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Mass and Weight

  1. Sep 23, 2012 #1
    I've always had some trouble with these terms and using them interchangeably but I feel like I've finally made the clarification for myself. In the most rudimentary terms, it seems that the mass of something is the amount of "stuff" occupying it, whereas weight is the force of gravity on something (please correct me if I'm wrong on any of these two terms).

    So here's my question, would it be appropriate to state my weight in Newtons?

    For example, I weigh 140 N
  2. jcsd
  3. Sep 23, 2012 #2
    Yes, but why?

    The idea of using weight is that it's what a scale reads. Even though mass is invariant under spatial changes and weight is specific to the gravitational field it's being measured in, it's more practical to use weight because in our everyday lives it's what we measure.
  4. Sep 24, 2012 #3


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    Since gravity is fairly uniform (to within about 0.5 percent) on the surface of the earth, it does not matter much which quantity that you use -- "weight" as a mass or "weight" as a force are roughly identical, up to a constant of proportionality.

    The question of what quantity scales measure is a tricky one. It is made trickier by the fact that most bathroom scales are not accurate enough for a 0.5 percent difference to be significant. You need a more accurate scale for the question to be meaningful.

    For an accurate balance scale it is pretty clear that that the scale measures mass. It does so by using net gravitational force (or the torque induced thereby) as a proxy for mass. The accuracy of the measurement rests on an assumption that the acceleration of gravity is uniform from one side of the scale to the other.

    For an accurate spring scale (or one using electronic load cells), I consider that it is measuring mass as well. The spring scale also uses net gravitational force as a proxy for mass. The accuracy of the measurement rests on an assumption that the acceleration of gravity and the spring constant are uniform between the place and time where the scale is calibrated and the place and time where it is used.

    For legal use in commerce, scales are typically subject to periodic calibration/certification at the place where they will be used. The calibration/certification process involves the use of standard weights whose mass is known. For purposes of commercial labelling in the U.S. the pounds and ounces used to specify "weight" are defined as units of mass.
  5. Sep 24, 2012 #4

    Andy Resnick

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    Yes- Newtons converts to pounds. The Imperial unit of mass is 'slugs'. While it's true that common scales convert pounds to kilograms, this invariably causes confusion when mass/weight is discussed in class.
  6. Sep 24, 2012 #5

    D H

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    This is a common misconception. Newtons convert to pounds-force, not pounds. There are many quantities called pounds in the imperial system with varying units. The pound sterling is a unit of money, the pound-force is a unit of force, and the avoirdupois pound, or just pound for short, is a unit of mass. Without any qualifier, the word "pound" should be interpreted as the avoirdupois pound.

    The slug, while not an official unit, is commonly used in some engineering disciplines that still use English units. Some but not all. It's a bit of a pain in the rear when one group uses pounds, another group uses slugs, yet another uses kilograms, all to describe different aspects of the same object.
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