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What is "mass"?

N

NeoPTLD

Platinum Member
How can you accurately measure the mass of something and what is mass defined as?

I just read that there's about 0.5% variation in gravitational acceleration due to latitude which means that it will throw off measurement by 5grams for every 1000g, but you can buy electronic scales that measures accurate down to 0.1g and range as high as 2400g.

All except for real balance that weighs the sample by mean of counter-weights, weighing is done by sensing the force and they're not affected by gravity.

So, even if an electronic scale claims to be accurate within 0.1g at 1,000g, there can be a 0.5% variation in force pulling the weight down on the scale, which would be 5grams. So, how can it be accurate down to 0.1g??


The platinum world reference for 'kg' maybe 1.000kg at whereever it was made, but when you take it elsewhere, it can have as much as 5g of difference due to gravitational difference.


 
Realize that mass and weight are two different things. Weight is a force. Mass of a body is the characteristic that relates a force on a body to a resulting acceleration. This is where Newton's Second Law comes in, you might have seen it before (F = ma). If you look at the law, you can see mass can only be physically sensed when there is acceleration on it.

Secondly, to clarify the difference between mass and weight (which is a force), I've listed the SI and British units below:

System Force Mass Acceleration
SI newton kilogram m/s^2
British pound slug ft/s^2

As you have seen the accepted measurement of a physical quanity in British units has been the pound, but the British unit for mass is really the slug. The SI system uses both. In both systems, mass times acceleration will give you the force.

So now comes to answering your question about deviations in mass in different parts of the planet. If you look at Newton's Second Law in terms of mass (m = F/a), you will see that as the acceleration on a mass varies from location, so does the force. If you know the acceleration at a point and you found the force, you can find the mass which will remain constant.

Most scales assume that acceleration = 9.81 m/s^2, so they measure the force and use Newton's Second Law to get the mass. If the scale was adjustable you could set it exactly to the acceleration at a point on the planet and the mass would still read constant. Hope this helps.
 
Also consider that scales as accurate as you mention must undergo periodic calibration to guarantee that accuracy. They do this by placing units of known mass on the scale and making adjustments. Unless the calibration takes place a long distance from where it is going to be used, the error in gravity that you mention will not be sufficient to be measurable. Most calibrations are done on site BTW, so any error in measurement would have little if anything to do with a difference in gravity.
 
Additionally, you can use inertial balances to find mass.... These can be used on earth, and could also be used in an environment that is "weightless." i.e. an inertial balance could be used on the space shuttle to accurately find the mass.... it is independent of gravitational forces.

edit: it would still have to be calibrated, but it's results wouldn't vary as you moved it to different locations on earth.
 
Simple definition from physics long ago

mass = the amount of matter in something, such as a solid. This is constant (or essentially constant for most cases)
weight = the force of gravity on a mass.

If you use a spring scale to measure 'mass' then you will get errors due to gravity, as springs rely on force and convert that to mass with a known conversion factor (g). If you use an inertial balance with the little masses on one side and your object on another, you will get the same mass regardless of gravitational force (it would read the same on earth and the moon).

The above is a very simple definition of mass that will work for 99% or more of the times you need to think about mass unless you get into advanced physics and the like.
 
Mass is the gravitational charge, basically. It determines the strength of gravitational interactions between two objects.

It also seems to be the same or close to the same as the inertial resistance of the object. That is, the constant that relates force and the resultant acceleration.
 
The commercial electronic scales that I am familiar with correct for local gravity not just for the effects of latitude but for local geographic features such as mountains. It is just one of the parameters that is entered during the calibration process. Considering that commercially available scales can weigh to microgram accuracy this gravitational effect is a pretty gross error. Commercial freight scales such as used to weigh trucks must be certified to .1% +or- 1 count and can weigh hundreds of tons to this accuracy.
 
Originally posted by: rjain
Mass is the gravitational charge, basically. It determines the strength of gravitational interactions between two objects.

It also seems to be the same or close to the same as the inertial resistance of the object. That is, the constant that relates force and the resultant acceleration.
Click to expand...

Excellent point, Rjain.

To the best of my knowledge, there's no consensus as to why inertial mass (the one in F=ma, which describes how matter "resists" acceleration) happens to be the same as gravitational mass (the one in F = G(m1)(m2)/r^2, which explains how matter attracts other matter).

But for all known stuff, it's true that inertial mass = gravitational mass within the marign of measurement error.
 
Originally posted by: NeoPTLD
So, where on the earth would 1.000E03cc of water exert a downward force of 9.806kN?
Click to expand...
Nowhere 🙂 . The average force it would exert would be 9.806N.

As for where it would exert that force? There are an infinite number of places, but I can't name any of them off the top of my head. Best bet would be to go somewhere 'heavy' and increase your altitude (increasing the radius from the center of the earth) until you get the correct value. Of course as an engineer 9.81 is close enough.
 
Originally posted by: KalTorak
Originally posted by: rjain
Mass is the gravitational charge, basically. It determines the strength of gravitational interactions between two objects.

It also seems to be the same or close to the same as the inertial resistance of the object. That is, the constant that relates force and the resultant acceleration.
Click to expand...

Excellent point, Rjain.

To the best of my knowledge, there's no consensus as to why inertial mass (the one in F=ma, which describes how matter "resists" acceleration) happens to be the same as gravitational mass (the one in F = G(m1)(m2)/r^2, which explains how matter attracts other matter).

But for all known stuff, it's true that inertial mass = gravitational mass within the marign of measurement error.
Click to expand...

