THE PULLEY. 



87 



Alleys for experiments must be very carefully made, and are 

 best purchased of an instrument- maker. The cords must be very 

 flexible ; twisted silk is the best material. The weights may be 

 placed in two light square boxes made of cardboard, threads being 

 fixed to the corners and tied together above. 



I 



A pulley like that shown in fig. 60, J., which is 

 attached to some support and cannot change its position 

 is called & fixed pulley. The two forces applied at the 

 ends of the cords will describe equal spaces, if motion 

 takes place, and it follows that there will be equilibrium 

 if the forces are equal. The fixed pulley serves chiefly 

 for changing the direction of a force ; a downward pull 

 on one side produces an upward motion on the other. 

 In the moveable pulley, fig. 60, B, only ^ 



one force acts along the cord ; the "* ~ == ^' '^^ ^ 

 other acts at the axis of the pulley. If 

 the two portions of the cord after pass- 

 ing round the pulley are parallel, and 

 it were required to raise the pulley 

 through 10 cm , each portion of the cord 

 will have to be shortened by this 

 length ; but as one end is fixed, the 

 other end will have to be pulled up- 

 wards through twice that length, or 

 ;20 cm . The force at the end of the 

 cord describes twice the space of the 

 force applied at the axis of the pulley ; 

 this force must therefore be twice as 

 ^reat as the former if the work done is 

 to be equal. Since weights can act only 

 lownwards, the free end of the cord, 

 be passed over a fixed pulley, if 



V 



Fio. 61. 



