44 



PROPERTIES AND MAKE-UP OF MATTER 



FIGURE 16 



line because a force called gravity pulled it down toward the 

 earth. When a train is moving along a straight level track, 

 we do not expect it to stop until the friction of the track or 

 some other force stops it. A bullet fired 

 from a gun will continue to move until 

 it hits some unyielding object or is 

 pulled to the earth by gravity. Thus 

 we see that a moving body does not stop 

 unless some force compels it to stop. 



We may sum up these observations in 

 the following words : A body at rest 

 remains at rest unless acted upon by some force; a body 

 in motion continues to move in a straight line at the same 

 speed unless acted upon by an outside force. This property 

 of matter is called inertia. Sir Isaac 

 Newton first stated these facts, and so 

 they are sometimes called Newton 's First 

 Law. We see this law frequently illus- 

 trated when standing passengers are 

 jostled off their feet by the sudden 

 starting or stopping of a car, or the 

 swinging of the car around a sharp curve. 



Experiment 12. Suspend a heavy ball by 

 a string not much too strong to hold it. 

 (Place a pad beneath it to catch it if it 

 drops.) Attach a similar string to the 

 bottom of the ball. (Figure 17.) Attempt 

 to lift the ball suddenly by the upper string. 

 What happens? Suspend the ball again and FIGURE 17 



lift it very gradually by the upper string. 

 What happens? Now pull down suddenly on the lower string. 

 What happens? Suspend the ball again and pull down gradually 

 on the lower string. What happens? 



