IN NATUBAL PHILOSOPHY. 19 



boat toward him. How much does the ship move to meet tht 

 boat? 



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

 heavier than the boat. 



31. Suppose a string, fastened at one end, will just support 

 a weight of '25 Ibs. at the other. Unfasten it, and let two per- 

 sons pull ripon it in opposite directions. How much can each 

 pull without breaking it ? 



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

 much as the nail did in the former. 



32. Can a man standing on a platform-scale make himself 

 lighter by lifting up on himself? 



He cannot ; because action is equal to reaction, and in an 

 opposite direction. As much as he lifts up, so much must he 

 press down. 



33. Why cannot a man lift himself by pulling up on hi\ 

 boot-straps f 



See last problem. 



34. If from a gun placed vertically, a ball were Jired into 

 perfectly still air, where would it fall? 



It would return into the gun. 



35. With what momentum would a steamboat weighing 

 1,000 tons, and moving with a velocity of 10 feet per second, 

 strike against a. sunken rock f 



1000 tons x 10 a = 100,000 tons. 



36. With what momentum would a train of cars weighing 

 loo tons, and running 10 miles per hour, strike against an 

 obstacle? 



The velocity per second is 14 2 / 3 ft. 100 tons x (14V,)' = 21,511% tons. 



37. What would be the comparative striking-force of two 

 hammers, one driven with a velocity of '20 feet per second, ana 

 the other lofeet? 



20 2 = 400. 10 2 = 100. 400 : 100 . : 1 : 4. Hence one will strike four times as 

 hard a blow as the other. This principle is of great importance in chopping 

 wood, splitting rails, and in all cases where percussion is concerned. The 

 highest attainable velocity is to be sought. 



