GENERAL PRINCIPLES 



21 



FIG. 37. 



Fw. -1R 



(fig. 38). That resultant must be zero in the case of equilibrium, 



that is to say, their rectangular components, X, Y, Z, must 



satisfy the condition that X = 0, Y == 0, Z = 0. But a system 



ot forces may not be concurrent. The directions of some may 



meet at the point 0, whilst others do not ; in this case equal and 



parallel forces are directed to the point 0, and [the forces which 



do not meet at are produced in like manner. The new 



system will have a general resultant in relation^ tolthe point 



; at the same time, the forces 



which do not meet at 0, such 



as F 4 and F 5 tend to produce 



movement of the solid round 



the point ; they have a 



moment (from movimentum) 



in relation to 0. There is, 



therefore, as well as the 



general resultant, a moment 



of each of these forces, or a 



resultant moment of all these 



forces if they are united. 



The moment of a force, such 



as F 4 ,in relation to the point 



0, is the product of that 



force by its distance from 



that point (vide fig. 39). 



We see that by its moment a force tends to turn the body 

 round the point 0, or an axis which would pass through it, with 

 a radius or leverage equal to d. In resolving the resultant 

 moment along the three rectangular axes, the condition of 

 equilibrium demands that the three components L, M and N of 

 the moment should be zero, i.e. ! 



L = 0, M = N = 0. 



FIG. 39. 



