58 FIRST YEAR COURSE IN GENERAL SCIENCE 



inertia, the ball and the boy have a tendency to continue 

 their motion in the same direction with the same velocity 

 as at first. But they do not do so. They gradually roll or 

 slide more slowly, until each finally comes to rest. Gravity 

 caused them to press upon the surface below, and this rub- 

 bing of two surfaces, or friction, is a hindrance to motion. 

 If two moving parts of a machine rub together, it takes more 

 power to make them move. Oil or some other lubricant 

 between the parts reduces the friction, because oil makes 

 smoother surfaces. Friction may cause reduced speed or 



loss of motion, that is, rest. 

 It is like an opposing force 

 in its effect. 



59. Work. If, in spite 

 of friction, a stone is moved 

 along the ground, work is 

 done. Overcoming resist- 

 ance to force is work. 

 Muscular . work is done in 

 lifting a body in spite of 

 the resistance offered by the 



4cm. 



FIG. 19. Two FORCES ACTING AT 

 RIGHT ANGLES 



AC may represent a force of 100 kg. 



(1 cm. = 25 kg.) acting toward the east downward pull of gravity. 



upon the point A^ i. what force and Thework of the steam in an 



what direction does AB represent' 



2. If the force AC acted alone, to what engine is to push the piston 

 point would a body at A move? If rod which U Qr h 



AB acted alone? 3. With the forces * ^ 



acting together, the body moves to D. other parts of the machine. 



In what direction does it move, and as 60< Energy. The ability 

 if acted upon by what force? &J J 



to do work is called energy. 



It is possessed by a moving body or by a body in such a 

 position that it may be made to move. A baseball which 

 has been struck has energy; it may overcome the resistance 

 offered by a pane of window glass. In breaking the glass, 

 the ball is doing work, and that requires energy as well as 

 putting a new pane in place. Suppose that an iron beam 

 is being lifted to. its place in the framework of a building. 



