H8' Statics. 



- 260, 6 X j X 0,24 lb 



1 __ i x 0,24 3 ' J ' 



Thus the tension of the cord 5, which without friction would be 

 only 100, is equal to 130, 3 -f 13, 2 = 143 lb , 5. 



243. In the determination, which we have given, of the effect 

 of friction upon the moveable pulley, we have not allowed for 

 any increased pressure at D arising from the augmentation of 



Jig. 130. fa e power, although this is done by some writers. The reason, 

 is, that this augmentation of the power contributes nothing to the 

 pressure at D ; indeed, setting aside the stiffness of the cords, and 

 certain other obstacles, whenever the power is greater than is 

 necessary to an equilibrium, the body of the pulley is elevated 

 by this excess. The augmentation of the power does not con- 

 tribute any thing to the friction against the axis, if no regard is 

 had to the velocity that may thus be produced, which together 

 with the inertia of the weight and pulley are at present left out 

 of consideration. It is only the weight which presses. This is 

 not the case with the fixed pulley ; and the method which we 

 have given comprehends, with respect to this point, every thing 

 which ought to be taken into the account, although it differs from 

 the course heretofore adopted, in which something has been taken 

 for granted. 



We will not deny, that in the calculation which we have given 

 of the effect of friction, if we would know the effect of friction 

 Fig. 92> very rigorously, it would be necessary to consider the subject a 

 little differently. In fact, by determining in the way we have 

 done, the particular tensions of each of the cords, it is taken for 

 granted that each pair of cords acts as it would do in the case 

 of a simple pulley, which is not strictly true, perhaps. But this 

 approximation will suffice for the present. 



244. In order to determine upon the inclined plane, the ratio 

 of the power to the weight, when the former is upon the point of 

 causing the body to move, we proceed thus. Through the point 



^ 13J of meeting F, of the directions of the power 9, and weight />, we 

 suppose the line FI drawn, making with the plane JIB an angle 

 FIA equal to that of friction. In order that the power q may be 

 upon the point of causing motion, it is necessary, (\.) That the 



