50 STATICS. [92, 



92. The product Rr represents twice the area of the triangle 

 having R for its base and 6" for its vertex ; Pp, Qq can be 

 interpreted similarly. This remark leads to another simple 

 proof of Varignon's theorem, which may serve to make its 



meaning better understood. With the 

 notation of Fig. 18 we have 



SOR = SOQ + SQR + QOR, 



or since ST+ TU= SU=p, 

 Rr=Qq + Pp. 



93. If the point 5 be taken on the resultant R, we have r=o, 

 hence Pp= Qq ; i.e. the sum of the moments of two forces about 



any point on their resultant is zero. 





 i .*<' 



94. The forces of nature receive various special names 

 according to the circumstances under which they occur. 

 Thus the weight of a mass has already been defined (Art. 67) 

 as the force with which the mass is attracted by the mass of 

 the earth. 



When a string carrying a mass at one end is suspended with 

 its other end from a fixed point, it will be stretched, i.e. sub- 

 jected to a certain tension. This means that if the string. were 

 cut it would require the application of a force along the line of 

 the string to keep the weight in equilibrium. This force, which 

 may thus serve to replace the action of the string, is called its 

 tension. 



When the surfaces of two physical bodies A, B are in con- 

 tact, a pressure may exist between them ; that is, if one of the 

 bodies, say B, be removed, it may require the introduction of 

 a force to keep A in the same state of rest or motion that it 

 had before the removal of B. This force, which will obviously 



