Mass vs Mass as a Force (Weight)

[Mister T]

would it not?

No, it would not. The weight of an object does not include any effect due to buoyancy. The comparison is assumed by definition to be made in a vacuum.

 

[cmb]

No, it would not. The weight of an object does not include any effect due to buoyancy. The comparison is assumed by definition to be made in a vacuum.

I would have to disagree with you then, because your reply does not mention what is actually 'weighed'.

If you put an object on scales consisting of a 2kg mass suspended from a 500g 1m^3 helium balloon, then it'd weight about 1 kg.

If we break down why that is, the presence of helium in an enclosed envelope displaces the air above its lower surface. Now consider the total mass of material bearing down from above the lower surface to those atmospheric loads from below.

The balloon has displaced a volume of air that means the atmospheric pressure from above is now less than that from below, and if you measure that on weight scales you'll find it'll be less, or even negative (floats off).

Same with any other mass, but obviously considerably less noticeable for solid masses because the masses involved are a lot more and the displaced air is a lot less.

Let's consider a 1kg 1 litre mass sitting on scales. Let's assume gravity is a perfect 10m^2/s. The mass's force on the scales due to gravity is 10N.

But the volume of the mass has displaced 1 litre of air above its lower surface. Therefore, there is 1 more litre of air bearing on its underside to its top, being 0.01N more from the underside directed upwards.

If we say 1 litre of air is 1 gramme, so a 1 litre 1 kg mass will actually 'weight' 9.99N. A 2 litre 1kg mass will 'weight' 9.98N.. etc..

For a 1000 litre 1kg mass, it'd 'weigh' nothing because it'd then be 1 gramme per litre deducted from the 1kg mass, and it'd be neutrally buoyant.

So, when you get on those bathroom scales, you have to add about one gramme for every litre of your volume, to translate a 'weight' read-out (in kilogrammes) into mass. No adjustment is required for lbs because that sort of 'weight' reading also includes buoyancy.

If you add 1 gramme per litre to the balloon example above you'll get the right answer for the mass (if all those numbers were precise).

 

[Dale]

I am with @Mister T on this. The weight is the force due to gravity. If buoyancy effects are present then a scale will not measure the weight. If your mass is 105 kg then you do weigh the same as a helium balloon with a mass of 105 kg.

 

[cmb]

I am with @Mister T on this. The weight is the force due to gravity. If buoyancy effects are present then a scale will not measure the weight. If your mass is 105 kg then you do weigh the same as a helium balloon with a mass of 105 kg.

Therein lies the whole discussion point of the thread.

I take 'weight' to mean, self-evidently, one's weight measured on scales at the Earth's surface in ambient STP conditions, or

.. it isn't.

If not, please define how and when weight can be measured, because it sounds like it'll be nothing like any lay person understands to be 'weight'.

Medical person; "What is your weight?"
Mister T; "Sorry, I have already explained in public discussion that weight is measured in a vacuum and I have never measured that in a vacuum."
Medical person; "Then I have to record your weight"
Mister T; "No way! I am not going to stand in a vacuum vessel just for that!".

Do you see why I think this is fundamentally part of the thread discussion?

My Chambers dictionary of Science and Technology says;
"Weight; (phys) The gravitational force acting on a body at the Earth's surface"

The ambiguity in that definition is whether it is the gravitational force for 'only' that body, or all sources of gravitational force (that therefore includes the immersive fluid it is in).

I would say the clue there is 'at the Earth's surface' and there is no such point on the Earth's surface that is in a vacuum.

 

[pbuk]

If you put an object on scales consisting of a 2kg mass suspended from a 500g 1m^3 helium balloon, then it'd weight about 1 kg.
...
If we say 1 litre of air is 1 gramme, so a 1 litre 1 kg mass will actually 'weight' 9.99N. A 2 litre 1kg mass will 'weight' 9.98N.. etc..

The word weight in these sentences is not correct; it should be weigh (it'd weigh about 1 kg...will weigh 9.98N).

For a 1000 litre 1kg mass, it'd 'weigh' nothing because it'd then be 1 gramme per litre deducted from the 1kg mass, and it'd be neutrally buoyant.

