166 MECHANICS 



a. Fasten about six inches of the window-roller 

 spring to a ring of the ring stand by means of string and 

 bend the bottom turn of the spring into the form of a 

 hook. Holding the rule against it, hang a ten gram 

 weight upon the spring by means of string. How much 

 is the spring stretched? Hang a twenty gram weight 

 upon the spring and see how much the total stretch is. 

 Repeat with the twenty and the ten together. Does the 

 stretch vary with the pull? Does the spring always return 

 to its first position when the weights are removed? If you 

 cannot obtain a roller spring you may wind enough 

 brass wire upon a round pencil to make a roll 

 about six inches long. 



b. Examine the spring balance. Are the division 

 marks the same distance apart? Is this right? Explain. 

 When you weigh anything on a spring balance what are 

 you really measuring? 



A pull tends to separate the parts of a body. We 

 say we can pull something apart. A push tends to make 

 the parts of a body closer together and we often speak of 

 pushing things together. When we push the parts of a 

 body closer together we say that we compress the body. 

 Compression requires force and the force which is needed 

 increases with the surface of the body. It takes more 

 force to compress a body of three square centimeters of 

 area than a body of one square centimeter of area, in fact 

 it takes three times as much force. Yet the force for each 

 square centimeter is the same in both cases. We call 

 the force per unit of area pressure. The pressure needed 

 to accomplish a certain compression is always the same 

 for the same body, but the total force which is needed 

 increases with the area. 



