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Does a scale *display* weight or mass?

  1. Mar 1, 2015 #1
    I know that scales measure weight (a force) via displacement of a spring. I also know the difference between a force and a mass. I know that, on the moon, the numbers on a scale might be different (depending on the answer to this question). I've read through the few threads on this matter here, and there isn't a clear answer to my question:

    What are the numbers that a bathroom scale actually displays? Are the numbers displayed a force, or does the scale implicitly assume it is on Earth, account for this—and thus display a mass?

  2. jcsd
  3. Mar 1, 2015 #2


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    There's been quite a few threads on this subject over the years
    have a look down at the bottom of your thread page and you will see some of the similarly titled threads
    have a look through and see if they answer your questions .... if there is still anything unclear
    then ask specific questions :smile:

  4. Mar 1, 2015 #3
    The scale measures force, that is weight. But instead of displaying force or weight, the numbers you see are weight divided by a factor, the downwards accelaration on the earth's surface, so as to display mass.

    If you used such a scale on the moon, the values would be incorrect, although you could easily make the correction by simply multiplying the correct factor.
  5. Mar 1, 2015 #4


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    Is there no unit displayed or written on your scale?
  6. Mar 2, 2015 #5


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    Most scales are calibrated for one value of g. So if you put a known mass on a scale it will read slightly differently at different places on the earth (eg near a mountain where g is very slightly higher or on the moon where it's lower). That demonstrates they are measuring and displaying weight not mass.

    If you wanted them to measure weight but display mass they would have to automatically correct for variations in g.

    Edit; You could make one that measured and displayed mass - for example by counting atoms or perhaps by using a horizontal spring and vibrating the mass.
    Last edited: Mar 2, 2015
  7. Mar 2, 2015 #6
    Scales assume a local g level of 9.80665 (m/s)/s, the result is mass (kg) and weight (kgf) provided your at the earths surface radius.
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