I read a paper by some brazilian scientist who theorized they were in fact different. His equations basically said that if you can get an object to absorb radiation at a low enough frequency, you can drive these two masses from equality, even to the point of an object having no inertial mass (free acceleration). I can't remember where it is though.
 
Originally posted by: silverpig

I read a paper by some brazilian scientist who theorized they were in fact different. His equations basically said that if you can get an object to absorb radiation at a low enough frequency, you can drive these two masses from equality, even to the point of an object having no inertial mass (free acceleration). I can't remember where it is though.
Click to expand...

Check out http://www.metaresearch.org . I don't know if this is the person you are talking about but Tom Van Flandern has some theories like that.
 
Originally posted by: NeoPTLD
So, where on the earth would 1.000E03cc of water exert a downward force of 9.806kN?
Click to expand...
At what temperature? 4 degrees C? (That's the temp when 1 mL of water is EXACTLY 1cc)
 
LurchFrinky - Howard is correct. The ml and the cc units were derived under different conditions. In general usage, the difference is small enough not to matter.
 
See Here

This nonsense was stopped in 1964 when it was ruled that the word "litre" may be employed as a special name for the cubic decimetre...
Click to expand...
I stand by my previous statement.

It was way past 1964 when I started learning about the metric system, but they still defined a litre as a kilogram of water. I wonder how many of today's textbooks still have it wrong.
 
1mL = 1cc, plain and simple (at least in modern metric system conventions)

now as temperature fluctuates, 1g of water may not be 1cc or 1mL, but saying 1mL is not the same as 1cc is claiming that the same object/substance instantaneously occupies two different amounts of volume.
 
LurchFrinky and Zeronine8 are correct.

What you're confusing it with is that at 3.98 degrees celsius, liquid water is densest, and 1 cubic centimeter of water has a mass of 1 gram.

From the CRC Handbook of Chemistry and Physics, (this is the first time I've used this book in years!)

The density of water at 3.98 degrees celsius is 1.00000 grams per cubic centimeter. As temperature increases from 3.98 to 100, the density drops to .95838 grams per cc. At 0 degree C, the density is .99987 g/cc.

Nonetheless, a milliliter is a unit of volume defined as a 1 cubic centimeter.
 
I wasn?t confused, just educated too early. I had Chemistry 101 as a freshman in 1958. In the lab, I recall the professor cautioning us against mixing glassware calibrated in cc versus those calibrated in ml. Of course I think in ?fluid ounces?, so I?m sure this new fangled metric stuff is just a passing fad.

Live and learn, even for us white-haired folk.



 
Originally posted by: DrPizza
LurchFrinky and Zeronine8 are correct.

What you're confusing it with is that at 3.98 degrees celsius, liquid water is densest, and 1 cubic centimeter of water has a mass of 1 gram.

From the CRC Handbook of Chemistry and Physics, (this is the first time I've used this book in years!)

The density of water at 3.98 degrees celsius is 1.00000 grams per cubic centimeter. As temperature increases from 3.98 to 100, the density drops to .95838 grams per cc. At 0 degree C, the density is .99987 g/cc.

Nonetheless, a milliliter is a unit of volume defined as a 1 cubic centimeter.
Click to expand...
There's more water in 1cc @ 4C than at 1C or 10C, isn't there?
 
Originally posted by: rjain
Originally posted by: silverpig

I read a paper by some brazilian scientist who theorized they were in fact different. His equations basically said that if you can get an object to absorb radiation at a low enough frequency, you can drive these two masses from equality, even to the point of an object having no inertial mass (free acceleration). I can't remember where it is though.
Click to expand...

Check out http://www.metaresearch.org . I don't know if this is the person you are talking about but Tom Van Flandern has some theories like that.
Click to expand...

Nope, but I found the link anyways. Interesting read.
 
Originally posted by: silverpig

Nope, but I found the link anyways. Interesting read.
Click to expand...
Looks dubious to me. Only a single sample at the critical wavelength per experiment. He should really be doing a number of samples around that wavelength to show that the anomaly isn't just an accident. For so few trials, he has a really suspicious degree of accuracy. I'm not convinced, but I won't dismiss it.
 
Originally posted by: rjain
Originally posted by: silverpig

Nope, but I found the link anyways. Interesting read.
Click to expand...
Looks dubious to me. Only a single sample at the critical wavelength per experiment. He should really be doing a number of samples around that wavelength to show that the anomaly isn't just an accident. For so few trials, he has a really suspicious degree of accuracy. I'm not convinced, but I won't dismiss it.
Click to expand...

It's the theory that is neat. Furthermore, I seem to remember the story being that he came up with this, published it, and started working on prototypes for a "mass separator" or whatever you want to call it. He went through a number of revisions which failed before coming up with one he was sure was going to work. Then the Brazilian government stepped in and he kinda went missing...
 
Hi,

I tried your link. Can anyone tell me whether any of the papers he's written have been published? It's just that all of the links to his papers are either pre-published editions, not labelled with the journal or haven't been published.

Cheers,

Andy
 
I checked his paper on "<SPAN lang=EN-US style="mso-ansi-language: EN-US"><o😛>The Gravitational Mass at the Superconducting State" and even though I can't say I read it very carefully but I studied the part where he writes about superconductivity and found several errors (I am not an expert on gravity but I know a thing or two about superconductors), I think it it safe to say this is just another "GUT-internet-nut", although this one actually seems to own a few books on physics.
And no, it seems he hasn't published anything (which is a bit strange since he is suppose to be a "Professor").
</o😛></SPAN>
 
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