Conservation of Energy 67 



going in the direction in which the automobile was 

 going. 



When you jump off a moving street car, you have to 

 run along in the direction the car was going or you fall 

 down ; your body tries to keep going in the same direc- 

 tion it was moving, and if your feet do not keep up, 

 you topple forward. * 



Generally we think that it takes force to start things 

 to move, but that they will stop of their own accord. 

 This is not true. It takes just as much force to stop a 

 thing as it does to start it, and what usually does the 

 stopping is friction. 



When you shoot a stone in a sling shot, the contract- 

 ing rubber pulls the stone forward very rapidly. The 

 stone has been started and it would go on and never 

 stop if nothing interfered with it. For instance, if 

 you should go away off in space say halfway between 

 here and a star and shoot a stone from a sling shot, 

 that stone would keep on going as fast as it was going 

 when it left your sling shot, forever and ever, without 

 stopping, unless it bumped into a star or something. 

 On earth the reason it stops after a while is that it is 

 bumping into something all the time into the particles 

 of air while it is in the air, and finally against the earth 

 when it is pulled to the ground by gravity. 



If you threw a ball on the moon, the person who caught 

 it would have to have a very thick mitt to protect his 

 hand, and it would never be safe to catch a batted fly. 

 For there is no air on the moon, and therefore nothing 

 would slow the ball down until it hit something; and 

 it would be going as hard and fast when it struck the 



