COMPOUND MOTION. 



73 



niustra 

 pie M. 



" Sim- 



What is the 

 course of a 

 body acted up- 

 on by two 

 forces called t 



Fig. 43. 



A body floating upon the water is driven exactly south by 

 a wind blowing south. A ball fired from a cannon takes the 

 exact direction of the bore of the cannon, or of the force 

 which impels it. 



A\iiat is Com- l^^. When a body is acted upon by two 

 pound Motion f f^^^^^ ^^ ^j^g ^^^^ ^^^^^ ^^^ ^^ different di- 

 rections, as it can not move two ways at once, it takes a 

 middle course between the two. Such motion is termed 

 Compound Motion. 



155. The course in which a body, acted 

 upon by two or more forces, acting in different 

 directions, will move, is called the Resultant, 

 or the Resulting Direction. 



In Fig. 43, if a body, A, be acted upon 

 at the same time by two forces, one of 

 which would cause it to move in the di- 

 rection A y, over the space A B, in one 

 second of tune, and the other cause it to 

 move in the direction A X, over the space 

 A C, in one . second ; then the two forces, 

 acting upon it at the same instant, will 

 cause it to move in a Resultant Direction, 

 A D, in one second. This direction is the 

 diagonal of a parallelogram, which has for its sides the Unes A B, A C, over 

 Vvhich the body would move if acted upon by each of the forces separately. 



156. The operations of every-day hfe afford numerous exam- 

 What are fa- , „^ , ,, . -r^ . , ^ 



miliar Eiam- ples of Resultant Motion. If we attempt to row a boat across 



pli-s of Kesiilt- ^ rapid river, the boat will be subjected to action of two forces; 

 ant Motion ? ^ ' ..... , 



viz., the action of the oare, which tend to drive it across the 



river in a certain time, as ten minutes, in a straight line, as from A to B, Fig. 



43, and the action of the current, which tends to carry it down the stream a 



certain distance in the same time, as from A to C. It will, therefore, under 



the influence of both these forces, move diagonally across the river, or in the 



direction A D, and arrive at D at the expiration of the ten minutes. When 



we throw a body from the deck of a boat in motion, or from a railroad ca:; 



the body partakes of the motion of the boat or the car, and does not strike at 



the point intended, but is carried some distance beyond it. For the same rea» 



son, in firing a rifle from the deck of a vessel moving rapidly, at some object 



at rest upon the bank, allowance must be made for the motion of the vessel, 



and aim directed behind the object. 



157. The principles of the composition and 

 Science of Pro- rcsolution of different forces acting upon a 

 jec es body to produce motion, constitute the basis 



