8 LABORATORY EXERCISES 



a. Suspend the ball or other object by means of a light 

 thread or string, so that it will swing freely. The thread may 

 be tied to a gas fixture or a similar support. The ball or weight 

 so suspended is called the "bob." What position do the bob 

 and string take when left at rest? Why are they together called 

 a plumb line? For what is a plumb line used? When does it 

 become a pendulum? What is the meaning of pendulum? 



b. Draw your plumb line aside, and let it go. What happens? 

 When a body has been set in motion, the amount of the motion 

 is called the momentum of the body. Momentum depends on 

 both, the mass (weight) of the body and its velocity. A body 

 weighing 10 grams and having a velocity of 10 meters a second 

 has the same momentum as a body weighing 1 gram and having 

 a velocity of 100 meters a second. What makes the pendulum 

 swing? Why does it not stop when the bob is at its lowest 

 point? Will the pendulum ever stop swinging? What has the 

 air to do with stopping it? If the pendulum string or thread 

 rubs (has friction) against its support, what will the effect of 

 this rubbing be? What device keeps a clock pendulum in 

 motion? 



Why is a hammock a pendulum? When you are swinging 

 in a hammock, what do you do that corresponds to the slight 

 push of the clock spring? When, in a swing, you "let the old 

 cat die," what do you depend on to keep you swinging? What 

 finally stops the swing? 



c. Make the thread or string of a pendulum about 120 cm. 

 (4 ft.) long, and tie it so that the distance from the point of 

 suspension to the center of the bob is very close to 1 meter 

 (39.37 in.). Set the pendulum swinging, and count the number 

 of swings (vibrations) that the pendulum makes in 1 minute. 

 Do this 3 times, add the results together, and divide by 3. 

 What is the average number of swings per minute? Adjust 

 the length of the thread so that the pendulum shall swing once 



