140 Statics. 



others, the one perpendicular to the plane AB, and the other in 

 the direction of this plane. These efforts will evidently be the 

 same as those which were directed according to KH and KL. 

 Moreover the first will be destroyed, especially if it meet the plane 

 AB in some point / common to this plane and the surface of the 

 body. As to the second, since it is in the direction of friction, 

 it will not be destroyed unless it happen to be exactly equal 

 to the force of friction. 



We hence see how the value of friction is to be determined 5 

 we take successively for q different weights until we find one that 

 is just sufficient to cause a motion in the body p. But not to 

 comprehend, in estimating the friction of the body/), effects for- 

 eign to that which is sought, .it is necessary to attend to several 

 particulars; (1.) The pulley M should move with the greatest 

 ease, and the cord KM q should be as flexible as it can be made. 

 (2.) The cord CM should be attached to some point C as near 

 as possible to the plane AB. The necessity of this precaution 

 arises from the circumstance, that other things being the same, 

 the point /, where the effort in the direction KI meets the plane 

 AB, will approach so much the nearer to the extremity E of the 

 base of the body, or will fall without the base so much the far- 

 ther from the extremity , according as the point C is the more 

 elevated above the plane AB. Now in the case where the point 

 / falls without the base, the effort perpendicular to the plane, 

 not being entirely destroyed, there will hence result a tendency 

 in the body to rotate ; and the friction thence arising would be 

 somewhat more considerable than the proper friction in question. 



Fig 127. 237. Let us now consider a weight p, put upon an inclined 

 plane, and retained by the effect of friction alone. The action 

 of gravity directed according to the vertical GZ passing through 

 the centre of gravity G of the body, and meeting in / some point 

 of the plane AB, may be decomposed into two parts, one in the 

 direction of the plane, and the other perpendicular to it. The 

 second will be destroyed, if the point / does not fall without the 

 base DE, and the first in order to be destroyed must be equal to 

 the force of friction. Now it is evident that by forming the 

 parallelogram IHZL, IZ will represent the weight of the body. 



