56 



MOTION AND FORCE 



the center A, the ball moves no nearer to and no farther from h than 

 the length of the string allows. As soon as you let go, both forces 

 cease to act and the ball obeys the first law of motion ( 55). 



Effects of centrifugal force are common. A pail of 

 water may be whirled in a circle overhead, centrifugal 

 force holding the water against the bottom of the pail 

 so that none is spilled. The same force 

 may cause a carriage or car to tip over in 

 rounding a sharp curve. The wheels are 

 held in place by the track or road, while 

 the e.g., tending to go on in a straight line 

 (Fig. 42), passes outside the base. In all 

 cases, note that the force is greater if the 

 body moves rapidly or the curve is sharp. 

 Water would spill from the pail which 

 was swung slowly, and freight trains take 

 curves much more easily than expresses. 



FIG. 42 



66. Falling Bodies. Bodies fall be- 

 cause gravity pulls them. Now since the 

 attraction of gravity depends upon the amount of matter 

 contained in any body, it follows that the greater the 

 mass, the more strongly gravity will pull it ; that is, a 

 heavy body will be acted upon greatly and a lighter 

 one less strongly. The result is that all bodies will fall 

 equal distances in equal periods of time, when not hindered 

 by any other force. Most bodies do fall equally fast ; 

 but a few (such as feathers, leaves, and paper) have so 

 large a surface, compared with their weight, that their 

 falling is greatly hindered. In a vacuum a penny and 

 a feather would fall exactly together (Fig. 43). 