You got the grammar right this time, but you are confusing three things:

• what a scale is designed to measure (which is mass)
• the mechanism any particular calibrated scale employs to measure mass (which is generally by comparison of the force exerted by a known mass), and any corrections which which may need to be made to the measured mass e.g. correcting for buoyancy using the difference between the densities of the measured mass and the calibrating mass
• the meaning of the weight of an object, which depends on the context e.g. 'an object in orbit is weightless'; 'you weigh less under water', but always refers to a force and is never therefore measured in kg.

So, when you get on those bathroom scales, you have to add about one gramme for every litre of your volume, to translate a 'weight' read-out (in kilogrammes) into mass. No adjustment is required for lbs because that sort of 'weight' reading also includes buoyancy.

How could this possibly be true? Do my bathroom scales magically acquire knowledge of the density of the object that is placed on them when I switch them from kg to lbs?

 

[Motore]

In science and engineering, the weight of an object is the force acting on the object due to gravity.

Weight = Mass x Acceleration due to gravity.
Example: Weight of a 100 kg (person, helium ballon) approx equals to 100 kg x 10 m/s² = 1000 N

Of course in everyday speak when I ask someone how much the weigh I mean how much mass do they have, but otherwise for me it's a pretty straight forward definition.

P.S.: I don't know anything about pound mass, pound force, pound weight, lbs, feets etc.

 

[Dale]

"Weight; (phys) The gravitational force acting on a body at the Earth's surface"

The ambiguity in that definition is whether it is the gravitational force for 'only' that body

There is no ambiguity. Buoyancy is not a gravitational force.

What you are talking about is called the “apparent weight”

 

[cmb]

Either this is a confusion of definitions, or of understanding.

Let me say this, then;-
If you have two objects each of 1kg mass, the first occupying a volume of 1 litre (at 1g/cc density) and a second 1kg mass occupying a volume of 2 litres (at 0.5g/cc density), then, when measured with the same balance or strain gauge type scales on a laboratory desk in STP atmospheric conditions, the larger one will return a 'weight measured' by the same scales that is about 1 gramme less than the other.

Do we all understand this is the case, or disagree about that?

 

[pbuk]

Do we all understand this is the case, or disagree about that?

I disagree with the words 'weight measured' because scales are used for measuring mass. But yes, before correction for buoyancy there will be a difference of about 1.2g between the measurements.

 

[pbuk]

Either this is a confusion of definitions, or of understanding.

It appears to be a confusion of definitions, or rather a confusion that the word 'weight' only has one definition regardless of context.

In physics, the definition "Weight; (phys) The gravitational force acting on a body at the Earth's surface" can be a good one. However in everyday speach, the word 'weight' is usually synonymous with 'mass'.

 

[cmb]

It appears to be a confusion of definitions, or rather a confusion that the word 'weight' only has one definition regardless of context.

In physics, the definition "Weight; (phys) The gravitational force acting on a body at the Earth's surface" can be a good one. However in everyday speach, the word 'weight' is usually synonymous with 'mass'.

OK. But it'd be better if words didn't have more than one meaning, else there can be a disagreement over the applicable context.

I disagree with the words 'weight measured' because scales are used for measuring mass. But yes, before correction for buoyancy there will be a difference of about 1.2g between the measurements.

OK, but it's clearly not measuring 'mass' then, if you can put two like masses on it and get different answers.

Hey, maybe it is just me, with a ridiculous expectation that words can be used to mean something specific and non-ambiguous? Scales that measure 'mass', but read differently for two objects with the same mass? OK, so long as we agree that is the outcome, the rest is 'definition' which does not seem well resolved to me and a source of potential disagreements. I'll leave it there.

 

[pbuk]

OK. But it'd be better if words didn't have more than one meaning, else there can be a disagreement over the applicable context.

Perhaps, but there is not a lot you or I can do to change the fact that at least 1 billion people say 'weight' when they mean 'mass'.

OK, but it's clearly not measuring 'mass' then, if you can put two like masses on it and get different answers.

No, but it's the best we've got. Can you suggest a method of directly measuring mass?

Hey, maybe it is just me, with a ridiculous expectation that words can be used to mean something specific and non-ambiguous? Scales that measure 'mass', but read differently for two objects with the same mass?

