Basic question about mass and weight

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Discussion Overview

The discussion centers on the concepts of mass, weight, and the units associated with them, particularly in the context of how home scales measure and display these quantities. Participants explore the implications of using scales in different gravitational fields and the calibration of scales.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about whether the scale shows mass or weight, questioning the appropriateness of the units displayed (kg vs. N).
  • Another participant asserts that the scale measures weight, which is converted to mass using the gravitational force on Earth, suggesting that the displayed units are appropriate under those conditions.
  • A further contribution clarifies that a spring scale measures the displacement of the spring, which correlates to the force exerted, and that the scale is calibrated to reflect mass based on Earth's gravity.
  • One participant provides an example of using a scale on Jupiter, arguing that the scale would inaccurately display mass due to the different gravitational field, emphasizing the importance of understanding the gravitational context when inferring mass from weight.
  • Another participant reiterates the point about the dangers of inferring mass from weight without knowing the gravitational field strength and the calibration of the scale.

Areas of Agreement / Disagreement

Participants generally agree that scales measure weight but display mass, though there is no consensus on the implications of this for different gravitational environments. The discussion remains unresolved regarding the accuracy and reliability of scales in varying gravitational fields.

Contextual Notes

Participants highlight the dependence on gravitational acceleration (g) for accurate conversion between weight and mass, as well as the calibration specifics of the scales, which are not universally applicable.

fluidistic
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I'm confused about mass, weight and kilogram force. Not that I don't know what the 2 firsts are but here I give the example that is perturbating me.
Say a home scale gives my "weight" as 67kg. I know that a weight is a force and therefore should have units of force, like the Newton (N) and not the kilogram which is a unit of mass.
So what is giving me the scale? Is it my mass? Is it my weight but with the wrong units? Or is it my weight in kilogram force (unit is kgf if I recall well)? I'm not even sure that a force can be given in kilogram force, but I guess yes...
 
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It's your weight because it uses gravity to produce the number. And it's also the right units (67 kg = 670 N), at least if we allow particle physicists to use eV as a unit of mass!
 
Just to add to atyy's answer...a weigh scale measures weight. If it operates using a spring, then what is being measured is the displacement of the spring, which is directly proportional to the force pressing down upon it. The scale has simply been *calibrated* to show you what mass (in kg) ON EARTH would be required to provide that force and therefore that displacement. In summary, the scale is displaying the mass of an object having the measured weight ON EARTH.
 
For clarity, here's an example:

If you took your scale to Jupiter, where you weigh FAR more than you do on earth, the weigh scale would show you as having a much higher mass than your actual mass. That is because it would be displaying the mass an object would have to have in order to have such a large weight on earth. The labelling of the tick marks on the scale would be quite literally incorrect (for Jupiter), and this weigh scale would be useless as an instrument for determining mass by measuring weight.

MORAL: It's dangerous to infer mass from weight unless you know how strong the gravitational field you're in is.
 
Ok, so it measures weight but shows mass.
MORAL: It's dangerous to infer mass from weight unless you know how strong the gravitational field you're in is.
And you also need to know how is calibrated the spring into the scale, more precisely you need to know the value of g that it has been assigned to use for the conversion weight-mass.
That mean that if one day I have to play with E=mc^2, m would be what shows the scale.
Thanks to both.
 

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