Friction* 161 



ing from the perpendicular force BF, the body would turn, and 

 the centre of gravity G would advance in a line parallel to the 

 tangent, as if the point B were drawn in that direction by means 

 of a thread attached at this point, and with a power equal to the 

 force of friction. 



248. Let us suppose that a hard, spherical body ABC, falls Fig.134. 

 freely upon a horizontal plane HR, and that it receives from some 

 cause or other, a motion of rotation about its centre of gravity ; 



if there were no friction, this body, after meeting the plane would 

 preserve only its rotatory motion, and its centre of gravity would 

 be at rest. But in the case of friction, when the body has reached 

 the plane, it will roll from / toward R, or from /toward //, accord- 

 ing as the rotatory motion is in the direction CAB, or in that of 

 BAC ; since the resistance of friction, which is exerted in the direc- 

 tion of the plane,is equivalent to a force acting upon this body in 

 a direction opposite to its motion ; and as it does not pass through 

 the centre of gravity of the body, it must give it a motion paral- 

 lel to the plane, and a rotatory motion, both in the direction 

 contrary to the actual motion of rotation. Now of these two 

 motions, the latter diminishes continually the original motion; 

 and on the other hand, the motion of the centre will be acceler- 

 ated to a certain point, after which it will be diminished till it is 

 destroyed with the motion of rotation. 



249. We are hence enabled to explain several phenomena ; 



as (1.) Why a spherical body ABC, struck in the direction DB, Fig. 135 

 after having advanced in the direction IE, returns afterward 

 from E toward 7, and may even pass beyond / toward H. The 

 impulse in the direction DB causes it to turn (on account of fric- 

 tion at B), according to ABC, and to advance in the line IE-, but 

 the friction upon the plane, being now a friction of the first kind, 

 the motion of the centre of gravity is soon destroyed, and the 

 motion of rotation gives rise to another in the opposite direction, 

 as in the preceding case. 24g 



(2.) We are moreover furnished, upon the same principles, 

 with the reason why a cannon ball, which had apparently lost 

 nearly all its force, seems on striking to recover it again, and 

 often with violence. When it is impelled by the force of the 



