LAWS OF MOTION 



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The force of a spring is not exactly uniform in its action, 

 for it has its greatest energy when most bent or most tightly 

 wound. Since the elastic force of a spring is not 

 a fleeted by the force of gravitation, it is used to 

 ascertain the amount of the earth's attraction 

 (pul! or weight) in various places. This is done 

 by the use of a cylindrical spring balance to which 

 a hook or ring is fastened (Fig. 41). The object 

 to be weighed is hung from the hook which pulls 

 the spring in proportion to the weight. From 

 graduations on the scale it is possible to read 

 directly the weight of the commodity. 



65. Weights as a Source of Power. Weights 

 are used as a source of energy when uniform 

 pressure or action is desired. The proper tension 

 is maintained on a rope by means of a weight 

 suspended on a movable pulley. There are 

 many applications of weights as a motive force, 

 but when they are used, the action is comparatively slow. 



FIG. 41. 



Spring 



Balance. 



They are sometimes employed as the motive force for large clocks, 

 such as those installed in towers. 



A clock or watch contains three important pieces of mechanism 

 or elements: (1) the source of energy to move the parts, which is 

 a suspended weight in large clocks or a spring in small clocks and 

 in watches; (2) the series of wheels, called a train of wheels, or 

 gears, operated by the driving force; and (3) a device for control- 

 ling the movement of the train of gears. 



66. Accumulated Energy. We know that energy tends 



to accumulate in our muscles while at rest and that it can 



then be expended either gradually or by one effort, but to no 



greater extent than the reserve force that has been accumu- 



s 



