Scale? -- mass or weight

  1. When I step on a scale I see that it says....70kg. We typically call this value weight? However, kg is used for "mass" and not "weight". So when we say 70kg do we mean that my mass is 70kg OR do we mean that my weight is 70 N?

    So if it does mean mass, then my ACTUAL weight is 686N!!! lol
    And if it means weight, then my mass is roughly 7kg.

    See where I'm coming from?

    And just to let you guys know, b4 u waste ur time, I def. know the difference between mass and weight. I'm just uncertain about what we are actually measuring.

    I truly appreciate anyones help! thanks
  2. jcsd
  3. pao

    pao 1

    you are measuring your mass!!:biggrin:
  4. Doc Al

    Staff: Mentor

    Since on the Earth's surface weight and mass are proportional, for everyday usage it doesn't matter which units you use. You can say that you "weigh" 70kg or 686 N (not 70 N!). (But in physics, weight is a force, measured in N not kg.)

    What a scale actually measures depends on the type of scale. A typical "bathroom" scale measures force--your "weight". Bring it to the moon and you'll weigh less according to the scale, but your mass is unchanged. But a balance scale compares masses; it will read the same on the moon.
  5. arildno

    arildno 11,265
    Science Advisor
    Homework Helper
    Gold Member

    Well, what the scale actually actually measures is how much a spring within the apparatus has been compressed, or, if a torsional spring has been used, the displacement angle.

    And then that is correlated to the effective weight the person standing upon it must have exerted upon the scale..
  6. D H

    Staff: Mentor

    A spring scale (e.g., the typical bathroom scale) comes close to measuring your weight (within about 0.3% on the Earth). What a spring scale measures is the normal force needed to keep a person from sinking into the floor. This normal force is your apparent weight, not your "actual" weight. The difference: weight is tautologically the force due to gravity while apparent weight is the sum of all forces acting on a body except gravity.
  7. um, if you're saying that it actually measures "weight" as we know it is in physics, then why not say 70N? B/C if it is truly measuring weight then the gravatational pull is already factored saying 686 in that case would be incorrect.

    So if I'm reading this right are you guys saying that my mass IS 7kg?
  8. Doc Al

    Staff: Mentor

    You're the one who said it reads 70 kg in your first post. In everyday usage 70 kg = 686 N (:yuck:). (I thought you said you understood the difference between mass and weight? :wink:)
  9. You have to differentiate between the formal definition in physics and the everyday definition in commerce. The weight of a head of lettuce will be in kg as will the weight of a baby at the pediatrician. In America, both will be in pounds. That's just the system that exists.
  10. you said that you can say that you weigh 70kg or 686N....which doesn't make any sense. What can be said is that one is Mass and the other is weight, respectively. Everyone is saying that the scale measures weight, so therefore it would be more correct to say 70N. I don't get it.

    I'm not trying to be stupid but i'm trying to understand this 100%

    thanks for your patience
  11. ...and what is the formal definition in commerce?
  12. Doc Al

    Staff: Mentor

    As I said in my initial post, what it really measures is force. (Jump up and down and your scale "weight" will change.) What units you use to express that force is arbitrary. When they say your "weight" is 70 kg, that's the same as saying your weight is the same as that of a 70 kg mass--the weight of which is 686 N.
  13. That will be determined in each case by the agency having legal jurisdiction. In interstate commerce, it may be the ICC. In the grocery store, it may be the state bureau of weights and measures. In most cases, these will be traceable back to a standards body such as NIST.

    There is, I think, no understanding available in such matters. Like the difference between aircraft heading and engineering standard angles, it's just a matter of usage.
  14. i'm gonna try one more time to understand this.

    I now understand that it is measuring your force. I see that it's dependant on the gravataional pull. The scale is reading your mass affected by gravity. So therefore, gravity is by default factored in? So that's my problem. I don't see why you would multiply it by 9.8 when gravity was already factored in to give you the reading of 70kg. That's why I was saying that it would make more sense if we would say 70N.

