260 Introduction to the Study of Science 



ance ? Explain. The same fact is illustrated in the locomotive. An 

 engineer in starting a train may sometimes let too much steam into 

 the cylinder, so that the driving wheels slip and spin on the rails. If 

 he supplies more steam while the wheels are slipping, will the engine 

 start the train? What must be done to give the wheels a chance to 

 take hold ? Does this agree with the facts observed in the case of the 

 sliding block? Why and how is the track sanded? 



Advantages of friction. The facts presented in the experi- 

 ments show both the advantages and disadvantages of sliding 

 and rolling friction. They emphasize especially the necessity 

 for a certain amount of friction between the wheels and the 

 track in order that the train may move. A frictionless contact 

 of wheels and track would make a start on a level track impos- 

 sible. Imagine, for instance, a locomotive trying to start on 

 greased rails. Furthermore, locomotives and many other 

 moving bodies must stop. What is the necessary condition 

 of stopping a train, a wagon, an automobile, or a boat on the 

 water? Explain each particular case. 



There is friction also between the axle and its bearings. 

 This is much less than that between the wheels and the track. 

 Is this friction of the axle and its bearings advantageous or 

 not? Suppose that instead of greasing the axle, the track 

 were greased. What would be the result? Consider this in 

 another way. The diameter of a wheel is two feet. What is 

 its circumference? The diameter of the axle is four inches. 

 What is its circumference? On the basis of these dimen- 

 sions, how many times as far as the axle slips in its bear- 

 ings will the wheel roll on the track ? WTrich will be the more 

 advantageous, reducing the friction between the wheel and 

 the track, or that of the axle in its bearings? Explain your 

 answer. 



Friction is commonly reduced in axle bearings by the use of 

 lubricants such as oil and graphite. Other means of over- 

 coming such friction are by the use of what are called friction- 

 reducing metal alloys. One of the most familiar anti-friction 

 alloys is Babbitt metal. If we examine the bearing of a small 



