6o 



ENGINEERING ON THE FARM 



rope, and (4) the ultimate strength of the rope and parts of 



the blocks, for as soon as we strain any part to the breaking 



point, the blocks will give way. 



Suppose that the two plies of rope leading from block A 



in Figure 87 are cut at the points a and b, and a spring 



scale tied between the 

 ends of each ply. Now 

 if a pull of 100 pounds is 

 exerted on the fall-rope, 

 each of the scales will 

 show it is holding a force 

 of approximately 100 

 pounds, the small error 

 being due to friction and 

 the stiffness of the rope. 

 This would show that 

 all parts of the rope are 

 under the same strain or 

 tension. Since each of 

 the scales lifts on block 

 A with a force which is 

 nearly equal to 100 

 pounds, the block A will 

 lift on a load with a force 

 which is a little less than 

 2 00 pounds. We thus see 

 that the block A lifts 

 with a force which is 

 nearly equal to the pull 

 on the fall-rope times the 

 number of plies leading 



Fig. 87 



from block A. If the pull on the fall-rope is 100 pounds and 

 directly downward, parallel to the other plies, we then have 

 three plies pulling down on block B, so that block B pulls 

 down on its support with a force of approximately 300 



