20 ANSWERS TO PRACTICAL QUESTIONS 



/y/9, or 3 times that at the surface. Hence, the body must be 

 12,000 miles from the center, or 8,000 miles from the surface 

 of the earth. The problem may be solved directly by propor- 

 tion, thus : 



x* : 4000 2 : : 192 inches : 21| inches. 



x = 12000 miles (distance from the center). 



12000 miles 4000 miles = 8000 miles. 



89. How far will a body fall in 8 seconds ? 1,O24 feet. 

 In the 8th second? 24O feet. In 1O seconds? 1,6OO 

 feet. In the 3Oth second ? 944 feet. 



4O. Hoiv long would it take for a pendulum one mile in 

 length to make a vibration ? 



According to the" 1 second law of pendulums (Physics, p. 69), 



1 sec. : x : :^S9 : A /5280x 12 in. 

 x = 40 + sec. 



41. Wliat would be the time of vibration of a pendulum 

 04 meters long ? 



(1 sec.) 2 : y? : : 1 meter (nearly) : 64 meters. 

 x = 8 seconds (nearly). 



42. A ball is dropped from a height of 64 feet. At the 

 same moment a second ball is thrown upward with suffi- 

 cient velocity to reach the same point. Where will the two 

 balls pass each other ? 



At the end of one second. The first ball would fall 64 feet 

 in 2 seconds ; the second would rise for 2 seconds, and they 

 would pass in 1 second, 48 feet above the ground. 



43. Explain the following fact : A straight stick loaded 

 with lead at one end can be more easily balanced vertically 

 on the finger when the loaded end is upivard than when 

 it is doivnward. 



When the loaded end is upward a slighter motion is needed 

 to bring the line of direction within the base. The principle is 

 similar to that of the balancing-pole of the gymnast. 