Yes these are ridiculous expectations. The rest of the world is quite happy with using context to resolve any difficulties; for instance the bouyancy correction is important in studying atmospheric pollution, see https://doi.org/10.1080/00022470.1980.10465130.

 

[Dale]

the larger one will return a 'weight measured' by the same scales that is about 1 gramme less than the other.

Do we all understand this is the case, or disagree about that?

We all (or at least I) agree. The standard scientific term for your “weight measured” is “apparent weight”.

See: https://en.m.wikipedia.org/wiki/Apparent_weight

 

[Mister T]

Either this is a confusion of definitions, or of understanding.

It's a confusion in your understanding. It is common to specify procedures done "in a vacuum". It's quite easy to pull a good vacuum, and has been for over 100 years. I don't need to place myself in a vacuum chamber to determine the buoyant force exerted on me by the air. All I have to do is measure my volume, take another object with the same volume, and place it in a vacuum chamber. Or use the density of air and my volume to calculate it. When I go to the doctor and they weigh me, the slight error in the reading due to buoyancy is small enough that they ignore it.

When we speak of the weight of an object, we don't specify the object because it's understood that it's the object of concern. For example, if I speak of the weight of my desk, it's understood that it's my desk that I'm referring to, and doesn't include any other objects that it may or may not be attached to.

And by the way, the pound used in the United States is defined, by law, to be exactly 0.453 592 37 kg.

 

[Mister T]

Perhaps, but there is not a lot you or I can do to change the fact that at least 1 billion people say 'weight' when they mean 'mass'.

Actually, if you are engaged in commerce you are obliged by law to use the word weight to refer to what physicists call mass.

 

[cmb]

I just want to add one point, which seems to make this forum quite a hostile place for me (and others like me, I have noticed).

It's a confusion in your understanding. It is common ...

'Common' ... ergo, frequent but not always ...

As an autistic person with a great deal of difficulty in understanding the ambiguities that people like to lever their arguments on, I have a tough time accepting this sort of outright subjectivity as being 'scientific'.

I'm just adding this comment because there really should be some consideration and reasonable adaptions made here for people with (as society labels it) 'communication difficulties'. (though it seems from my POV to be communication difficulties on behalf of others)

It is not a confusion on my behalf, but a confusion foisted on me by society that expects me to understand different things from the same word given (or more often as in what you are saying, NOT given) in different circumstances.

 

[jbriggs444]

'Common' ... ergo, frequent but not always ...

You quoted @Mister T as saying:

It's a confusion in your understanding. It is common ...

But what @Mister T actually said was:

It's a confusion in your understanding. It is common to specify procedures done "in a vacuum".

While technically correct, the quote you provided was misleading. It suggests that @Mister T was saying that your confusion was common rather than that specifying procedures be done in a vacuum is common. That destroys the meaning of the quotation.

As an autistic person with a great deal of difficulty in understanding the ambiguities that people like to lever their arguments on, I have a tough time accepting this sort of outright subjectivity as being 'scientific'.

It does not need to be scientific. Science is scientific. Diatribes about the language that we use to talk about science are not.

It is not a confusion on my behalf, but a confusion foisted on me by society that expects me to understand different things from the same word given (or more often as in what you are saying, NOT given) in different circumstances.

You seem to want to distinguish between a confusion that is your fault and a confusion that is the fault of some one or some thing else. But such a distinction is unimportant. What matters is identifying the confusion, addressing and working to eliminate it.

Assignment of blame is for managers and lawyers. Not for scientists and engineers.

Yes, it is less challenging for one's self-image to point to society at large and say "My confusion is their fault, that is where we should look for a solution". But that just keeps the self-image polished. It does nothing to alleviate the confusion.

 

[cmb]

It does not need to be scientific. Science is scientific. Diatribes about the language that we use to talk about science are not.

You seem to want to distinguish between a confusion that is your fault and a confusion that is the fault of some one or some thing else. But such a distinction is unimportant. What matters is identifying the confusion, addressing and working to eliminate it.

Assignment of blame is for managers and lawyers. Not for scientists and engineers.

Yes, it is less challenging for one's self-image to point to society at large and say "My confusion is their fault, that is where we should look for a solution". But that just keeps the self-image polished. It does nothing to alleviate the confusion.