    And I do understand the difference. Mass is absolute....doesn't change. Weight is dependant on it can change.
  15. Doc Al

    Staff: Mentor

    Think of multiplying by 9.8 as doing a units conversion. A mass of 1 kg weighs 9.8 N. So, can refer to that same mass using either units.
    But that's the weight of a 7.1 kg mass. You can't just take the number 70 and change the units from 70 kg to 70 N, since kg and N are already defined to be related by w = mg. (Note that we are talking about "everyday" usage, not technical usage in physics.)
  16. Bathroom scales assume you are under the influence of Earth's gravity at 9.8m/s/s, and calculate your assumed mass off of that assumption. A quick way to fool your scale is to get on an elevator that accelerates upwards and downwards, making it look like you weigh more then less of what you actually do.
    Should your scale read 70kg, it believes you have a mass of 70kg, as you are probably exerting a force of 686N on the scale. If the world were less complicated, your scale would read out in Newtons, or we would say you mass yourself on a scale, instead of weighing yourself. Do keep in mind, though, that pounds are a unit of force, so you can weigh yourself in pounds or newtons, and mass yourself in kilograms (or slugs).
  17. D H

    Staff: Mentor

    The people at NIST know the difference between mass and force. The English system units of mass and force are pounds-mass and pounds-force, respectively. Note the qualifiers. We say pounds in the vernacular. So what? Scientists and engineers who are forced to use English units do use the precise terms (or their abreviations, lbm and lbf).

    Derived units can be used in lieu of the standard ones to enable the use Newton's second law in the form applied by our metric system-using cohorts (i.e., F=ma). With lbm and lbf, one has to use the more generic form F=kma.
  18. that's sort of what I've been trying to get at this whole time. You're saying that 1kg = 9.8N.

    How did we get the 1kg measurement? Obviously it had to come from weighing it on a scale to get the mass...NOT the weight. B/C if the scale gave us our weight then there would be no need to multiply it by 9.8

    I emailed the admin at "" b/c he had an article on weight vs. mass which can be found at

    He said and I quote "The scale estimates your mass based on the force your body exerts on it.

    If the scale measures kg, you can work out how much force your body is
    exerting on it by multiplying by 9.8 (to convert kg into Newtons)."

    According to him the scale measures mass and not weight. This is more consistent than saying that the scale measures weight. B/c if you said that the scale measures weight than you would be multiplying by 9.8 two times (one from the scale figuring out your weight and the other from multiplying it by 9.8).

    So I think by weight, you REALLy mean mass.
  19. D H

    Staff: Mentor

    Dr. Al is most certainly not saying 1kg = 9.8N. Your bathroom scale measures force, not mass. Pick the scale up and give it a good squeeze with your hands. It will register some value.

    If you want to be real picky, it isn't even measuring force. As Arildno said earlier, it is actually measuring the displacement of a spring. The spring has a known spring constant, so the displacement of the spring varies linearly with the force that caused the displacement. The normal force that keeps a person standing on the scale from sinking into the scale is proportional to the person's mass; the constant of proportionality is the Earth's gravitational acceleration. Your scale automatically converts the spring displacement (units=length) to mass via the known spring constant and the known gravitational acceleration.
    Last edited: Feb 17, 2008
  20. Doc Al

    Staff: Mentor

    We're making a mountain out of some trivial semantics. When I say weight, I mean weight: the force of gravity.

    He's just saying it "measures" mass because the units it's marked in are mass units! :rolleyes: Cross them out and write the units in Newtons. Now it measures weight!

    Again, what the scale actually does is measure the force you exert on it. We use it to measure our mass or weight--since they are proportional, you can choose which units you'd like your answer in. The same scale can be marked to show both units at the same time, if you like. (I've seen scales that give readings in both pounds and kilograms.)

    As far as weight goes, it's most accurate to say the scale measures "apparent" weight (as D H mentioned). If you jump up and down on the scale, your apparent weight changes. Obviously, the scale is not directly measuring your mass since your mass doesn't change. (Of course we can use it to measure our mass, again, because weight and mass are proportional.)

    I would say that it's more accurate to say the scale measures weight, not mass--since I can take it to the moon and it will show your weight (in whatever units the scale is marked with) on the moon. If it uses kg, then your weight will no longer show as 70 kg, but only about 11.6 kg. Do you really think your mass has changed?

    You might want to read this: Weighing Scale
  21. I emailed a professor of Physics @ a University in Ohio and he had this to say:

    "If your bathroom scale gives a readout in kg, then it is telling you your mass. If you want your weight in newtons you have to multiply by 9.8 m/s^2. If your bathroom scale gives a readout in newtons, it is telling you your weight. If you want to know your mass in kg you have to divide by 9.8. Assuming the scale is manufactured correctly and calibrated correctly it can read mass in kg or slugs, and it can read weight in newtons or pounds. If it is working correctly, you should take the reading exactly as it says. If it says 70 kg then your mass is 70 kg and your weight is 686 N."

    That makes complete sense to me! However, everything else I've read on here just sounds completely contradictory. I'm just gonna stick w/ what the professor says b/c it makes more sense to me.
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