148 



USING MACHINES 



When a body is at 



in that condition unless an outside 



it tends to continue 

 _ is applied. 



FIG. 231 



Experiment 107. What is centrifugal force? 



Place some water in a small pail which has a handle or 

 bail. Swing the pail and water around in a circle with the 



pail at arm's length. Does 

 the water fall out of the 

 pail? Can you notice a 

 tendency for the water to 

 pull outward against your 

 inward pull on the handle 

 of the pail ? 



Tie a small weight on a 

 fairly strong string and 

 whirl this around. Be sure 

 you are away from win- 

 dows and fellow students 

 when you do this. The best 

 place to try it is outside 

 the school building. Whirl 

 the string and weight 

 around several times and 



then let it go. Observe the direction it takes by repeating 

 the experiment several times. 



Secure a rubber band and cut it to make a single strand 

 of rubber. Tie this to a small weight and whirl the weight 

 around. Compare this result with the one above, where the 

 weight was attached by a string. 



Record the results of this experiment carefully. Answer 

 the following questions. How many forces seem to be at 

 work in each of the above cases? Why did the water stay 

 in the pail? Did you observe the forces in the case of the 

 string and weight? What happened when you released the 

 weight? Why did this happen? What was the difference 

 between the string and weight and the rubber band and 

 weight? The force pulling toward the center in these ex- 

 periments, that is, the force with which you were pulling 

 on the pail and the string, is known as centripetal force. 

 The force pulling away from the center of rotation is known 

 as centrifugal force. 



READINGS AND ACTIVITIES WHICH WILL HELP 

 -^ ANSWER THE PROBLEM QUESTIONS 



[Is perpetual motion possible? This has been a prob- 

 lem of the ages. Since man began to build machines 

 to aid him in his conquest of natural forces, he has 

 been puzzled with the problems of producing per- 

 petual motion. About the time of the American Revo- 

 lution the French patent offices were so flooded with 

 models and requests for patents on devices which 

 were supposed to achieve continuous motion that they 

 refused to take more of them. Some of the machines 

 proposed for perpetual motion were interesting and 

 most ingenious. Figure 232 shows one type. 



Exercise. After studying the drawing of a suggested 

 perpetual motion machine carefully, write your conclu- 

 sions as to why you believe it would or would not pro- 

 duce continuous motion. Record in your notebook. 



Exercise. By reference to other books see how many 



FIG. 232. PERPETUAL 

 MOTION DEVICE 



ideas and drawings for perpetual motion devices you can 

 find. Study them and see if 

 you can find the reason that 

 they will not work. 



Though man has been 

 lured by the appeal of 

 creating a machine that, 

 once set in motion, would 

 never stop, our present un- 

 derstanding of the prin- 

 ciples that control per- 

 petual motion in a me- 

 chanical device would 

 seem to indicate that it is 



not possible. Whenever moving parts of a machine 

 touch, there is friction, and to overcome this friction 

 energy from some outside source must be supplied. 

 This seems to be the difficulty with most of the 

 schemes proposed. In many instances models have 

 been made in which the friction has been reduced to 

 the very lowest point, and the machine would seem- 

 ingly run on forever, but after while the friction over- 

 comes the motion and the machine is stopped. Even 

 today hardly a year goes by that the United States 

 Patent Office does not receive drawings of fantastic 

 mechanical devices for the accomplishment of con- 

 tinuous motion. 



What are the advantages and disadvantages of fric- 

 tion? Friction is a force which both aids and hinders 

 us. A person walking on an icy pavement welcomes 

 the increased friction of sand or cinders. An automo- 

 bile is prevented from slipping on wet pavements by 

 the treads on the tires and on icy pavements by 

 chains. The locomotive engineer puts sand on the 

 tracks when they become wet or slippery. These are 

 aids of friction. On the other hand, in the auto- 

 mobile and every other modern machine man tries 

 to reduce friction. Wherever moving parts of a ma- 

 chine touch there is friction which tends to slow down 

 the machine and make it run harder. A new automo- 

 bile must not be driven fast for the first five hun- 

 dred miles. This is because the moving parts of a new 

 car are so tight that the great heat produced by 

 friction might cause damage to them. 



Exercise. We have industries whose efforts are de- 

 voted to increasing friction and others devoted to re- 

 ducing it. Can you give examples of both kinds of indus- 

 try. 



What causes friction? If you were to examine 

 even the smoothest surfaces with a powerful micro- 

 scope you would find that actually they are not 

 smooth at all, but quite rough. When two such sur- 



