238.] GENERAL PRINCIPLES. ! 3I 



F approaches oc (Art. 5) ; in this case it is strictly true that the 

 effect of ordinary forces can be neglected when impulsive forces 

 act on the body. 



If the rigid body be originally at rest, it will be convenient 

 to denote by x, y, z the components of the velocity of the par- 

 ticle m just after the action of the impulses. We may also 

 denote by R the resultant of all the impulses, by H the result- 

 ant impulsive couple for the reduction to the origin of co- 

 ordinates, and mark the components of R and H by subscripts, 

 as in the case of forces. With these notations the effect of a 

 system of impulses on a body at rest is given by the equations 



(19) 

 H g . (20) 



In the equations (19) we have, of course, ^mx=Mx, 

 z=Mz, where x, y, ~z are the components of the velocity of 

 the centroid, and M is the mass of the body ; i.e. the momentum 

 of the centroid is equal to the resultant impulse. The meaning 

 of the equations (20) can be stated by saying that the angular 

 momentum of the body about any axis is equal to the moment of 

 all the impttlses about the same axis. 



