152 



USING MACHINES 



pound and 2.2046 pounds in a kilogram. The metric 

 table of weights is as follows: 



10 milligrams 

 10 centigrams 

 10 decigrams 

 1000 grams 



1 centigram 



1 decigram 



1 gram 



1 kilogram 



In Experiment 105 you found that equal volumes of 

 different substances weigh different amounts. For 

 example, lead will weigh eleven times as much as the 

 same volume of water, and aluminum about 2.56 times 

 as much as an equal volume of water. The weight of 

 a unit volume of any substance is known as its dens- 

 ity. Density is an important factor in scientific work 

 because it provides one way of determining the purity 

 of substances. Density is expressed as a decimal or as 



Wood 



0.6 



Water 



1.0 



Aluminum 



Iron 



7.8 



Lead 



113 



Gold 



19.3 



FIG. 241. EQUAL WEIGHTS OF DIFFERENT SUBSTANCES 

 HAVE DIFFERENT VOLUMES 



Density is the weight of a unit volume of any substance. 



a whole number. Figure 241 shows the densities of 

 some common substances. 



Why does one pitch forward in a car that is sud- 

 denly stopped and why are turns on railroads and 

 highways banked? No doubt you have wondered why 

 these two questions were asked in one problem, and 

 you will certainly be surprised to learn that they are 

 closely related ; in fact, their causes are identical. The 

 cause is the inertia of bodies. 



Inertia is a property possessed by all bodies that 

 tends to make them remain as they are. If they are in 

 motion, inertia tends to keep them in motion in a 

 straight line, and if they are at rest, inertia tends to 

 keep them at rest. Perhaps you have seen a sleight-of- 

 hand performer remove the tablecloth from a table 

 set with dishes without disturbing them. By giving 



the tablecloth a quick pull he sets it in motion, but the 

 inertia of the dishes leaves them undisturbed. 



If you have ever been riding in an automobile or 

 train that is suddenly stopped, you have observed 

 the tendency of a body to remain in motion when 

 suddenly stopped. And when the automobile or train 

 is suddenly started, the inertia of your body while 

 at rest causes you to be jerked backward. You use 

 inertia when you pick one book from under a pile 

 or when by a sudden jerk you remove a board from 

 under a pile of other boards. 



We make many other uses of inertia in daily life. 

 Often the head of a hammer, or a pick, is driven on 

 to the handle by pounding the end against a stone. 



FIG. 242. AN EXAMPLE OF INERTIA 



The boy removes the tablecloth with a sudden pull. 



The force of the blow sets the whole body in motion. 

 The motion of the handle is stopped by the rock, 

 but the inertia of the head tends to continue its motion 

 and thus drive it more tightly on the handle. 



Exercise. Infer a .cause or reason based on the above 

 study for each of the following: 



1. A person tends to feel lighter for a moment when 

 an elevator suddenly starts downward, and heav- 

 ier when it suddenly starts upward. 



2. A heavy hammer or weight is often placed behind 

 a springy board into which a nail is to be driven. 



3. Rugs are shaken to remove dust and dirt. 



4. It is unsafe to step from a moving street car or 

 automobile. 



If a body is in motion its inertia tends to keep it 

 moving in a straight line unless some other force acts 

 to change this direction. An automobile turning a 

 curve on a slippery pavement often skids. This is an 

 example of the tendency to remain in motion in a 

 straight line. In Experiment 107 you learned that 

 when you whirled the weight on the string and then 

 let go the string the weight flew outward in a straight 

 line. This tendency of whirling bodies to fly outward 



