# B Let's change the definition of 'weight'

1. Oct 7, 2016

### dbc

Since 97% of everyday weight scales (both in doctor offices and at home) measure our actual mass either in lbm and/or kg, and NOT force (lbf or N), then why does oxford choose to define weight to be relative?

"a body's relative mass or the quantity of matter contained by it, giving rise to a downward force; the heaviness of a person or thing."

Its not relative at all. It is an estimated absolute unchangeable value. My at home scale defines my "weight" to be 150 lbs (which is actually lbm not lbf although they dont say it). It takes into account and factors out earth gravity but still 150 lbm is my mass and if I were on the moon I would still be 150 lbm. So... its not relative at all.

2. Oct 7, 2016

### Staff: Mentor

In physics, "weight" generally refers to the force of gravity exerted by something (such as a planet) on an object.

3. Oct 7, 2016

### Krylov

Weight is a force, unlike mass. Of course you can opt to change the definition of "weight" to be more in line with everyday language use, but I don't think in this case that is a good idea. In fact, I think it is really important to keep weight and mass apart, at least when discussing physics.

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4. Oct 7, 2016

### Staff: Mentor

Actually, the typical "bathroom scale" measures the force pushing down on it, even though it can be labeled in mass units.

5. Oct 7, 2016

### ZapperZ

Staff Emeritus
Is this really a physics question, or a "how-things-are-in-use-everyday" question?

There are NO ambiguities in the definition of "weights", "mass", etc in physics. So what are you asking us to change? The US still can't get everyone to agree to use MKS/SI, unlike most parts of the world already!

Zz.

6. Oct 7, 2016

### Staff: Mentor

If you were on the moon, your scale would show a different weight.

If you could somehow lug your scale to the top of Mt. Everest, the reading would be different there, as well, although the scale might not be precise enough to show the difference.

7. Oct 7, 2016

### jbriggs444

In commerce, the word "weight" generally has an operational definition that corresponds to the mass of the goods being exchanged.

Which is to say that OP's wish has already been granted!

8. Oct 7, 2016

### CWatters

Thats not correct. They effectively measure weight not mass. Eg they measure the extension of a spring. Then they assume a value for g and attempt to display your mass.

They don't always do a good job of converting weight to mass. If you want accurate scales they have to be calibrated depending on where on the planet they will be used. Partly because the earth isn't spherical and partly because it rotating.

If they really did measure mass then they wouldn't need calibration as your mass is constant regardless of where you measure it on earth.

9. Oct 7, 2016

### ZapperZ

Staff Emeritus
This may depend on the equipment being used. If they used those "balance scale" type equipment, where they shift the position of the weights till it balances out, then they do measure your mass directly without having to assume the value of "g".

I actually had a conversation with my doctor on this (he knows that I'm a physicist and we always talk about medical stuff related to physics). They have an electronic scale at the office, and he said that the SOP for the office is that the scale is calibrated by a standard, known mass "every few months". He told me this when I offered the slim possibility that my gain in weight since my last visit might be due to an error in his weighing scale (it wasn't).

I still don't see the issue that the OP is having with this.

Zz.

10. Oct 8, 2016

### dbc

This is absolutely incorrect sir and all this does is confuse the heck out of someone when you say it. It may use the force pushing down on it by procedure but it is not what is ultimately measured. The mass is measured and correctly labeled in kg or lb (lbm).

Oxford defines measure as:

"Ascertain the size, amount, or degree of (something) by using an instrument or device marked in standard units."

It defines measure as the quantity given as the end result of the procedure not during.
Therefore a bathroom scale measures the mass not the force although the force is used to calculate it.

It is pointless and ultimately distracting when someone says a scale measures the force. It doesn't measure the force at all. It uses force, which is the correct statement.

11. Oct 8, 2016

### dbc

This is absolutely incorrect sir and all this does is confuse the heck out of someone when you say it. It may use the force pushing down on it by procedure but it is not what is ultimately measured. The mass is measured and correctly labeled in kg or lb (lbm).

Oxford defines measure as:

"Ascertain the size, amount, or degree of (something) by using an instrument or device marked in standard units."

It defines measure as the quantity given as the end result of the procedure not during.
Therefore a bathroom scale measures the mass not the force although the force is used to calculate it.

It is pointless and ultimately distracting when someone says a scale measures the force. It doesn't measure the force at all. It uses force, which is the correct statement.

The reason I bring this up is because when I was in school learning all this someone said that and led the class to believe that kg is actually the force you're putting on the scale! This was and is completely wrong and should never be said to a physics student ever.

12. Oct 8, 2016

### Staff: Mentor

You seem determined to make things as confusing as possible.
The point is that the bathroom scale responds to force, not mass. Bring the bathroom scale to the moon, and it will read less than on earth, yet mass has not changed.

