MECHANICAL FORCES 



31 



The three classes of levers, expressed in terms of the relative positions of 

 applied force, F a , resultant force, F r , and fulcrum, with directions denoted 

 by the arrows, are given in a classical example in Figure 2-1. 



2nd class 



weight of jaw 



weight of body 



weight of 

 head 



Figure 2-1. First-, Second-, and Third-Class Levers. 



The muscular-skeletal system of the human body is a complex system of 

 levers. The majority of these are third-class levers. A runner on tiptoe has 

 a second-class lever in his foot: the ball is the fulcrum, F a is at the heel, ap- 

 plied by Achilles' tendon and the calf muscle, and F T is exerted near the 

 instep. The jaw, the forearm, and the fingers of the hand are all third-class 

 levers. However Jiu-jitsu is a study in first-class levers, and the arm and leg 

 locks used in wrestling are almost invariably first-class levers. While doing 

 push-ups the body is operating as a second-class lever. The pump of an old- 

 fashioned well and a wheelbarrow are second-class levers, and there are 

 countless other examples of each among man's tools. Simple levers were 

 man's first machines. 



Compressed Gas 



Pressure is mechanical force per unit area (Figure 2-2). Atmospheric 

 pressure is simply the weight force of a column of air 1 cm 2 in area and of a 

 height, h, equal to the effective height of air above the earth. From basic 

 definitions P = p gh, where p is the average density over the height, h. The 

 units of pressure are dynes cm -2 , and of g, cm sec -2 . 



However, it is common practice, where differences or ratios of pressure are 

 involved, to ignore the factor, g, which is constant at any particular spot on 

 the earth's surface. The weight of the column of air is about 1 ,050 g or 1 5 lb 

 above 1 in. 2 The common unit is 15 lb (force) per sq in. (15 psi) = 1 at- 

 mosphere (1 atm) at sea level. 



