of levers that you have probably noticed by now. Since they can be used to magnify the applied force, they provide  positive  mechanical  advantages.  The  third-class lever provides what is called a fractional mechanical advantage,  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  a 50-pound pull actually overcame the 200-pound weight Figure  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 the wheelbarrow   is   4.   Expressing   the   same   idea   in mathematical  terms, MECHANICAL  ADVANTAGE  = RESISTANCE EFFORT or Thus, in the case of the wheelbarrow, This  rule—mechanical  advantage  equals  resistance divided by effort —applies to all machines. The mechanical advantage of a lever may also be found by dividing the length of effort arm A by the length of resistance arm a. Stated as a formula, this reads: or How does this apply to third-class levers? Your muscle  pulls  with  a  force  of  1,800  pounds  to  lift  a 100-pound   projectile.   So   you   have   a   mechanical advantage of which is fractional-less than 1. 1-5