THE LEVER AND BALANCE 39 



Compare the numbers in columns 3 and 6, and state in words the law 

 or rule indicated by the results. 



The experiments show that there is a definite proportion between the 

 weights on the two sides of a lever, and their distances from the fulcrum 

 The proportion as : 



left right . right left 



load ' load distance ' distance. 



Or, expressed in another way, the loads are inversely proportional to their 

 distances from the fulcrum. 



The turning effect of any force acting upon a lever, as each load did in 

 the experiment, is termed the moment of the force. The comparisons of 

 columns 3 and 6 prove that the moment or turning effect is measured by the 

 product of load into distance from the fulcrum. 



(b) Hang two bags on the same side of the lever at different distances 

 and one bag on the other side. Move the single bag until equilibrium is 

 obtained. Do this several times with the bags in different positions, and 

 compare the sum of the moments of the forces acting on one side with the 

 moment of the force on the other side. 



(c) Hang a small book or a bag with shot or nails in it on one side of 

 the fulcrum, and the 100 gram bag from the other. Move this bag along 

 the lever until equilibrium is obtained. Then, remembering that 



, i distance _, , distance 2 



acl1 x from fulcrum ~ ' *** x from fulcrum, 



calculate the weight of the book or bag of nails. Repeat the experiment, 

 using the 200 gram bag. 



In the preceding experiments the fulcrum has been between the forces 

 due to the loads hanging from the lever. The forces may, however, both 

 act on one side of the fulcrum. It is convenient to call one of the forces 

 the effort and the other the resistance. 



Machines. A machine is a contrivance by means of which a given 

 force is made to resist or overcome another force acting in a contrary 

 direction. 



What are termed " the mechanical powers " are really simple 

 machines which can be used to overcome resistance. The mechanical 

 principles underlying the action of such a simple machine as the lever 

 have now to be considered. 



The lever. A lever is a rigid bar which can be turned freely about 

 a fixed point. The fulcrum of a lever is the fixed point about which 

 the lever can be turned. The force exerted when using a lever is often 

 described as the power and the body lifted or resistance overcome as 

 the weight. These words are convenient, but they are not correctly 

 used in connection with levers, as their true meanings are confused 

 by so doing. It is better to substitute the word effort for power, and 

 rrxtstnnce or load for weight. It should be borne in mind, that, so 

 far as mechanical principles are concerned, there is no difference between 

 the power and the weight ; both represent forces, and as such they 

 must be considered in the action of levers. 



