340 CASKS i\ WHICH 



(G) Let us suppose the bodies in contact to 1 .ami 



Oft upon the other 



the ttreoeding GSM. The- < ach body on the nth, -r will 



not 1 '.-il nlonir tin 1 normal A\. but may 1 



into two, one along A\ and the other .! A. 



irtnal moment of the former force v.mMies, as we have 



i in tin- preceding case; and since ti tion of the 



Straight line joining A and rr ultimately coinridrs wit: 

 and is therefore perpendicular to the. second lone, the virtual 

 moment of the second force vanishes in the same man: 

 in the third case, 



Tin- result depends on the hypothesis that tin- bodies 

 on each other; if there is sliilimj the. virtual moment of the 

 force at right angles to ANvt\\\ not vanish. 



(7) Suppose an inextensible string to have on*' end at- 

 tached to a fixed point, and the other end to a particle either 

 isolated or forming part of a rigid body ; one of the forces of 

 the system is then the tension of this string which acts . 



its length. Let tin- particle be so displaced as to keep the 

 st ring stretched, then it may pasa from its first to it 

 position by moving over an arc of a circle, and in 

 manner as in the third case, we see that the virtual v< 

 of the particle with respect to the tension which the string 



. is indefinitely small compared with the absolute virtual 

 velocity of the. partirle. Hence, the tension of the MI in 

 axs from the equation of virtual velocities. 



(8) Suppose an inextensiMc string connecting two parti- 

 cles of the system, and let the particles be displaced alon^r the 

 direction of the string, the string being 1 



.tide be displaced through a space /?, and 7M- 

 the tension of the string, and then f'.re the IT I by 



the string on this particle, P&p is the virtual iiinmciit .{' the 

 force which the string exerts on thi will 



be the virtual moment of the force which the, string 



cornl particle. Hence, by taking the .- ,' virtual 



moments for the two p; the tension of the string dis- 



appears from the equation of virtual velocities. 



(9) If we suppose a fur: lacemcnt of the system in 

 the preceding case, by keeping one particle fixed and ma 