This is precisely 'not making allowances' for how someone communicates.

OK, then , science is scientific.. Please go ahead and give me the scientific definition of weight, with proper refences (requesting references to claims and definitions usually being the MO here for things the moderators are unsure about).

With that objective, referenced definition of 'weight' (which will surely have to unambiguously include all this stuff about 'in a vacuum' etc) and I'll be on my way, question duly answered.

 

[jbriggs444]

This is precisely 'not making allowances' for how someone communicates.

OK, then , science is scientific.. Give me the scientific definition of weight, with proper refences (references to claims usually being asked here for things the moderators are unsure about).

With that objective, referenced definition of 'weight' (which will have to unambiguously include all this stuff about 'in a vacuum' etc) and I'll be on my way, question duly answered.

That is not how the world works. One can look up "weight" in a dictionary and find multiple meanings. The word is ambiguous. If a writer wishes to nail down a word so that the usage is unambiguous in context then the writer can state a definition. If a reader wishes to understand the meaning then he can look for such a local definition, look for contextual clues or make a guess at which dictionary definition applies.

If you want to use "weight" to mean "effective weight" then say so. We can all go home happy.

If you want to use "weight" to mean "gravitational force" then say so. We can all go home happy. This is a meaning that is good enough for the physics classroom but not for real life. I'll happily adopt it for a particular course if the textbook asks me to.

If you want to use "weight" to mean "effective gravitational force in the lab frame" then say so. We can all go home happy. Some textbooks use this definition. I prefer it over the gravitational force one. This meaning makes the term "weightless" apt.

If you want to use "weight" to mean "mass" then say so. We can all go home happy. This is the nominal meaning for commerce in the U.S.

If you want to use "weight" to mean "mass measurement corrected for local gravity but not for atmospheric buoyancy" then that's fine too. This is the usual operational definition for commerce and the doctor's office, at least in the U.S.

 

[Dale]

Please go ahead and give me the scientific definition of weight, with proper refences (requesting references to claims and definitions usually being the MO here for things the moderators are unsure about).

You yourself cited it earlier, but for common definitions Wikipedia is a good reference:

“In science and engineering, the weight of an object is the force acting on the object due to gravity.”

https://en.m.wikipedia.org/wiki/Weight

The same article goes on to explain “Several definitions exist for weight, not all of which are equivalent.”

It is a fact that most words have multiple meanings and that this can lead to confusion. Unfortunately, the human voice can make only a small number of sounds, and most languages use only a small subset of those sounds. The number of concepts that we would like to assign brief sounds to far exceeds the number of brief sounds available. This problem is therefore essentially unavoidable.

I am sorry to hear that it is a particular challenge for you individually. Unfortunately, I think that a solution is fundamentally impossible. We are constrained by biology and further constrained by existing languages.

The word weight has multiple meanings, but for clear communication the best meaning in science is the force due to gravity. You are not wrong to use one of the other meanings, but then you are putting yourself in a situation where a word is used with multiple meanings.

 

[Richard R Richard]

No es así como funciona el mundo. Uno puede buscar "peso" en un diccionario y encontrar múltiples significados. La palabra es ambigua. Si un escritor desea precisar una palabra para que el uso no sea ambiguo en el contexto, entonces el escritor puede establecer una definición. Si un lector desea comprender el significado, entonces puede buscar una definición local, buscar pistas contextuales o adivinar qué definición de diccionario se aplica.

Si desea utilizar "peso" para referirse a "peso efectivo", dígalo. Todos podemos irnos felices a casa.

Si desea utilizar "peso" para referirse a "fuerza gravitacional", dígalo. Todos podemos irnos felices a casa. Este es un significado que es suficientemente bueno para el aula de física, pero no para la vida real. Con mucho gusto lo adoptaré para un curso en particular si el libro de texto me lo pide.

Si desea utilizar "peso" para referirse a "fuerza gravitacional efectiva en el marco del laboratorio", dígalo. Todos podemos irnos felices a casa. Algunos libros de texto utilizan esta definición. Lo prefiero a la fuerza gravitacional. Este significado hace que el término "ingrávido" sea apropiado.

