ISO 



USING MACHINES 



in modern life. Almost all the food we buy is pur- 

 chased on the basis of its weight. It is also true of 

 many of the other necessities of life. In spite of the 

 fact that we use weighing so much, not many per- 

 sons could give a really scientific answer to the ques- 

 tion of this problem, "Why do substances have 

 weight?" From your everyday experience you are 

 acquainted with many of the facts which will help 

 you answer this question if you can put them together 

 in the right manner. Everyone has thrown stones or 

 baseballs into the air and observed that no matter 

 how hard they are thrown they always return to the 

 earth. Even the shells from cannons propelled at tre- 

 mendous velocities and things released from airplanes 

 and balloons fall soon to the earth. 



Over two hundred fifty years ago Isaac Newton, 

 one of the greatest thinkers of all time, was ponder- 

 ing over this same question, "Why do substances 

 have weight?" and with his keen ability to think 

 clearly and see the causes for certain phenomena, he 

 was able to offer an explanation which is as accept- 

 able today as it was when he first proposed it long 

 ago. 



Newton believed that every body in the universe 

 attracts every other body with a certain force. That 

 is, when a ball is thrown into the air it pulls on 

 the earth and the earth pulls on it with a certain 

 force. The ball seems to fall toward the earth, but 



FIG. 237. THINGS FALL TO EAETH 



really the earth moves up a little way to meet the 

 ball. However, the earth pulls so much harder on 

 the ball than the ball can pull on the earth that the 

 ball moves farther toward the earth than the earth 

 does toward the ball, before they meet. 



Why should the earth pull harder on the ball than 

 the ball on the earth? Newton believed that this 

 attractive force which exists between bodies and 

 which he called gravitation was due to the amount of 

 material in a body, or its mass. Therefore the earth, 

 having greater mass than the ball, exerted the greater 

 gravitational pull of the two. 



Newton with his genius applied this principle of 

 gravitation to explain why the planets and other 

 bodies of the solar system follow fixed orbits or path- 

 ways about the sun. 



But what has this force of gravity to do with the 

 weight of substances? Everything! The weight of any 

 body is merely the measure of the pull of gravity 

 for that substance. When you step on a pair of scales 

 you are measuring the extent to which the force of 

 gravity is pulling you to the earth. Things have 

 weight because gravity is pulling on them. If one 

 could travel outward in space to a point about 

 160,000 miles from the earth toward the sun, the 

 force of gravity of the sun pulling the earth would 

 be exactly equalled by the force of gravity of the 

 earth pulling the sun. At this point things would 

 have no weight. 



Exercise. Can you suggest some of the effects if the 

 force of gravity were suddenly to disappear? 



The weight of a substance is determined by com- 

 paring it with a standard of weight that has been 

 determined according to some system. Figure 238 

 shows a photograph of the standard of weight main- 

 tained at the Bureau of Standards in Washington, 

 D.C. This is the standard kilogram, a unit of weight 

 used in the metric system, and it is equal to 2.2046 

 pounds. Weights for use with weighing devices are 

 checked with this or other standards. 



Two types of scales are used to determine the 

 weights of substances, one known as the spring scale 

 and the other known as the platform scale or balance. 

 In using the spring scale the weight of an object 

 is read directly from the scale, which has been marked 

 off by using standard weights on it. In the use 

 of the platform balance the weight of an object is de- 

 termined by comparing it with or balancing it against 

 a set of weights. Since the force of gravity pulls 

 equally on bodies that have equal mass, when the 

 scales are in equilibrium the weights of the bodies 

 are equal and the weight of the body being weighed 

 may be determined from the total of the known 

 weights. 



Bodies do not weigh the same at all places on the 



