338 CASES ix 



virtual moment ol v.hirh the plane exerts on the 



body vanishes. 



(3) Let two smooth bodi< -s i 



a force on the other alon^ their c.'mmn normal. Sir 

 one of them .so displaced, that the point in it which was 



.allv in c>ntart with tin- other body still remains in 

 tact with it; the case is similar to that of a body i 

 with a fixed plane; the virtual velocity of the point of CO 

 relative to the normal force is not aero, lut is indefinitely 

 small compared with the absolute virtual velocity. 



Let BA C be a section of one body made ly a plane which 



contains the common normal to the surfaces, and ])AE tli-- 

 section of the other made by the same plane ; A the point of 

 contact. Suppose the body BA C displaced into the p -iiion 

 ' ", so that the point A is moved to A'. Draw . I'M per- 

 pendicular to the common normal to the surfaces. Then AM 

 represents the virtual velocity of the p,,int nf contact with 

 respect to the normal foree, while the straight line i 

 and A' is the absolute virtual velocity. Sine.- MA A' is ulti- 

 mately a right angle, AM vanishes compare.! with AA'. 



(4) Su ' - in contact at a single point, and 



.em be both displaced so that they .still remain with th<; 



point of each body in contact. Let P denote the force 



in the normal on one body, and therefore P that on the 



other; then, if Pbp denote the virtual moment of the normal 



