FORCE AND MOTION 



63 



of apparatus or machine called a balance, and the process of 

 finding the weight is really an experiment with gravity as a 

 force. 



64. Weighing. There are two common forms of ma- 

 chines for weighing: the spring balance and the equal-arm 

 balance. In the spring balance, the body to be weighed is 

 hung on a hook attached to a spiral spring. This causes the 

 spring to extend, more or less, according 



to its stiffness and the weight of the 

 body. By means of a scale near the 

 spring, we can learn the pull which 

 the earth exerts. We call this weigh- 

 ing; it is measuring the force with 

 which gravity acts upon the body. 

 Therefore weight is the measure of the 

 earth's attraction for a body. 



Another method of weighing is by 

 the use of some form of an equal-arm 

 balance. A horizontal bar is supported 

 at the middle in such a way that it is 

 free to swing up or down at either end. 

 The bar will be horizontal if bodies 

 of equal weight are placed on the pan 

 or platform attached to each end. To 

 weigh a body, we place it on one pan 

 or platform, and on the other we place 

 certain bodies of known weight that 

 bring the bar to a horizontal position. 

 (LABORATORY MANUAL, Exercise V.) 



65. Density. One of the important 

 properties of a substance is its density. 

 The term density has an exact mean- 

 ing. We say that the weight of a body is the amount of 

 matter which, however large or small, it contains. Density 

 is the amount of matter in a unit of volume. For example, 



FIG. 25. THE SPRING 

 BALANCE 



1. JVhen a body is 

 suspended from the 

 hook, the spiral wire to 

 which it is attached is 

 partially straightened. 

 What straightens it? 



2. When the body is 

 removed, what property 

 of the wire is shown? 



3. Would steel or lead 

 be the better wire for a 

 balance? Why? 



