Conservation of Energy 71 



thrown ; it would be impossible to jump ; there would 

 cease to be waves on the ocean; and the moon would 

 come tumbling to the earth. The earth would stop 

 spinning; so there would be no change from day to 

 night; and it would stop swinging about in its orbit 

 and start on a rush toward the sun. 



But there is always inertia. And all things everywhere 

 and all the time tend to remain stock still if they are 

 still, until some force makes them move ; and all things 

 that are moving tend to keep on moving at the same 

 speed and in the same direction, until something stops 

 them or turns them in another direction. 



Application 20. Explain why you should face forward 

 when alighting from a street car ; why a croquet ball keeps 

 rolling after you hit it ; why you feel a jolt when you jump 

 down from a high place. 



Inference Exercise 

 Explain the following : 



81. It is much easier to erase charcoal drawings than water-color 



paintings. 



82. When an elevator starts down suddenly you feel lighter for a 



moment, while if it starts up quickly you feel heavier. 



83. You can draw a nail with a claw hammer when you could not 



possibly pull it with your hand even if you could get hold 

 of it. 



84. When an automobile bumps into anything, the people in the 



front seat are often thrown forward through the wind 

 shield. 



85. Certain weighted dolls will rise and stand upright, no matter 



in what position you lay them down. 



86. Some automobile tires have little rubber cups all over them 



which are supposed to make the tires cling to the pavement 

 and thus prevent skidding. 



87. It is hard to move beds and bureaus which have no castors 

 ^ or gliders. 



