DYNAMICS. 25 



58. When one body changes its place relatively 

 to another, it is said to have a relative motion with 

 respect to that other body. 



Two bodies if they move with equal velocities toward 

 the same side, in the same or in parallel lines, will 

 have no motion relatively to another. 



If two bodies move with equal velocities in the sanfe 

 straight line, but in opposite directions, their rela- 

 tive motion is the sum of their real motions ; if in 

 the same direction, it is the difference of their real 

 motions. The same is to be said of their relative 

 velocities. The use of the signs + and brings 

 the two parts of this and all similar propositions 

 under one enunciation. 



59. The path of one moving body relatively ta 

 another is determined, by supposing the latter to 

 stand still in any point of its path, and by inqui- 

 ring in what line the other must move, and with 

 what velocity, so that it may approach to the for- 

 mer (supposed at rest) or recede from it, at the same 

 rate, and in the same direction, that it actually does- 

 when they are both in motion. 



SECT. 



