24 OF CENTRAL FORCES. 



LECTURE II. 



OF CENTRAL FORCES." 



Motion or WE have already mentioned it as a necessary conse- 

 i^ndiffer- quence arising from the deadness or inactivity of mat- 

 ent to all ter, that all bodies endeavour to continue in the state 

 they are in, whether of rest or motion. 



If the body A B C 



were placed in A \,J~ 



any part of free space, and nothing either draws or im- 

 pels it any way, it would for ever remain in that 

 part of space, b^ause it could have no tendency of 

 itself to remove any way from thence. If it receives a 

 single impulse any way, as suppose from A towards B, 

 it will go on in that direction; for, of itself, it could 

 never swerve from a right line, nor stop its course. 

 When it has gone through the space A B, and met with 

 no resistance, its velocity will be the same at B as it 

 was at A; and this velocity, in as much more time, 

 will carry it through as much more space, from B to C ; 

 and so on for ever. Therefore, when we see a body in 

 motion, we conclude that some other substance must 

 have given it that motion ; and when we see a body fall 

 from motion to rest, we conclude that some other body 

 or cause stopped it. 



All motion As all motion is naturally rectilineal, it appears, that 



rectilineal, a bullet projected by the hand, or shot from a cannon, 



would for ever continue to move in the same direction it 



received at first, if no other power diverted its course. 18 



Note 17. Every body moved in a circle, has a tendency to fly off 

 from its centre, which is called the centrifugal force, and it is opposed 

 to the centripetal or gravitating force, which invariably draws a body 

 towards the nearest or greatest mass that occurs within its sphere of 

 operation. Tnese two forces are called central forces. 



Note 18. The art of gunnery depends principally on a due atten- 

 tion to the projectile and gravitating lorces, operating on the body to 



