60 STATICS OF SYSTEMS OF PAETICLES 



will move or will remain at rest, when systems of forces act from 

 outside on its different particles. We shall have to remember 

 throughout that the forces applied from outside are not the only 

 forces acting, but that these are accompanied by actions and reac- 

 tions between the different particles. 



45. One consequence of this last fact appears at once. Applying 

 a force to one particle A of a body is not the same thing as apply- 

 ing an exactly similar force to another particle B. For the systems 

 of internal actions and reactions will be different in the two cases. 

 Any simple example will show that the resulting motion will, in 

 general, also be different; e.g. a horizontal force applied to the 

 middle point of the back of a chair will probably cause the chair 

 to overturn. A similar force applied to one foot will drag it along 

 the ground and also cause it to turn about a vertical axis. 



The position occupied by the particle to which a force is applied 

 is called the point of application of the force. The line drawn 

 through this point in the direction of the force is called the line 

 of action of the force. 



Clearly, in order to have full data as to the action of a force, 

 we must know 



(a) its magnitude; 



(b) its point of application j 



(c) its line of action. 



MOMENTS 



46. DEFINITION. The moment of a force about a line at right 

 angles to the line of action of the force is defined to be the product 

 of the force and of the shortest distance between the two lines. 



This moment, as we shall soon find, measures the tendency to turn around 

 the line about which the moment is measured ; e.g. if the arm of a balance 

 is of length I, a weight w at its end has a moment Iw about the pivot of the 

 balance, and we shall find that this measures the tendency of the arm to turn. 



DEFINITION. The moment of a force about a line L which is not 

 at right angles to the force is defined to be the same as the moment 

 about L of the component of the force in a plane perpendicular to L. 



