276 Introduction to the Study of Science 



a penny or any similar body. Snap the card directly on the 

 edge ; it is driven away, leaving the penny in practically the 

 same location. Passengers in a car which starts suddenly 

 tend to stay where they are or fall in a direction opposite to 

 that of the moving car. Passengers on a car which suddenly 

 stops do not cease instantly and entirely their motion in the 

 direction they were traveling. Cite other examples from 

 your experience and observation. Do they not all point to 

 the fact that whether in motion or at rest things tend to con- 

 tinue in their given state? This characteristic of things is 

 generally called inertia. 



Law of inertia. Sir Isaac Newton (1642-1727) formulated 

 what he called the Law of Inertia, which is one of his Three 

 Laws of Motion. He stated the law of inertia in these words : 

 " Every body continues in its state of rest or of uniform motion 

 in a straight line, unless it is compelled by other forces to 

 change this state." This is exactly what your observations 

 lead you to conclude. Since we are concerned with motion, 

 we may limit the statement as follows : " A body in motion 

 tends to move in the same direction unless it is compelled by 

 external forces to change that direction." 



116. Elevation of the outside rail. In the construction of 

 curves or bends on railroads where speed, safety, and comfort 



are necessary, the inertia of 

 moving bodies is accurately 

 computed and provided for. 

 The track is elevated on the 

 outside of the curve exactly 

 enough to compel the train 

 FIG. si. Track on curve, showing easily and gradually to 



lines of forces which keep the car in change its direction, to allow 

 a position normal to the track. ,. . . , ., 



the safe maintenance of a 



given rate of speed, and to secure the equal distribution of 

 the load upon both sets of wheels and rails. To have the load 

 bear too heavily on the outside wheels, their flanges, and the 