13. Oct 8, 2016

### Staff: Mentor

Just because you had a confused instructor is no reason to redefine things that are perfectly well-defined as they are. Weight is a force! (The everyday use of the terms weight and mass should not drive the definitions that physicists and physics books use.)

14. Oct 8, 2016

### ZapperZ

Staff Emeritus
Bring that scale with you next time you get into an elevator. Pay attention to the reading on the scale when the elevator starts, and when it is about to stop. Do you think the reading will change? Did your mass change during those times? If your answer is yes/no, then what exactly is that scale measuring?

It is you who is confused as to what a typical weighing scale like that is DIRECTLY measuring. You don't have to believe what is being said here. You can actually TEST this yourself!

Zz.

15. Oct 8, 2016

### dbc

I said 150 lbm is my mass and if I were on the moon it would STILL BE my mass (150 lbm). Is that confusing?

Actually sir, I contend your statement is confusing and if I didnt know anything about physics and you said ....

.... I would take that to mean - by reading it literally as you wrote it - that the scale is labeling the force in mass units. This is WRONG! It is estimating the mass by the spring tension.

16. Oct 8, 2016

### dbc

If it was labelling force in mass units it would show "667 kg" (would correctly be labelled Newtons) instead of 68 kg.

17. Oct 8, 2016

### ZapperZ

Staff Emeritus
You don't seem to be getting it.

The weighing scale measures FORCE, i.e. "weight" as defined in mechanics, i.e. mg. However, because it has been CALIBRATED so that the value of "g" is taken care of, it can DEDUCE the value of the mass, because "g" is a constant for every single person who steps on the weighing scale! That is the reason why it gives you your "mass" in kg!

However, if you take it elsewhere with a different local value of "g", or if you bring this onto an elevator that is accelerating upwards or downwards, then the value of the "mass" that it reads is no longer accurate, because the value of "g" that it was calibrated for previously is no longer valid!

Any of this getting through to you?

Zz.

18. Oct 8, 2016

### dbc

I COMPLETELY understand that. I also understand that if you want the most accurate measurement of your mass then you need to stand on it in the factory where it was calibrated since gravity's force is not constant.

My replies in this thread has changed the subject to me taking a stand against people misrepresenting what a scale attempts to display which is mass NOT force although some have said, and not only in this thread, that a scale is displaying force and incorrectly labelling it in mass units - which is completely wrong.

19. Oct 8, 2016

### ZapperZ

Staff Emeritus
WHAT?

A scale measures the NORMAL FORCE acting on the object being measured! This gives you the APPARENT WEIGHT. It changes if you accelerate upwards or downwards! Your mass doesn't change!

So why would you claim that an instrument that directly measures a quantity x, can produce different values of x under different circumstances, and yet, x remains constant? This makes no logical sense!

So the scale does NOT directly measure mass. It measures the normal force! It is only via calibration that you get the mass!

Why is this so difficult?

Zz.

20. Oct 8, 2016

### dbc

Here is another perfect example of what I am contending here.

In this thread the guy is trying to understand what a scale is measuring and as usual Doc Al comes in and throws the following wrench in and confuses EVERYONE including me and has me second guessing my understanding.

Doc Al: ...... A typical "bathroom" scale measures force--your "weight". .........

Then I think.... wait that cant be right because it displays kg not N. Thats because its NOT right and the scale IS displaying the estimated mass.

21. Oct 8, 2016

### ZapperZ

Staff Emeritus
You are FIXATED on the units it displays and are somehow blind to the MECHANICS of how it is able to display such a unit. Somehow, explaining to you how a weighing scale is able to DEDUCE the mass that is being measured has not sunk in.

Several threads here have tried to get through to you on how a weighing scale is able to give you the value of a "mass", and even why that value may not be accurate under different conditions. If you are unable to comprehend something as simple as that, there's nothing else that can be done.

Zz.

22. Oct 8, 2016

### dbc

Like I said. Oxford and I and many others understand the word 'measure' to be the end result of a measurement procedure.

Oxford

"ascertain the size, amount, or degree of (something) by using an instrument or device marked in standard units or by comparing it with an object of known size."

Therefore I ask you Zz, what is displaying on the scale as the end result.... the FORCE or the MASS. And like you just said.... "you get the mass!"
Therefore since the "mass" is the end result .... THE MASS is what is MEASURED by the SCALE.

23. Oct 8, 2016

### ZapperZ

Staff Emeritus
24. Oct 8, 2016

### dbc

I already understood all that coming into this. So why not say "a scale measures mass by using the force applied on it", instead of it saying it "measures mass by measuring force" (very confusing statement).

25. Oct 8, 2016

### Staff: Mentor

I think you're the only one confused here. This is standard stuff.

I don't care what the scale is labeled, it obviously only knows about the force exerted on it. It only displays mass if you assume (quite naturally) that it is used on the earth. It doesn't know anything about mass directly.