Chap. XL.] PARALLELOGRAM OF FORCE. 



491 



position, and suppose the weight to represent a force of 

 1 pound. Then o is under the influence of three forces ; 

 but it is at rest, so that the forces are in equilibrium. 

 The forces OA and OB are both tending to draw o 

 upwards, and they are completely counterbalanced by 

 the 1 pound weight. To put it in another way, 

 the weight is tending to pull o downwards, but 

 is counterbalanced by OA and OB. But the weight 



Fig. 201. Parallelogram of Force, 



would be counterbalanced exactly by a force of 1 

 pound acting in the direction directly opposed to it, 

 that is, in the direction of the straight line drawn up 

 from o. If, therefore, OA and OB be withdrawn, 

 and one force substituted equal to the weight oppo- 

 sing them, equilibrium will still be maintained. So 

 the two forces OA and OB can be replaced by a 

 single force, which is called the EESULTANT FORCE. 

 If a parallelogram be constructed on OB- OA, as indi- 

 cated in the figure, it will be seen that the resultant 

 force is the diagonal of the parallelogram. This is 

 represented also in Fig. 201, where two forces OA OB are 

 represented acting on a particle. To find the direc- 

 tion in which the particle will move, a parallelogram 

 is constructed of which OA and OB form two sides, and 

 then the diagonal OR of the parallelogram is drawn. 

 It gives the direction which the particle takes ; it is 



