22 ANSWERS TO PRACTICAL QUESTIONS 



eration of gravity is 27 times what it is at the earth's sur- 

 face ? 



t' = .19 + second. 



51. How many vibrations per minute would be made 

 at the surface of the moon by a pendulum 4O ft. long ? 



First find the time of a single vibration. 



x 1 2 

 . / . . A/ ""'^ . A / " ' 



mt ' V ~W ' V ~T8~~ 



t = 9 + seconds. 



Hence, in a minute, the number of vibrations will be not 

 quite 7. 



52. A pendulum vibrates 2OO times in 15 minutes. 

 irJiat is its length? 



15x60 



= 4 seconds, the time of a single vibration. 



x 791.8 inches, or 66 feet nearly. 



53. For a certain clock in New York the pendulum was 

 made 5OO Ibs. in weight. JFJiat was the object in making 

 it so heavy ? 



To secure regularity of motion by means of the large mass, 

 so that variations in resistance may be avoided as nearly as 

 possible. 



54. Pendulums are often supported by knife-edges of 

 steel resting on plates of agate. Why ? 



Because the friction between steel and agate is less than if 

 any other substances are used. It is desirable to avoid friction 

 as completely as possible. 



55. The acceleration of gravity at the equator is 32.O88 

 ft. ; at the pole, 32.253ft. If a pendulum vibrates 3, GOO 

 times an hour at the equator, how many times an hour 

 will it vibrate at the pole ? 



