46 PROPERTIES AND MAKE-UP OF MATTER 



bear the double strain of the weight of the ball and the 

 steady pull of the hand. 



'It is the inertia of the water which enables the small, 

 rapidly revolving propeller to move the big ship. The re- 

 sistance which the particles of air offer to being thrown 

 suddenly into motion, their inertia, enables the propeller 

 to pull the airplane along, and keeps the craft from falling 

 to the ground as long .as it is moving rapidly. It is owing 



FIGURE 18 



to inertia that the heavenly bodies keep on moving in space. 

 Once in motion they must keep on forever unless some force 

 stops them. 



Experiment 13. Place a glass globe partly filled with water and 

 a small amount of mercury on a rotating apparatus. (Figure 18.) 

 Rotate the globe rapidly. What do the water and mercury tend 

 to do? 



In Experiments 11 and 13 it was seen that revolving 

 bodies tend to move away from the center around which 

 they are revolving. This is a manifestation of inertia 

 which is sometimes called centrifugal force. The weight 



