Transportation on Water 351 



that it is this principle which is used in explaining the relation 

 of the sun and the earth, of the moon and the earth, and of 

 every body falling toward the earth or of every force exerted 

 upon the earth. Its significance is so far reaching that it has 

 been accepted as one of the great laws of motion. Sir Isaac 

 Newton, one of the greatest scientists, made this his third law 

 of motion and expressed it in this general form: "To every 

 action (or force) there is an equal and opposite action (or 

 force)." 



SUMMARY 



A floating body is one that displaces its own weight of water; a 

 sinking body is one that displaces less than its weight. 



The amount of buoyant force exerted by any liquid on any body 

 is equal to the weight of the liquid displaced by the body. This is 

 Archimedes' principle. 



Buoyant force is due to the pressure of the liquid. 



The density of a substance is its weight per unit of volume. 



Relative densities are often given in which water is taken as the 

 standard. 



The stability of a boat depends upon the relative positions of its 

 center of weight and its center of buoyancy. It is secured by the form 

 of the boat and the proper placing of ballast. 



Stability is especially necessary in a sailboat, because of the force 

 of the wind on the sail. 



Small boats are generally propelled by paddles or oars, which are 

 forms of the lever and illustrate its principle. 



The propulsion of a boat illustrates Newton's third law of motion : 

 Action and reaction are equal and in opposite directions. 



REVIEW QUESTIONS 



1. What are the relative advantages and disadvantages of travel 

 by sailing boat and steamship? 2. Describe some primitive boats. 

 3. Under what conditions does an object float in the water? Under 

 what conditions does it sink? 4. State the principle of buoyancy. 

 5. (a) Tell the story of Archimedes and the golden crown. (6) What 

 was the new method employed in determining the purity of the gold? 

 (c) Of what use is this method to us? 6. What facts regarding pres- 

 sure can be demonstrated by means of a tin can full of holes ? 7. What 

 is the relation of pressure to depth? 8. What is the relation of pres- 



