6 ANSWERS TO PRACTICAL QUESTIONS 



1,000 tons = 2,000,000 Ibs. 2,000,000 x 10 = 20,000,000 units 

 of momentum. 



32. With what momentum would a train of cars weigh- 

 ing 1OO tons, and running 1O miles per hour, strike against 

 an obstacle ? 



The velocity per second is 14f feet. 100 tons = 200,000 Ibs. 

 The momentum is 200,000 x 14f = 2,933, 333^ units of momen- 

 tum. 



33. Wliat would be the comparative kinetic energy of' 

 two hammers, one driven with a velocity of 2Q feet per 

 second, and the other 1O feet ? 



20 2 = 400. 10 2 = 100. 400 : 100 : : 4 : 1. 



Hence the kinetic energy of the first, or its ability to ac- 

 complish work, will be four times that of the second. This 

 principle is of great importance in all cases where percussion is 

 concerned. The highest velocity attainable is to be sought. 



34. If a 1OO horse-poiver engine can propel a steamer 

 5 miles per hour, ivill one of 20O Jiorse-poivcr double its 

 speed ? 



By no means. Resistance is proportional to / y 2 . (See Popu- 

 lar Physics, p. 65.) To double the velocity would require over 

 400 horse-power. 



3<>. Why are ships becalmed at sea sometimes floated 

 toy strong currents into dangerous localities without the 

 Knowledge of the crew ? 



As there are no fixed objects with which to compare their 

 motion, the officers are not sensible of any movement, and so 

 are drifted far out of their course. 



3856*. A man in a ^vagon holds a 50-lb. weight in his 

 hand. Suddenly the wagon falls over a precipice. Will 

 he, while dropping, bear the strain of the ^veight ? 



No. While on solid ground, his hand resisted the tendency 

 of the weight to fall toward the earth's center of gravity ; but 

 all are now descending freely under the influence of gravity, 

 and he no longer feels the pressure. 



