Conservation of Energy 73 



were ; for if any people survived the shock of the awful 

 collision, they would be roasted to death by the heat 

 produced by the striking together of the earth and the 

 moon. Moreover, the earth would be whirled swiftly 

 toward the sun, and a little later the charred earth would 

 be swept into the sun's vast, tempestuous flames. 



When we were talking about inertia, we said that if 

 there were no inertia, the moon would tumble down to 

 the earth and the earth, too, would fall into the sun. 

 That was because if there were no inertia there would be 

 no centrifugal force. For centrifugal force is not really a 

 force at all, but it is one form of inertia the inertia of 

 whirling things. Do this experiment : 



Experiment 25. Hold a pail half full of water in one 

 hand. Swing it back and forth a couple of times; then 

 swing it swiftly forward, up, and. on around, bringing it 

 down back of you (Fig. 36). Swing it around this way 

 swiftly and evenly several times, finally stopping at the be- 

 ginning of the up swing. 



It is centrifugal force that keeps the water in the 

 pail. It depends entirely on inertia. You see, while 

 the pail is swinging upward rapidly, the water is moving 

 up and tends by its inertia to keep right on moving 

 in the same upward direction. Before you get it over 

 your head, the tendency of the water to keep on going 

 up is so strong that it pulls on your arm and hand and 

 presses against the bottom of the pail above it. Its 

 tendency to go on up is stronger than the downward 

 pull of gravity. As you swing the pail on backward, 

 the water of course has to move backward, too ; so now 

 it tends to keep on moving backward ; and when the 



