CHAPTER VI 

 LAWS OF MOTION 



53. Three Laws of Motion. Some interesting facts 

 about the motion of bodies, which we ordinarily find out 

 only as the result of long experience, can readily be under- 

 stood by a knowledge of the laws of motion and momentum. 

 A body set in motion by a force, such as steam or electricity, 

 starts slowly and its speed increases in proportion to the 

 strength of the force and the resistance of the body. To 

 illustrate: When an electric car moves we experience a 

 heavy jarring; this is due to the seat starting before our 

 body and pulling us along. 



The natural state of inorganic or lifeless bodies is one of 

 rest, called inertia. Every body continues in a state of rest, or 

 when set in motion continues to move in a straight line, unless 

 acted upon by some external force. This is the first law of 

 motion. 



When an object is moving, its speed may be increased by 

 applying more force. If the force is applied in a different 

 direction from that in which the body is moving, the body 

 will either stop or change its direction of motion. This prin- 

 ciple may be expressed by stating that every change of motion 

 is in the direction of the new force applied to the body, and is 

 proportionate to it. This is the second law of motion. 



A force never appears singly. That is, there are always 

 two or more contending forces in every mechanical operation 

 and in all mechanical work. To illustrate : When a mechanic 



54 



