38 COMPOUND MOTION. 



A ball fired from a cannon is acted upon by two forces, the 

 one is that occasioned by the powder, the other is the force 

 of gravity. 



Circular motion is the result of two forces on a body, by 

 one of which it is projected forward in a right line, whilst 

 by the other it is confined to a fixed point. When you 

 whirl a ball, for instance, which is fastened to your hand 

 with a string, the ball moves in a circular direction ; be- 

 cause it is acted upon by two forces, that given it by your- 

 self, which represents the force of projection, and that of the 

 string which confines it to your hand. If during its motion 

 the string were suddenly to break, the ball would fly off in a 

 straight line ; being released from confinement to the fixed 

 point, it would be acted on but by one force, and motion 

 produced by one force is always in a right line. The force 

 which confines a body to a centre round which it moves, is 

 called the centripetal force ; and that force which impels a 

 body to fly from the centre, is called the centrifugal force. 

 In circular motion these two forces constantly balance each 

 other, otherwise the revolving body would either approach 

 the centre, or recede from it, according as the one or the 

 other prevailed. If any cause should destroy the centripetal 

 force, the centrifugal force would alone impel the body, and 

 it would fly off in a right line in the direction in which it 

 was moving, at the instant of its release. When a stone, 

 whirled round in a sling, gets loose, it flies off in a right linCj 

 called a tangent, because it touches the circumference of the 

 circle in which the stone was revolving. 



It is by the laws of circular motion that the moon and all 

 the planets revolve in their orbits. The moon, for instance, 

 has a constant tendency to the earth, by the attraction of 

 gravitation, and it lias also a tendency to proceed in a right 

 line, by that projectile force impressed upon it by the Crea- 

 tor ; now, by the joint action of these two forces it describes 

 a circular motion. If the projectile force were to cease, 

 the moon must fall to the earth ; and if the force of gravity 

 were to cease acting upon the moon, it would fly off into 

 infinite space. 



When you throw a ball in a horizontal or oblique direc- 

 tion, it describes a curve line in falling, and is acted upon 

 by three forces ; the force of projection, which you commu- 

 nicated to it ; the resistance of the air, which diminishes its 



