G6 WELLS'S NATURAL PHILOSOPHY. 



SECTION I. 



ACTION AND REACTION. 



^ . 138. When a body communicates motion 



Wliat IS meant , •' i /> • 



by Action and to anothei body, it loses as much of its own 



Reaction ? •' ' . . 



momentum, or force, as it gives to the other 

 body. We apply the term Action to designate the power 

 which a body in motion has to impart motion, or force, to 

 another body ; and the term Reaction to express the 

 power which the body acted upon has of depriving the 

 acting body of its force, or motion. 



What is the 139. There is no motion, or action, in the 

 Ac'tionindRe- univeisc without a corresponding and oppo- 

 action ? gj^g action of equal amount ; or, in other words, 



Action and Reaction are always equal and opposed to 

 each other. 



^y^ . - If a person presses the table with his finger, he feels a re- 



lustrations of sistance arising from the reaction of the table, and this coun- 

 actio'n?*"'^ ^°" ter-pressure is equal and contrary to the downward pressure. 

 "When a cannon or gun is fired, the explosion of the powder 

 which gives a forward motion to the ball, gives at the same time a backward 

 motion, or "recoil," to the gun. A man in rowing a boat, drives the water 

 astern with the same force that he impels tlie boat forward. 

 Towh.itisthe I'^O- The quantity of motion in a body is 

 Sin ^ in *^ a mcasurcd by the velocity and the quantity of 

 uonatef"^"'' matter it contains. 



A cannon-ball of a thousand ounces, moving one foot per 

 second, has the same quantity of motion in it as a musket-ball of one ounce, 

 leaving the gun with a velocity of a thousand feet per second. The momen- 

 tum, or quantity of motion, in the musket-ball being, however, concentrated 

 in a very small mass, the effect it wiU produce will be apparently much 

 greater than that of the cannon-ball, whose motion is diffused through a very 

 large mass. This explanation will enable us to understand some phenomena 

 which at first appear to contradict the law, that action and reaction are always 

 equal, and opposed to each other. 



Thus, when we fire a bullet fi-om a gun, the gun recoils back with as much 

 force as the bullet possesses, proceeding in an opposite direction. The reason 

 the effects of the gun are not equally apparent \\-ith those of the ball, is that 

 the motion of the gun is diffused through a great mass of matter, with a 

 ^maU velocity, and is, therefore, easily checked ; but in the ball the motion 



