MOTION. 65 



whole force of cohesion among the atoms of wood, but its force acts for so 

 short a time, owing to its rapid passage, that it is not sufficient to affect tlio 

 inertia of the door to an extent to produce motion. The cohesion of the part 

 of tlie wood cut out by the ball would have borne a very great weight laid 

 quietly upon it; but supposing the ball to fly at the rate of 1200 feet in a 

 second, and the door to be one inch thick, the cohesion being allowed to act 

 for only the minute fraction of a second, its influence is not perceived. 



It is an effect of this same principle, that the, iron head of a hammer may bo 

 driven down on its wooden handle, by striking the opposite end of tho 

 handle against any hard substance with force and speed. In this very simple 

 operation, the motion is propagated so suddenly through the wood of the han- 

 dle, that it is over before it can reach the iron head, which therefore, by its 

 own inertia, sinks lower on the handle at every blow, which drives the han- 

 dle up. 



How 13 the Mo- 135. The Momentum, or force, which a mov- 

 "ody^^c^ca- ing body exerts, is estimated by multiplying 

 lated? j|-g mass or quantity of matter by its velocity. 



Thus, a body weighing 10 pounds, and moving with a velocity of 500 feet 

 in a second, will have a momentum of (10X500) 5,000. 



What nn . ^^^' ^^^^ vclocity being the same, the mo- 



betwein ^''T ^^Qeutum, Or moviug force of a body, will be 



Momentum of dircctly proportionate to the mass, or weight ; 



weight and ve- and the mass or weight remainino: the same, 



locityf Ml 1 T 1 . 



the momentum will be directly proportionate 

 to the velocity. 



Thus, if 2 leaden balls, each of 5 pounds' weight, move with a velocity of 

 6 miles per minute, the momentum, or striking force of each, wiU be 25 ; 

 if now the two balls, molded into one of 10 pounds' weight, move with the 

 same velocity of 5 miles per minute, the momentum, or striking force, will 

 be 50, since with the same velocity the mass, or weight, will be doubled. I^ 

 on the contrary, we double the velocity, allowing the weight to remain the 

 same, f .e same effect will be produced ; a ball of 5 pounds, with a velocity 

 of 5, will have a momentum, or striking force, of 25 ; but a ball of 5, with a 

 velocity of 10, will have a momentum of 50. 



How can a 1^7. A Small, or hght body, may be made 

 ^tk.n^be'^raide ^^ striko with a greater force than a heavier 

 ^mXrcelsl ^'^dy, by giving to the small body a sufficient 



large one ? VClocity. 



Illustrations of these principles are most familiar. Hail-stonea, of small 

 mass and great velocity, strike with sufficient force to break glass, and de- 

 stroy standing grain ; a ship of huge mass, moving with a scarcely percept- 

 ible velocity, crushes in the side of the pier with which it comes in contact. 



