Conservation of Energy 



Your jerk was so sudden 

 that there was not time 

 to overcome the inertia 

 of the glass of water ; 

 so it stayed where it was. 



Experiment 24. Have 

 a boy on roller skates skate 

 down the hall or sidewalk 

 toward you and have him 

 begin to coast as he comes 

 near. When he reaches 

 you, put out your arm and 

 try to stop him. Notice 

 how much force ^it takes FIG 35 when a boy is moving rapidly> 



to Stop him in spite of the it takes force to change the direction of his 



fact that he is no longer motion - 

 pushing himself along. 



Now let the boy skate toward you again, coasting as 

 before; but this tune have him swing himself around a 

 corner by taking hold of you as he passes. Notice how 

 much force it takes just to change the direction in which he 

 is moving. 



You see the boy's inertia makes him tend to keep going 

 straight ahead at the same speed ; it resists any change 

 either in the speed or the direction of his motion. So it 

 takes a good deal of force either to stop him or to turn 

 him. 



If, on the other hand, you had no inertia, you could 

 neither have stopped him nor turned him; he would 

 have swept you right along with him. It was because 

 inertia made you tend to remain still, that you could 

 overcome part of his inertia. At the same time he 



