iv MASS AND ITS MEASUREMENT 49 



in equilibrium. It is found that ivhen the pans are at equal 

 distances from the supports there is equilibrium when the masses 

 are equal. 



EXFT. 52. Repeat the last experiment, but while keeping 

 the pan with the 20 grams in it in the same position, shift the 

 other pan and find how many grams must be put into it to 

 bring about equilibrium. Do this with the movable pan in a 

 variety of positions and show that when the lath is balanced, 



Mass on Distance _ Mass on Distance 

 one side from fulcrum ~~ other side from fulcrum. 



Moments. Another way of expressing the condition of things 

 when there is equilibrium in Expt. 52 is to say that the turning 

 effects of the forces, about the point of suspension, acting on the 

 lath at the points where the pans are supported, are equal. The 

 turning effect of a force is termed the moment of the force. 



Referring to the condition of things in our experiment we can 

 represent it very simply by a diagram (Fig. 21), where F is the 



,B 



M, 



FIG. 21. To Illustrate Moments of Forces. 



point of support of the lath, and M x . is a larger mass at a 

 distance AF in equilibrium with a smaller mass M 2 , at a greater 

 distance FB. 



The force at A is, as we know, the weight of the mass Mj act- 

 ing vertically downwards ; and the force at B is that of the weight 

 of the mass M 2 acting in the same direction. The moment of 

 the force acting vertically downwards at A is a product like that 

 obtained in Expt. 52, viz. the product of the force equal to the 

 weight of M t and the distance AF, which as the diagram shows 

 is measured at right angles to the direction in which the force acts. 

 Similarly the moment of the force equal to the weight of the 

 mass M 2 , about the point A, is equal to the product of this force 

 and the vertical distance BF. 



This is a rule of universal application for taking moments 

 which will have to be used several times later on, and must be 

 very carefully learnt : 



The moment of a force about any point is the product 



