FORCE AND MOTION 61 



The advantage in using the pulley at the left (Fig. 21) is simply change 

 of direction. A 50-lb. weight is balanced by P, which = 50 Ib. + enough to 

 overcome resistance in the machine. There are two advantages in the 

 use of the pulley at the right. W is supported by 2 cords, each of which 

 bears half the weight of W + the weight of the pulley block. P must 

 balance the weight supported by one cord. 3. Disregarding internal 

 resistance, what P will balance 550 Ib. in the first pulley? 4. What P in 

 the second? 



5. Where is P applied to the wheel and axle (Fig. 22)? In what direc- 

 tion and how far does P move? 6. In what direction does W move? How 

 far does it move for one turn of the axle? 



The thread of a screw fits a groove in the nut, which hi Fig. 23 is the 

 base of the screw. At the beginning of operation, the head of the screw 

 rests upon the nut; spokes are thrust into the head of the screw by which 

 to turn it. If used in lifting great weights, as in house moving, several 

 men can be employed. As- the head makes one turn, the screw and the 

 weight upon it rise as much as the distance between the threads. 7. The 

 threads of a set of jack screws are 2 in. apart; how many turns are needed 

 to raise a building 1 ft.? 



Complex machines, such as bicycles, sewing machines, egg 

 beaters, or gas engines, are combinations and modifications 

 of these simple machines and a few others. (See Fig. 24, 

 p. 62.) 



The force used in a machine is called the power (P), 

 whether it is the muscular force of man or animal, the pull 

 of gravity, or the push of steam. The resistance to be over- 

 come is called the weight (W). It may be a block of stone 

 to be lifted, a bale of hay to be compressed, or a building to 

 be moved. Any resistance which the machine itself furnishes, 

 on account of stiffness of ropes and friction between parts 

 of the machine, is included in W. 



62. Experiments. An experiment is an attempt to find 

 out the truth from observation of things themselves rather 

 than from what others have learned and written about them. 



When a piece of wood is placed on one pan of a balance, 

 that side goes down. We place a piece of stone there with 

 the same result. We exchange that for paper or feathers, 

 and always that side of the balance goes down. We ask 

 ourselves, "Why?" 



If we conclude, after many experiments, that it is because 

 there is more matter to be attracted toward the earth on 



