Conservation of Energy 81 



legs and body and arms, but not by touching the ground. 

 (Try to make it swing forward and backward only; when 

 you try to swing sidewise, the distance between the ropes 

 spoils the experiment.) See if you can figure out why the 

 swing will not move back and forth. Notice your bodily 

 motions ; notice that when half of your body goes forward, 

 half goes back; when you pull back with your hands, you 

 push your body forward. If you watch yourself closely, 

 you will see that every backward motion is exactly balanced 

 by a forward motion of some part of your body. 



Application 22. Explain why you push forward against 

 the table to shove your chair back from it; why a bird 

 beats down with its wings against the air to force itself 

 up; why you push back on the water with your oars to 

 make a rowboat go forward. 



Inference Exercise 



Explain the following : 



101. Water comes up city pipes into your kitchen. 



102. When you try to push a heavy trunk, your feet slip out from 



under you and slide in the opposite direction. 



103. When you turn a bottle of water upside down with a small 



piece of cardboard laid over its mouth, the water stays in 

 the bottle. 



104. You can squeeze a thing very tightly in a vise. 



105. There is a water game called " log rolling " ; two men stand 



on a log floating in the water and roll the log around with 

 their feet, each one trying to make the other lose his 

 balance. Explain why the log rolls backward when the 

 man apparently runs forward. 



1 06. The oil which fills up the spaces between the parts of a duck's 



feathers keeps the duck from getting wet when a hen 

 would be soaked. 



107. Sleds run on snow more easily than wagons do. 



108. In coasting down a hill, it is difficult to stop at the bottom. 



109. When you light a pinwheel, the wheel whirls around as the 



powder burns, and the sparks fly off in all directions, 

 no. You cannot lift yourself by your own boot straps. 



