I 



36 THE LAWS OP MOTION. i 



that human powers can produce. Accelerated motion takes j 

 place, when the motive power continues to act upon any 1 

 body, so that its motion is continually increased. When a 

 stone falls from a height, the impulse which it receives from j 

 gravity during the first instant of its fall, would be sufficient ^ 

 to bring it to the ground with a uniform velocity ; but the 

 stone is not acted upon by gravity merely at the first instant \ 

 of its fall, — this power continues to impel it during the whole 1 

 of its descent, and it is this continued impulse which acce- | 

 Jerates its motion. It has been found by experiment that \ 

 heavy bodies, descending from a height by the force of gra- ; 

 vity, fall sixteen feet the first second of time, three times that ; 

 distance in the next, five times in the third second, seven J 

 times in the fourth, and so on, regularly increasing their ve- 

 locities according to the number of seconds during which i 

 ihe body has been falling. Retarded motion is that of a : 

 ))ody which moves every moment slower and slower ; and it J 

 {3 produced by some force acting upon a body in a direction ,i 

 Opposite to that which first put it in motion, as when a stone \ 

 IS thrown upwards, its velocity is gradually diminished by ] 

 (he power of gravity. ^ 



The force, or power, with which a body in motion strikes i 

 against another body, is called its momentum. It is composed * 

 ^f its quantity of matter, multiplied by its quantity of motion ; { 

 or in other words, its weight and its velocity. A small body i 

 may have a greater momentum than a large one, provided its : 

 velocity be sufficiently greater ; the momentum of an arrow j 

 l?hot from a bow, for instance, must be greater than a stone 

 thrown by the hand. The momentum of bodies is one of j 

 the most important points in mechanics ; for you will find, | 

 that it is from opposing motion to matter, that machines de- \ 

 rive their powers. i 



When a body in motion strikes against another body, it  

 meets with resistance from it ; and the resistance of the body • 

 tt rest will be equal to the blow struck by the body in mo- \ 

 ^ion ; or to express the same in philosophical language, action \ 

 4nd re-action will be equal and in opposite directions. It 

 Appears, therefore, that one body acting upon another, loses 1 

 as much motion as it communicates, and that the sum of th« \ 

 Motions of any two bodies in the same line of direction, can- ; 

 "ot be changed by their mutual action. From the action \ 

 *M& re-action of bodies we may learn in what manner a bird, j 