Si desea utilizar "peso" para significar "masa", dígalo. Todos podemos irnos felices a casa. Este es el significado nominal del comercio en EE. UU.

Si desea utilizar "peso" para referirse a "medición de masa corregida por la gravedad local pero no por la flotabilidad atmosférica", también está bien. Esta es la definición operativa habitual para el comercio y el consultorio médico, al menos en los EE. UU.

Hi all

It is rare to read that, if we are in a science forum we answer as in a science forum, if this were a trade forum, perhaps our definitions would be more lax.

I think that no one can go home happily if they provide five different definitions, saying that it does not matter what they choose and that will be "weight" ... The underlying reasons for the question of why they do it are understood, but they do not have to give us a all the same.
Science defines weight in one way, but when it wants to apply it in everyday life, it presents deviations of the measure, which have nothing to do with "weight", deviations such as the hydrostatic pressure of the air and the rotation of the planet on its Its own axis as a function of latitude are topics that can be discussed and clearly defined, as a result we have that the scales always indicate the value of the normal force of contact with the body that is being measured, this is known as apparent weight.

But the US has nothing to do with how science does science. The US has very good technology but it is 200 years behind by not joining the international system of units. I do not know which idol will fall on the road , or perhaps any other geopolitical reason of the day. The same definition of pound that they use is already expressed in kg, the same "IS" units, so there is nothing logical to wait to join and share units with the rest of the world ... What the US believes to be mass is just that, what the US believes. The rest of us already know what mass and weight are and how to measure them. As I believe that here in the political forum there is no debate, I do not know why it has brought that issue to the thread. Here I leave it.

As commercial scales or scales can only measure weight, it is logical that the law requires that the items be sold by weight, and not by mass, at some point when the moon or Mars is traded perhaps, it is convenient to change that law.

This thread originates, someone wonders why the standards for weight have a unit of measurement in unit of mass.
A logical historical account not faithful to the events, could be made by explaining how science related the atomic number and the atomic mass (measured by comparisons between elements), and tried to establish a relationship between those numerical values and the macroscopic experimental ones, using as a factor conversion of Avogadro's number. But even so, with the definition of meter at that time as the unit of measurement and the second for time, the acceleration of gravity did not result in 1 unit but in 9.81 units, so the force necessary to accelerate a mass of one kg at $1 m/s^2$ it was not equivalent to the same force as to support its weight. That was the moment to decide whether the units of weight were related to the units of mass or the units of mass to the units of weight. The first was decided and today we have the acceleration of gravity and Avogadro's number as conversion factors between the atomic weight of the element with the weight of a body made up of one mole of atoms that single element.
Attempts to establish the kilogram-force https://en.wikipedia.org/wiki/Kilogram-force did not come to fruition in 1901, and the international system did not adopt it, but it did establish a single unit for force the Newton , which now everyone knows that the weight of a mass of one kilogram on the surface of the Earth in vacuum and without rotation corresponds to $9.80665 N$.
When that is put into commercial practice, there will be those who take into account or not, what influences the hydrostatic thrust, and the local centripetal force due to the Earth's rotation, to make a fair deal between parties, but that does not change the definition of "weight" as a result of gravitational interaction.

Regards

 

[Dale]

To all participants, please let’s keep political discussions out of the technical forums.

 

[jbriggs444]

It is rare to read that, if we are in a science forum we answer as in a science forum, if this were a trade forum, perhaps our definitions would be more lax.

In any context we are free to speak or write as we wish. If our words might be misunderstood, we are free to disambiguate by explicitly stating definitions.

I think that no one can go home happily if they provide five different definitions, saying that it does not matter what they choose

I am not sure what you mean to say here.

If both speaker and listener are clear on which definition is used then communication is achieved. Which of five (or ten) definitions is agreed upon is of no particular concern.

Science defines weight in one way, but when it wants to apply it in everyday life, it presents deviations of the measure, which have nothing to do with "weight", deviations such as the hydrostatic pressure of the air and the rotation of the planet on its Its own axis as a function of latitude are topics that can be discussed and clearly defined, as a result we have that the scales always indicate the value of the normal force of contact with the body that is being measured, this is known as apparent weight.

