INERTIA AND CENTRIFUGAL FORCE 9 



a second. About how long, then, is a pendulum that swings 

 once a second? 



Make the pendulum 25 cm. long (how many inches is this?), 

 and count the number of vibrations in a second. Get the aver- 

 age of 3 trials, as before. 



If you can find a high enough point of suspension (a stick 

 placed horizontally out of an upper window will do), make a 

 pendulum 4 meters long, and find out how many vibrations it 

 makes in a minute. What is the time of one vibration? If we 

 have two pendulums, and one is 4 times as long as the other, 

 compare the amounts of time which they will take to swing. 



EXERCISE 6 

 INERTIA AND CENTRIFUGAL FORCE 



Apparatus and Materials. Wooden blocks, calling card, coin, flat- 

 iron, strong cord, thread, ball with a rubber cord, small pail with a 

 handle, water. 



a. Try the experiment suggested in the text, 29, exercise 4. 



6. Hold out your left hand, palm upward. Near the tip of 

 your first or your middle finger place a calling card, and on the 

 card, just over your finger, put a coin, such as a nickel. Prac- 

 tice snapping the edge of the card with the thumb and a finger 

 of your right hand, until you can drive the card away horizon- 

 tally, leaving the coin on your finger. Explain why this is 

 possible. 



c. Suspend a flatiron by means of a stout cord, so that it 

 escapes the floor by a few centimeters (an inch or two). Have 

 the cord as long as is convenient. Tie a piece of thread (not 

 too strong) to the flatiron, and by gentle pulling, properly 

 timed, set the flatiron pendulum to swinging. When the 



