THE PRINCIPLE OF VIRTUAL WORK. 



>or 



I 8s ___ I COS( /?) I 



r 8J3 ~ (sin a- sin/2) 2 = i +cos(a 



Hence, finally, 



245. A weightless rod of length AB = 1 rests at C on a horizontal 

 .cylinder whose axis is at right angles to the vertical plane through the rod ; 

 its lower end A leans against a vertical wall, and from its upper end B a 

 weight W is suspended. Determine the reactions at A and C, and the 

 distance AC = *for equilibrium, if the distance CD = a of the point of 

 .support from the vertical wall is given (Fig. 75). 



(a) Let A glide vertically upwards, C remaining in contact. At A 

 .as well as at C the forces are perpendicular to the displacements; 

 Jience, putting EB =y, we have Wfy = o. 



,C 





whence, (/ .x).* 2 /(JT # 2 ) = o 



Fig. 77. 



or 



(b) Give the rod a vertical displacement to a parallel position : 



(c) Give the rod a displacement in its own directign : 



246. / ^ parallelogram formed by four rods with hinges at the ver- 

 Jices, elastic strings are stretched along the diagonals. Determine the 

 ratio of the tensions in these strings* 



