Transportation on Land 



275 



Exercise. On a track of narrow cardboard or sheet tin bent in 

 the shape of a trough, and curved to form a vertical loop as in Fig. 80, 

 a marble will loop the loop just as a skillful bicyclist, automobilist, or 

 aviator can do in his machine. 



Other instances may be cited, but all point to the same 

 principle, that is, a moving body tends to continue to move in the 

 same direction in which it was 

 started, until compelled by some 

 other force to stop or change its 

 direction. Complementary to 

 this is the familiar fact that a 

 body stays where it is put, 

 until moved by some external 

 force. There is a question FIG. 80. A trough made of card- 



board, tin, or wires bound together, and 



involved here Which must be shaped as in figure will guide a marble 

 answered before considering around the loop. What is the principle 

 ., 1-1, involved? 



now railroad curves are built 



to permit speed. When a body is moving in a circle about 

 a center, in what direction does it actually tend to move? 

 Does it move directly away from the center ? Or does it tend 

 to move in a straight line continuous with its direction at 

 any given moment? The answer to this question may be 

 found experimentally in the following way. 



Exercise. Attach a string eight or more feet in length to a wooden 

 ball. Take hold of the string two or three feet from the ball and swing 

 the ball in a circle, the plane of which is parallel to the floor and just 

 a little above it. Mark the center of the circle described by the mov- 

 ing ball. When the ball is moving steadily, release the string. Ob- 

 serve the course of the ball. Does it move away from the center of 

 the circle? Does it move in a straight line continuous with its direc- 

 tion at the moment of its release? 



115. Inertia. Consider now the behavior of bodies at 

 rest, which is complementary to the behavior of moving bodies. 

 Place on the table a visiting card or other small card with part 

 of it extending beyond the table's edge ; and place on the card 



