GALILEO AND THE NEW PHYSICS 



analogous experiments. It might readily be observed, 

 for example, that a stone dropped from a moving cart 

 does not strike the ground directly below the point 

 from which it is dropped, but partakes of the forward 

 motion of the cart. If any one doubt this he has but 

 to jump from a moving cart to be given a practical 

 demonstration of the fact that his entire body was 

 in some way influenced by the motion of translation. 

 Similarly, the simple experiment of tossing a ball from 

 the deck of a moving ship will convince any one that 

 the ball partakes of the motion of the ship, so that it 

 can be manipulated precisely as if the manipulator 

 were standing on the earth. In short, every-day ex- 

 perience gives us illustrations of what might be called 

 compound motion, which makes it seem altogether 

 plausible that, if the earth is in motion, objects at its 

 surface will partake of that motion in a way that does 

 not interfere with any other movements to which 

 they may be subjected. As the Copernican doctrine 

 made its way, this idea of compound motion naturally 

 received more and more attention, and such experi- 

 ments as those of Galileo prepared the way for a new 

 interpretation of the mechanical principles involved. 



The great difficulty was that the subject of moving 

 bodies had all along been contemplated from a wrong 

 point of view. Since force must be applied to an ob- 

 ject to put it in motion, it was perhaps not unnaturally 

 assumed that similar force must continue to be applied 

 to keep the object in motion. When, for example, a 

 stone is thrown from the hand, the direct force applied 

 necessarily ceases as soon as the projectile leaves the 

 hand. The stone, nevertheless, flies on for a certain 



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