168 



MECHANICS 



wheels of a clock are allowed to turn slowly by means of 

 a pendulum. Now we are going to see why a clock can 

 be regulated by the length of its pendulum. 



Experiment 70. The Pendulum. 

 Apparatus: String, stones of various sizes, rule, 

 a. Hang up a small stone by a string 25 centi- 

 meters long and count the number of swings per minute. 

 Tie on a large stone in place of the small stone, keeping 

 the length of the pendulum the same. Does the weight 

 of the stone make any difference in the number of swings 

 per minute? The time that it takes a pendulum to swing 

 from one side and return again to its 

 first position is called the time of 

 vibration. 



,* b. Hang up a stone, making the 

 I string 100 centimeters long and count 

 the vibrations per minute. How does 

 the number compare with the result 

 obtained in (a) above? If you want a 

 clock to go faster would you lengthen 

 or shorten the pendulum? The illustra- 

 tion shows a clock pendulum. The pend- 

 ulum is kept in motion by the push o/ 

 the projections on the wheel, but the 

 time is regulated by the length of the 

 pendulum. Do you think that a clock 

 would go faster or slower in hot 

 weather? Explain. See Section 18. 



65. The Lever. 



A stick, supported at a point between 



