4 MECHANICS 



to the weight times the length of the weight arm. That is, 

 in each of the forms of levers shown in Figs. 1 to 3, F:W = l:L, 



or FL=Wl. From this it follows that F = and W= . 



L I 



If the force and the weight are known, and it is desired to 

 calculate the lengths of the weight and force arms so that the 

 lever may balance, the following formulas may be used: 

 For the style of lever shown in Fig. 1, 

 Fa Wa 



For the style of lever shown in Fig. 2, 

 Fa Wa 



For the style of lever shown in Fig. 3, 

 Fa Wa 



FIXED AND MOVABLE PULLEYS 



A pulley consists of a grooved wheel on an axle held in a 

 frame, or block, and is very useful in moving or hoisting loads. 

 A fixed pulley is one whose block is not movable. Such a pulley 

 is shown in Fig. 1. A rope passes over the pulley and carries 

 the load W at one end, the hoisting force F being applied 



FIG. 1 FIG. 2 FIG. 3 FIG. 4 



at the other end of the rope. Neglecting the friction of the 

 pulley in its block, the force required to lift a load is equal 

 to the load; that is, F = W. 



A movable pulley is one whose block is movable, such as that 

 shown in Fig. 2. In this case, one end of the rope is fastened 



