FORCES 165 



body and the earth is five pounds. It would be neces- 

 sary to pull the body, from on top, with a force of five 

 pounds, to keep it from falling, or we could accomplish 

 the same result by pushing the body from underneath. 

 We have used a balance to find out the weights of bod- 

 ies. In that case we compared the weight of an object 

 with the weight of some standard. That is we com- 

 pared pulls. 



Some bodies move as a whole, that is, if one part is 

 pushed or pulled so that it moves the whole of the mass 

 moves. Other bodies, such as rubber and thin pieces of 

 many kinds of material, may be moved in one part while 

 the rest remains motionless. If we fasten a 

 rubber band at one end we may move the 

 other end a considerable distance without 

 causing the fastened end to move. When the 

 end, which has been moved, is released it 

 will return to its former position. Bodies 

 which act like this are elastic. Thin pieces of 

 wood fastened at one end, may have the othei 

 end bent quite a distance to one side without 

 breaking. The amount of stretching or bend- 

 ing depends upon the pull. If a weight of five 

 grams bends the stick or stretches the rubber 

 one centimeter, ten grams will cause a move- 

 ment of two centimeters. This gives us an- 

 other method of weighing. 



Experiment 69. Weighing by Elasticity. 



Apparatus: Ring stand, spring from an 

 old window roller, or some brass wire No. 

 20; rule, set of weights, spring balance as shown in the 

 illustration, string. 



