132 OUR PHYSICAL WORLD 



first time should go well away from buildings and companions, 

 for at first the stone is likely to be thrown in a direction quite 

 different from that intended, and it requires much practice to 

 become skilful in hitting a mark. 



This simple weapon is illustrative of several important scien- 

 tific principles. Primitive man, of course, did not comprehend 

 these. In fact, we usually acquire control over the forces of 

 nature by a trial-and-error method. We learn first how to do 

 things and later inquire why things behave as they do. It is 

 always interesting, however, when we can understand the reason 

 why. When any object is at rest it requires the application of 

 force to move it from this position of rest, and when a thing is in 

 motion it tends to continue that motion in a straight line unless 

 something acts upon it to stop it or start it moving along another 

 line. This is called the law of inertia. When the stone is 

 swinging rapidly around in the sling and one thong is released 

 the stone moves in a straight line in the direction that it was going 

 at the moment of release, and it keeps on going until it is stopped 

 by striking some object. If no other object is struck it is, of 

 course, striking particles of air all the time and gradually these 

 check its movement and it drops to earth pulled down by the 

 earth's attraction, what we call the force of gravity. The stone 

 is held in the sling because every moment it tends to fly off in a 

 straight line, and so presses against the leather which restrains 

 it. Probably most children have amused themselves by taking 

 a small pail partly full of water and holding the handle of the 

 pail in the hand have swung this around in a vertical circle. Of 

 course, when the pail is directly overhead with its mouth down, 

 the water would spill out of the pail if the pail were not being 

 swung rapidly. B ecause of the inertia the water tends to fly away 

 from the center of the circle in which the pail is being swung and 

 therefore presses against the bottom and sides of the pail, so remain- 

 ing in the pail, a demonstration of the so-called centrifugal force. 

 This experiment will help one understand why the stone stays in 

 the sling when it is merely laid in the leather and not fastened to it. 



