A spring scale is a weighing scale used to measure force, such as the force of gravity, exerted on a mass or the force of a person's grip or the force exerted by a towing vehicle. This force is commonly measured in newtons, for which one newton corresponds to the weight of about a tenth of a kilogram of mass, or about the weight of an apple. The spring scale can also be correctly called a Newton Meter, because it is used to measure newtons.
The spring scale apparatus is simply a spring fixed at one end with a hook to attach an object at the other. It works by Hooke's Law, which states that the force needed to extend a spring is proportional to the distance that spring is extended from its rest position. Therefore the scale markings on the spring scale are equally spaced.
Spring scales can be calibrated for the accurate measurement of mass in the location in which they are used, but many spring scales are marked right on their face "Not Legal for Trade" or words of similar import, due to the approximate nature of the theory used to mark the scale. Also, the spring in the scale can permanently stretch with repeated use.
If two spring scales are hung one below the other, the top will read the weight of the body hung on the lower scale, plus the weight of the lower scale itself.
Spring scales come in different sizes. Generally, small scales that measure newtons will have a less firm spring (one with a smaller spring constant) than larger ones that measure tens, hundreds or thousands of newtons.
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