IN POPULAR PHYSICS. 5 



24. Would it vary the effect if the two persons were 

 running in opposite directions ? 



The blow would then be the sum of both their momenta. 



If they were running in the same direction ? 



The blow would be equal to the difference of their momenta. 



25. Why can you not fire a rifle-ball around a hill? 



Because a single force always produces motion in a straight 

 line. 



26. Wliy does a heavy gun "kick" less than a light 

 one ? 



See problem 12. 



27. A man on the deck of a large steamer draws a 

 stnall boat toward him. Can yott express the ratio of the 

 ship's motion to that of the boat ? 



The ship moves as much less distance than the boat, as the 

 boat's mass is less than that of the ship. 



28. Suppose a string, fastened with a nail at one end, 

 will just support a weight of 25 Ibs. at the other. Unfasten 

 it, and let tivo persons piill upon it in opposite directions. 

 How much can each pull ivithout breaking it ? 



25 Ibs. The second person, in the latter case, can pull as 

 much as the nail did in the former. The tension in the string 

 is 25 Ibs., and the action of the one person is just balanced by 

 the reaction of the other. 



29. Can a man standing on a plat form- scale make 

 himself lighter by lifting up on himself? 



He can not ; because action and reaction are equal and op- 

 posite. 



30. Why can not a man lift himself by pulling up 

 on his boot-straps ? 



See last problem. 



31. With ivhat momentum would a steam-boat weigh' 

 ing 1,OOO tons, and moving with a velocity of 1O feet per 

 second, strike against a sunken rock ? 



