TRAVEL ON WATER 



303 



bend up the sides so as to make a boat. Place it in water. 

 Why docs it float? 



When a body floats it pushes out or displaces (more, 

 less) water than when it sinks. 



Experiment 178. Why do some substances float while 

 others sink in water? 



a) Secure a container such as the one shown in Figure 



504 which may be used 

 as an overflow can. Also 

 secure a small basin 

 which may be used to 

 catch any overflow 

 water, and a spring bal- 

 ance. Carefully weigh 

 the catch basin and re- 

 cord its weight. Weigh 

 a piece of iron in air and 

 record its weight. Fill the 

 overflow can as full of 

 water as possible and 

 lower the iron weight 

 suspended from the bal- 

 ance into the can. Read 

 the balance and record 

 the weight of the iron 

 in water. Weigh the 

 catch basin and water 

 and record their com- 

 bined weight. Calculate 

 504 the difference of weight 



of the iron in air and in 

 water. Find the weight of the water displaced. 



b) Repeat the experiment with a piece of wood which 

 will float. Record your results as in part a. Complete the 

 following statements. 



Bodies which sink in water displace an amount of water 



whose weight is equal to their of weight from to 



water. 



FIG. 



Floating bodies appear to lose 



of their weight in 



water. Floating bodies displace water equal to their weight 



in . 



Both floating and sinking bodies appear to be up by a 



force when they are in water. This force seems to be equal 

 to the weight of water which they 



Activity 179. Inspection of some modern boats. 



If there are any water transportation lines in or near 

 your community, visit a dock and inspect the types of boats 

 being used. Try to find answers to the following questions. 



How are they propelled through the water? How are 

 they steered? What kinds of engines are used? 



Experiment 180. How does the gyroscope work? 



Cut a disk five or six inches in diameter from a piece of 

 metal or from an old tin can. Locate the center accurately 

 and punch a small hole at this point with a nail. Tie a large 

 knot in the end of a piece of string and pull the string 

 through the hole so that the knot holds the disk. 



Swing the disk back and forth like a pendulum and ob- 

 serve how it behaves. Now set the disk rotating rapidly 

 about the string and again swing it like a pendulum. Ex- 

 periment with different positions of the disk. What differ- 

 ences do you notice in the behavior when the disk is spinning 

 and when it is not spinning ? Can you observe any tendency 

 on the part of the spinning disk to resist a force that tends 

 to change its plane of rotation? 



Plan a further experiment to test this point. Suggestion: 



c 



FIG. SOS. 



Try blowing on the spinning disk through a rubber tube. 

 Does the disk tip up at the point where the air blast strikes 

 it? How far away from this point does it tip up? Can you 

 suggest how such a rapidly rotating body might be of use 

 in preventing the rolling of a ship at sea? 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What is the history of transportation by water? 

 The savages of long ago, no doubt, were the first to 

 use water to get from one place to another. The first 

 boat was probably a floating log with the passenger 

 astride. Later it was found that groups of logs fast- 

 ened together in a raft could be used to transport 

 several persons at the same time. The log boat and 

 the raft were heavy and difficult to handle. To make 

 a lighter craft, the inside was burned out of the log 

 boat to make the first dugout. As skill and daring 

 developed man made the first bark canoe and then 

 larger and larger boats until today we travel on 

 liners which are floating hotels with every comfort. 



The earliest boats were propelled by man power 

 until the development of the sailboat which used the 

 force of the winds. From about 1000 A.D. down to 

 the use of the steam engine, the sailboat went through 

 many stages of development. From the combination 

 man-and-wind-power ship of the tenth century to the 

 clipper ship of the nineteenth century is a story of 

 progress with few equals. 



