MASS, WEIGHT, FORCE, WORK, AND POWER 537 



merely one form of FOBCE. It is now very evident that we do 

 not go to the grocery to purchase a certain weight of sugar or 

 to the meat market to buy a certain weight of meat. What we 

 do wish to buy is a certain mass of sugar or mass of meat. 



630. How Weights and Masses are Determined. The 

 easiest way to determine the mass of an object, however, is to 

 determine its weight. A 1-lb. mass has just 1 Ib. of weight. 

 Knowing this, we see that determining the weight of an object 

 tells us at once its mass. This is not at all new to us when we 

 stop to think of it. We have been used all our lives to seeing 

 masses determined by determining the weights of those 

 objects. We want to purchase a certain mass of meat; the 

 dealer determines the mass by determining the weight of the 

 meat. While it is easily possible to determine the mass of an 

 object without determining its weight at all, ordinary scales 

 and balances simply tell us the weight of the object and it is 

 because we accept the fact 



that a pound-mass weighs 

 just 1 Ib. that we are willing 

 to accept this method of de- 

 termining the mass of our 

 purchases. 



631. Beam Balances. 

 The most common, as well as 



the most accurate, devices FlG 3 35 ._ B ~ am balance . 



for determining the weights 



of objects are the various forms of BEAM BALANCES. A beam 

 balance consists of a rigid beam mounted horizontally upon a 

 sharp, hard support called a KNIFE-EDGE or FULCBUM (Fig. 335). 

 Each end of this beam carries a pan which is also suspended 

 from a "knife-edge/ 7 Great care is taken to eliminate fric- 

 tion. If the two arms of the balance are of exactly equal 

 lengths, a 1-lb. mass upon one scale pan will exactly balance 

 a 1-lb. mass upon the other pan. If the two arms of the beam 

 balance be of unequal lengths, in order to balance each other, 

 the two masses must be inversely proportional to the lengths of 



