CHAPTER XXV 

 FORCE AND MOVING BODIES 



143. Momentum and Inertia. When a body is at rest, it 

 cannot be moved without applying some external force to it; 

 that is, energy must act upon it in order to move it. When 

 it is once set in motion, however, it offers a similar resistance 

 to any effort to stop it. The resistance which a body thus 

 offers to any attempt to change its condition is called its 

 inertia. If a moving body meets with no opposition, it will go 

 on in a straight line forever. On the earth, however, bodies 

 ultimately meet with enough resistance in Drubbing against 

 the air or other forms of matter to bring their motion to an 

 end. When a body^ is in motion, the resistance which it 

 offers to being stopped is spoken of as its momentum. Momen- 

 tum is not the same as speed, however, for a heavy body, such 

 as a cannon-ball, moving slowly may offer more resistance to 

 being stopped than a lighter body moving with a much higher 

 speed. If a second force be applied to a moving body, its 

 effects depend upon the direction from which it is applied. 

 If applied in the direction in which the body is moving, it 

 increases its speed; if applied in the opposite direction, it re- 

 duces the speed, or, if large enough, may either stop it entirely 

 or cause it to move in the opposite direction. If applied in 

 any other direction, the body will take a new course which is 

 the exact average of the two courses which it would have fol- 

 lowed had each force acted separately upon it. We have an 

 illustration of this when any heavy object is tied to a string 

 and swung around in a circle. The moving body tends to fly 

 away in a straight line, but being constantly pulled out of its 



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