COMPOUND MOTION. 



79 



ihe statements already made (§ 155), the path of the moving body will be a 

 resultant of the two forces, or the diagonal of a parallelogram. 



How then, it may be asked, does the body attached to the 

 string and whirled round tho head, move in a circle ? This 

 will be clear, if we consider tnat a circle is made of an in- 

 finite number of httle straight lines (diagonals of parallelo- 

 grams) and that the body moving in it, has its motion bent 

 at every step of its progress by the action of the force which 

 confines it to the hand. This force, however, only keeps it within a certain 

 distance, without drawing it nearer to the hand. The two forces exactly 

 balancing each other, the course of the whirling body will be circular. 



165. The two forces by which circular mo- 

 tion is produced, are called the Centrifugal'^'* 

 and Centripetal Forces.f 



166. The Centrifugal Force is that force 

 which impels a body moving in a curve to 

 move outward, or fly off from a center. 



167. The Centripetal Force is that force 

 which draws a body moving in a curve toward 



the center, and assists it to move in a bent, or curvelinear 

 course. In Circular Motion the Centrifugal and Centri- 

 petal Forces are equal, and constantly balance each other. 



If the Centrifugal Force of a body revolving in a circular 

 path be destroyed, the body will immediately approach the 

 center ; but if the Centripetal Force be destroyed, the body 

 will fly off in a straight hne, called a tangent. 



Thus, in whirUng a ball attached by a string to tho fin- 

 ger, the propelling force, or the force of projection, is given by the hand, and 

 the Centripetal Force is exhibited in the stretching, or 

 tension of the string. K the string breaks in whirling, 

 the Centripetal Force no longer acts, and the ball, by 

 the action of the Centrifugal Force, generated by tho 

 whirUng motion, flies off in a tangent, or straight hne, 

 as is represented in Fig. 51. If, on the contrary, tha 

 whirling motion is too slow, the Centripetal Force pre- 

 ponderates, and the ball falls in toward the finger. 

 Familiar examples of the effects of Centrifugal Force 

 are common in the experience of every-day hfe. 



,.., , , The motion of mud flving fit)m the rim of a coach-whceL 



A\'i-t are fa- . r> -i 



miliar iiiiistra- movmg rapidly, is an illustration of Centrinigal Force. Fig. 



fu*'!a'ForceV'' ^"^ represents a coach-wheel throwing off mud ; o tho point at 

 which the mud flies off; ab, the straight hne in which it 

 • CentTifnf»al, compounded of center, and "fugio" to fly oft 

 t Centripetal, compounded of center and "peto," to seek. 



Kow may the 

 curve of a cir- 

 cle be consid- 

 ered as equiva- 

 lent to the 

 diagonal of a 

 parallelogram T 



What are the 

 two forces 

 which produce 

 Circular Mo- 

 tion called 1 



What is Cen- 

 trifugal Force T 



What is Cen- 

 tripetal Force 7 



■WTiat follows 

 if the Centri- 

 fajfal or Cen- 

 trip'jtal Forces 

 are destroyed f 



Fig. 51. 



