FORCE AND MOVING BODIES 175 



needed to move it. Oil, grease, and graphite fill up the irregu- 

 larities of surfaces in contact and thus reduce friction. The 

 area of the surfaces in contact does not usually affect friction, 

 nor does an increase in speed when a solid is moving over 

 another solid, but when the solid is moving in water or air, 

 friction increases with speed. It requires proportionately 

 more energy to drive a train or steamship at high speed than 

 it does to run them at a more moderate velocity. 



145. Advantages of Friction. Friction has its advantages 

 as well as its disadvantages. Were it not for friction, nails 

 would not hold, belts would slip over pulleys without turning 

 them, and walking would be impossible. We realize the 

 truth of the latter statement when we attempt to walk on 

 smooth ice or a highly polished floor. Sawing, filing, polish- 

 ing, and similar operations could not proceed without friction. 

 The sand-blast for cutting designs on glass and the like, also 

 owes its efficiency to friction. 



146. Gravity. An important force tending to move all 

 bodies is called gravity. This is a mysterious attraction 

 existing throughout the universe which tends to draw all 

 bodies toward one another. The pull of gravity is exactly 

 proportional to the mass of the body. A heavy body therefore 

 exerts a stronger pull than a lighter one, but it exerts no more 

 pull in proportion to its mass. Gravity also acts as if the 

 entire mass of the body were at the center of its mass. It 

 decreases rapidly as the distance between these centers in- 

 creases, but notwithstanding this, it is gravity that keeps the 

 stars, suns, and planets in their proper paths. The same force 

 acting between the sun, moon, and earth, is the cause of the 

 tides. It is also the attraction of the earth for all bodies upon 

 it that gives them weight. When we lift anything, we exert 

 sufficient force to overcome the pull of the earth upon it. 

 When gravity is the only force acting on a falling body, its 

 path is a straight line toward the center of the earth. Masons, 



