56 



ELEMENTS OF LABORATORY WORK 



Fig. 15. 



account the mass of the pulley, which is raised together with 

 the attached mass. 



The more completely friction is avoided, the more nearly 

 will a given total mass at A be 

 balanced by one-half its mass at 

 c, and the smaller will be the 

 excess over one-half which is 

 needed to cause the fall of c and 

 the rise of A. 



While there is no movement, 

 we have a system in equilibrium ; 

 the stress between the earth and 

 c balances the stress between the 

 earth and A by the aid of the cord. 

 It is easy to see that" in such 

 an arrangement, where the three 

 portions of the cord are all parallel, 

 the mass at c would descend through twice the distance 

 ascended by the mass at A. 



With a combination of pulleys, which consists of two 

 blocks, each with three pulleys, and a continuous cord passing 

 over each pulley, it is easy to see that the ascending mass 

 would move through one-eighth the distance of the descent 

 made by the smaller mass ; and, if it were not for the effect 

 of friction, which is much greater in such a system, a given 

 mass would balance one eight times larger. 



These experiments illustrate the need of considering both 

 mass and distance in all such mutual changes. 



41. The Lever. The lever, like the pulley, shows that both 

 mass and distance must be considered in all cases of mutual 

 change. Its principle is illustrated by a rigid rod supported 

 on an edge or small surface, called the fulcrum. If this rod 

 is of constant section and homogeneous material it will be 

 found to balance when supported at the centre, the stress on 

 the one side of the fulcrum being just equal to that on the 

 other. In any displacement from the horizontal position each 

 particle on one side of the fulcrum has a corresponding 



