58 



MOTION AND FORCE 



e gravity ceases to pull b downward ; but the bob then 

 has enough momentum so that it rises to d against the 

 opposing force of gravity. At d the bob stops, gravity 

 now pulls it to e, and it then moves on toward c. The 

 path in which b swings (ced) is called the arc of the 

 pendulum. A single complete sweep across this arc is 

 called one vibration. As a pendulum swings to and 



fro, its arc constantly be- 

 comes smaller, and in time 

 the bob comes to rest at e. 

 The air offers a slight 

 resistance to the moving 

 body, slowly bringing it 

 to rest. 



Experiment 52. Make two 

 pendulums of exactly equal 

 lengths, by tying string to 

 stones. Make them about two 

 feet long, using stones of very 



unequal weights. Start them exactly together and compare the 

 rates of their vibrations, that is, the number of swings made by 

 each in a certain period of time. What effect has the weight of 

 the bob upon the vibration rate of the pendulums ? 



Experiment 53. Swing a pendulum through a small arc and 

 count its vibrations for 15 seconds. Now swing the same pendu- 

 lum through an arc much greater, and count its vibrations for 15 

 seconds. What effect has the length of arc upon the rate of 

 vibration ? (The length of the arc makes a slight difference in 

 rate if one arc is much greater than the other, and none at all if 

 both arcs are small less than 3.) 



Experiment 54 . Make a pendulum 9 inches long and another 

 36 inches long. Carefully count the vibrations of each for 15 

 seconds and compare results. Now make one 4 inches long and 



