8 ANSWERS TO PRACTICAL, QUESTIONS 



42. If a man and a boy were riding in a wagon, and, 

 on coming to the foot of a hill, the man should take up the 

 boy in his arms, ^vould that help the horse ? 



No change would be produced in the weight of the entire 

 establishment drawn by the horse, as no readjustment of the 

 load would modify the attraction of gravity which produces the 

 weight. Also, action reaction ; so the man would press down 

 on the wagon an amount equal to the weight of the boy. 



4:3. If we ivliirl a pail of tvater swiftly around on its 

 oivn axis, ivhy will the water tend to leave the center of 

 the pail ? 



The centrifugal force is compounded with the force of grav- 

 ity, and at each point of the surface the water level is at right 

 angles to their resultant. The centrifugal force increases with 

 distance from the axis, hence the water surface becomes con- 

 cave. 



44. IVJiy will the foam collect at the holloiv in the cen- 

 ter? 



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

 tum, in proportion to its volume, and is forced back by the 

 heavier particles. 



45. If tivo cannon-balls, one weighing 8 Ibs. and the 

 other 2 Ibs., be /ired ^vith the same velocity, which will go 

 the farther ? 



The former has much less surface in proportion to its weight. 

 It will therefore go much farther against the resistance of the 

 air. 



46. Resolve the force of the wind tvhich turns a com- 

 mon ivindmill, and show hoiv one part acts to push the 

 tvJieel against its support, and one to turn it around. 



This case is exactly like that of the action of the wind 

 against the sail of a ship, as shown in the text on p. 29. 



47. When an animal is jumping or falling, can any 

 exertion made in mid-air change the motion of its center 

 of gravity ? 



