204 ANSWERS TO PRACTICAL QUESTIONS 



49. If we whirl a pail of water swiftly around with 

 our hands, why will the water tend to leave the centre of the 

 pain 



This is generally attributed to the action of the cen- 

 trifugal force. More correctly, the inertia of the water, 

 i.e., its tendency to continue to move in the straight 

 line in which it is at each moment passing, overcomes 

 the weak force of cohesion, and the molecules fly off 

 from the centre of motion and collect against the outside 

 of the pail. 



50. Why will the foam collect at the hollow in the 

 centre ? 



The foam, being lighter than the water, has less mo- 

 mentum, and is forced back by the heavier particles. 



51. If. two cannon-balls, one weighing 8 Ibs. and the 

 other 2 Ibs., be fired with the same velocity, which will go 

 the further 1 } 



The former has much less surface in proportion to its 

 weight and consequent momentum. It will therefore go 

 much further against the resistance of the air. 



52. Resolve the force of the wind which turns a common 

 wind-mill, and show how one part acts to push the wheel 

 against its support, and one to turn it around. 



(See Arnott's Physics, p. 226.) 



The toy-mill shown in Fig. 14, p. 32, illustrates the 

 principle perfectly. The vanes turn in a direction con- 

 trary to that in which they are inclined. Let GH, in Fig. 

 10, p. 31, represent the face of the vane, and the descrip- 

 tion in the text will then apply. 



