I2V POPULAR PHYSICS. 19 



33. A boy ivishing to find the height of a steeple lets flu 

 tin arrow that just reaches the top, and then falls to the 

 ground. Tt is in the air 6 seconds. Required the height. 



s = 16t*. s = 16 x 3 2 = 144 feet. 



34. An object let fall from a balloon reaches the 

 ground in 1O seconds. Required the distance. 



s = 16 x 10 8 = 1600 ft. 



35. In what time will a pendulum 4O feet long make a 

 vibration ? 



According to the 2d law of pendulums, and taking the 

 length of a seconds pendulum as 39 in., we have: 



1 sec. : x : : y^39 =^40 x 12 in. 



'480 

 39 

 x = 3. 5 + sec. 



36. Two bodies in space are 12 miles apart. They 

 weigh 1OO and 2OO Ibs. respectively. If they should 

 fall together by force of their mutual attraction, what 

 portion of the distance would be passed over by each 

 body ? 



The distance passed over by the two bodies is inversely as 

 their mass ; hence one moves 8 miles and the other 4 miles. 



37. If a body weighs 2,OOO Ibs. upon the surface of 

 the earth, what would it weigh 2,OOO miles above ? 



(6000 mi.) 2 : (4000 mi.) 2 : : 2000 Ibs. : x = 888f Ibs. 

 Hoiv much 5OO miles above ? 



(4500 mi.) 2 : (4000 mi.) 2 : : 2000 Ibs. : x = 1580 + Ibs. 



38. At what distance above the surface of the earth will 

 a body fall, the first second, 21\ inches? 



A body falls 16 feet* (192 inches) at the surface of the 

 earth. 21^ inches are \ of 192 inches. Now, as the attraction 

 is inversely as the square of the distance, the distance must be 



* According to the best authorities the distance is more nearly 16^$ feet. 



