27 



LECTURE III. 



ON ACCELERATING FORCES. 



l^E have hitherto only considered motion as ah-eady existing, without any 

 regard to its origin or alteration;' We have seen that all undisturbed motions 

 are equable and rectilinear; and that two motions represented by the sides of 

 a parallelogram, cause a body to describe its diagonal by their joint effect. 

 We are now to examine the causes which produce or destroy motion. Any 

 cause of a change of the motion of a body, with respect to a' quiescent space, is 

 called a force; that is, any catise which pr6duc'es motion in a body at res't, or 

 which increases, diminishes, oi* modifies it in a body which -tvas before in mo- 

 tion. Thus the power of gravitation, which causes a stone to fall to the 

 ground, is called a force ; but when the stone, after descending down a hill, 

 rolls along a horizontal plane, it is no longer impelled by any force, and its 

 relative motion continues unaltered, until it is gradually destroyed by the re- 

 tarding force of friction. Its perseverance in the state of motion or rest in 

 consequence of the inertia of matter, has sometimes been expressed by the 

 term vis inertiae, Or force of inertia; but it appears to be somewhat inaccurate 

 to apply the term force to a property, which is never the cause of a change of 

 motion in the body to which it belongs. 



It is a necessary condition in the definition of force, that it be the cause of 

 a change of motion with respect to a quiescent space. For if the change were 

 only in the relative motion of two points, it might happen without the opera- 

 tion of any force: thus, if a body be moving Without disturbance, its motion 

 with respect to another body, not in thehneofits direction, will be perpetually 

 changed ; and this change, considered alone, would indicate the existence of 

 a repulsive force: and, on the other hand, two bodies may be subjected to the 

 action of an attractive force, while their distance remains unaltered, in con- 

 sequence of the centrifugal effect of a rotatojy motion. (Plate I. Fig. 9-> 