Properly calibrated commercial scales do not normally work in the way you describe. They do not display a figure for apparent weight. Most display a figure for mass, uncorrected for buoyancy. This is an intended result of the way they are calibrated.

To be sure, it is possible to calibrate a scale to display a figure for apparent weight. All one needs is a reliable force standard. However, the typical portable standards are mass standards, not force standards.

 

[Richard R Richard]

In any context we are free to speak or write as we wish. If our words might be misunderstood, we are free to disambiguate by explicitly stating definitions.

Hi @jbriggs444 , Right, what I mean is that in a physics forum, it is better that we give the hard definition of gravitational interaction, and that the deviations of the "weight" measure that a scale can register, we can define them but never include them in the definition of weight, but yes in the one of apparent weight.

I am not sure what you mean to say here.

If both speaker and listener are clear on which definition is used then communication is achieved. Which of five (or ten) definitions is agreed upon is of no particular concern.

I reiterate my appearance ,when weight is said, it is not said apparent weight, nor mass, nor under what conditions the weight is measured, in any case it explains how the apparent weight is obtained, which is the main source of experimental data.

Properly calibrated commercial scales do not normally work in the way you describe. They do not display a figure for apparent weight. Most display a figure for mass, uncorrected for buoyancy. This is an intended result of the way they are calibrated.

I think that is not correct, the same scale will not indicate the same reading when measuring the same object, if one measurement is made at the equator and another at the pole, nor will it coincide with the one made in a vacuum.
You can calibrate the balance so that it measures on a scale proportionally directly to the mass, and it will only be useful in that place, if you move it from latitude, height, it will no longer have that proportionality, that is, it will no longer measure mass, but it will continue measuring apparent weight.
Not all the objects that you want to measure have the same volume nor do they have the same density, so you cannot calibrate the hydrostatic deviation.

To be sure, it is possible to calibrate a scale to display a figure for apparent weight. All one needs is a reliable force standard. However, the typical portable standards are mass standards, not force standards.

The standard will have a certain amount of mass, which allows you to compare other masses if no boundary condition is changed, if you alter a single condition then you will be comparing apparent weights and not masses.

 

[cmb]

In any context we are free to speak or write as we wish. If our words might be misunderstood, we are free to disambiguate by explicitly stating definitions.

Curious you'd say that.

I put

I take 'weight' to mean, self-evidently, one's weight measured on scales at the Earth's surface in ambient STP conditions

to then be told

It's a confusion in your understanding.

and for others to back that up.

You say I am free to disambiguate by this explicitly stated definition, so how can I be confused over my own definition?

 

[hutchphd]

You can calibrate the balance so that it measures on a scale proportionally directly to the mass, and it will only be useful in that place, if you move it from latitude, height, it will no longer have that proportionality,

Can we at least keep the physics right? A balance will balance on mars. This differs from a scale which contains a spring. This is the third time I've said this in this thread. Can somebody decapitate this thread?? Please?

 

[Richard R Richard]

Can we at least keep the physics right? A balance will balance on mars. This differs from a scale which contains a spring.

Ok, see if we can achieve the same balance on Mars as on Earth ...
, could you tell me

• the air pressure there,
• the acceleration of gravity,
• the radius of the planet,
• the rotation period,
• and the latitude to which you will compare the measurements?

When you define all that, the only thing you can calculate is the apparent weight,
The mass to be measured will remain the same.
The weights only depend on the mass of the planet Mars, the mass of the object, and its relative distance between its centers of mass. , calibrating a balance to measure on a certain scale is a matter of engineering, not physics. .
As other times has happened, can you tell me which part of physics you think I have been overlooked, I do not redefine physics, I do not even try to do so, I think I am not wrong in this case either
The same spring on Mars, comparing the same mass as on Earth, will give different elongation since there is a different apparent weight. What is not understood, what physics have I changed?

 

[Dale]

The same spring on Mars, comparing the same mass as on Earth, will give different elongation since there is a different apparent weight. What is not understood, what physics have I changed?

A spring is a scale, not a balance. Indeed, a balance measures mass (in vacuum), not weight.

Can somebody decapitate this thread?? Please?

Since it is over 200 posts at this point, I agree that it has outlived its usefulness. Recent posts have been heading in a bad direction.

Thread closed.