OF SIMPLE ACCELERATING FORCES. 7S 



motion in another body, and not to the motion of any od< 

 body considered separately. 



Scholium 2. It is a necessary condition In the defi- 

 nition 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 operation of any force : thus, if a body be 

 moving without disturbance, its motion with respect to 

 another body, not in the line of its direction, will be per- 

 petually 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 consequence of the centrifugal effect of a rotatory mo- 

 tion : the inertia here becoming a relative force, which 

 tends to increase the distance of the body from a point out 

 of the line of its direction, with an accelerated motion, 

 unless counteracted by an attractive force. 



Scholium 3. The muscular exertion of an animal, the 

 unbending of a bow, and the impulsion produced by the 

 apparent contact of a moving body, are familiar instances 

 of the actions of forces. We must not imagine that the 

 idea of force is naturally connected with that of labour or 

 difficulty ; this association is only derived from habit, since 

 our voluntary actions are in general attended with a cer- 

 tain effort, leaving an impression almost inseparable from 

 that of the force which it calls into action. f 



Scholium 4. It is natural to inquire, in what imme- 

 diate manner any force acts, so as to produce motion ; for 

 instance, by what means the earth causes a stone to gravi- 

 tate towards it. In some cases, indeed, we are disposed 

 to imagine that we understand better the nature of the 

 action of a force, as, when a body in motion strikes ano- 



