36 FORCE AND ENERGY 



The mass of a body is the amount of matter in it. 



The amount of attraction between bodies depends on the masses of 

 the bodies and on the distance they are apart. 



The weight of a body represents the earth's attraction, at any given 

 place, for the mass of the body. 



A body weighs more at the earth's poles than at the equator, and 

 more at the earth's surface than "up in the air." The mass does not 

 change. 



In a vacuum all bodies fall with the same speed. 



If a falling body is acted upon by a horizontal force, its path is 

 curved; but the time taken is the same as if gravity acted alone. 



Force is exerted on a body by the muscles of men and animals, by 

 the earth (gravity), by the wind, by other bodies (as resistance or fric- 

 tion), by expanding gases, etc. 



A body on which work has been done has energy, or the capacity for 

 doing work on other bodies. 



There is energy of position and energy of motion. 



Work (against gravity) = weight X vertical distance. 



Power is the rate of doing work. 



Inertia is the helplessness of matter to alter its condition of rest or 

 motion. 



"Centrifugal force" is the pull of matter, when revolving, against 

 whatever holds it to the center. 



The force that is exerted between the particles of matter and holds 

 matter together is called cohesion, or adhesion. 



The cohesion of liquids causes them to form drops, and to have elas- 

 tic surfaces. 



The upper surface of a liquid that does not adhere to the vessel is 

 like a, flattened drop; that is, convex. 



If the liquid wets the vessel, the surface is concave. 



The cohesion of liquids and their adhesion to solids (wetting) 

 causes the liquids to rise in narrow spaces or tubes. This is "capillary 

 action." 



The density of a body or a substance is the quantity of matter in a 

 given volume. Pure water at 4 C. has a density of 1. 



Buoyant force is the supporting, or lifting, power of liquids and gases. 



