Understanding Weight vs. Mass: Clarifying the Debate

[pb23me]

weight or mass?

I have a friend here who is totally convinced that the scales we use at home to weigh ourselves our actually measuring our mass not our weight...Could somebody please enlighten her hahaha

 

[russ_watters]

A spring scale can only directly measure mass, but it may have a button on it that makes it read in kg...

 

[K^2]

Most bathroom scales measure weight, because they are essentially either spring scales or pressure sensors, both of which measure forces.

I've seen quite a few medical scales, however, that use counterweights. While these still measure a force, they measure it in units of force produced by reference mass, meaning that they do ultimately measure mass rather than weight.

Edit: Russ, I think you might have mistyped.

 

[Danger]

A spring scale can only directly measure mass

But that weight is entirely dependent upon mass, as measured in a particular gravity environment.

 

[pb23me]

ok so what is it? I am in america and have always had the idea that weight scales we use at home, weight scales fighters use before fights measure "weight" or mg. while scales used in chem and physics labs measure mass...

 

[Danger]

Weight is just the effect upon mass of a gravitational source.

 

[pb23me]

i know weight is mass*gravity and that's is what "weight" scales measure...Gravity's effect on mass. am i wrong or right?

 

[Danger]

i know weight is mass*gravity and that's is what "weight" scales measure...Gravity's effect on mass. am i wrong or right?

You're right, but now I'm starting to wonder about the semantics myself. Those same scales on the moon would read 1/6th of what they do at sea level on Earth, and would read nothing at all in zero gravity, so I've revised my opinion to think that they do actually measure weight rather than mass. Something that relies upon inertia or gravitational influence would measure mass.
I agree with K^2 and Pb23me that a balance beam would more properly be considered a mass-measuring device, since it compares one side to the other within the same gravitational influence rather than acting against pre-calibrated springs.

 

[Buckleymanor]

You're right, but now I'm starting to wonder about the semantics myself. Those same scales on the moon would read 1/6th of what they do at sea level on Earth, and would read nothing at all in zero gravity, so I've revised my opinion to think that they do actually measure weight rather than mass. Something that relies upon inertia or gravitational influence would measure mass.
I agree with K^2 and Pb23me that a balance beam would more properly be considered a mass-measuring device, since it compares one side to the other within the same gravitational influence rather than acting against pre-calibrated springs.

There is a problem when relying on inertia or gravitational influence to measure mass in as much that gravity is different depending on where it is measured and if you use this measurement to produce your test mass.
Using inertia has similar problems as this can be influenced by gravity.
Big Gs constant is difficult to measure as the Earth's gravity plays it's part.http://physicsworld.com/cws/article/news/2628
Little g varies depending on where it is measured.
So as far as your scales are concerned on Earth it would depend on what reference mass was used to calibrate them against.

 

[pallidin]

Interesting posts. I must admit that I was not aware that there could be a good discussion on this issue. I learned a few things from this.

 

[D H]

Weight is just the effect upon mass of a gravitational source.

i know weight is mass*gravity and that's is what "weight" scales measure...Gravity's effect on mass. am i wrong or right?

Wrong and wrong, at least in terms of what a spring scale measures.

You're right, but now I'm starting to wonder about the semantics myself. Those same scales on the moon would read 1/6th of what they do at sea level on Earth, and would read nothing at all in zero gravity, so I've revised my opinion to think that they do actually measure weight rather than mass.

Getting closer. Zero gravity, at least in the terms of astronauts floating around the space station, does not mean that there is no gravitational force. Those astronauts gravitational weights (mass times acceleration due to gravity) are about 88% of their weight on the surface of the Earth.

So why do we call it "zero gravity" and what do spring scales measure? The latter question first: Spring scales measure everything but gravitational force. There is no device that can directly measure gravity. Einstein's equivalence principle comes into play.

Suppose you took a spring scale on the Vomit Comet. As the plane starts its climb in preparation for a zero-g parabolic arc the scale will register your weight as higher than normal. As the plane starts its parabolic arc your scale weight will decrease and reach zero. You feel weightless, and that is exactly what the scale registers. When the plane starts to level out your scale weight will increase, eventually surpassing your normal weight. Time for the next parabolic arc. The gravitational force changes by a tiny, tiny amount between the bottom and top of those arcs. Your gravitational weight, m*g, hardly changes at all. The scale obviously is not weighing m*g.

When you are standing still on the surface of the Earth the scale is exerting an upward force on you that nearly cancels the downward force due to gravity. By Newton's third law, you are exerting a downward force on the scale that is equal but opposite to that upward force exerted on you by the scale. It is this downward force exerted by you on the scale that the scale measures. Note that I said "nearly cancels". Even on when you are standing on the surface of the Earth a spring scale does not quite measure your gravitational weight (m*g). You are rotating at one revolution per day about the Earth's axis, after all. The difference between your scale weight and gravitational weight is rather small when standing still on the surface of the Earth, and is even smaller when standing still on the surface of the Moon.So why do we call it zero g? As far as those astronauts are concerned, and as far as Einstein's equivalence principle is concerned, those astronauts are weightless. Just as a scale measures everything but gravity, people feel everything force acting on them but gravity.

 

[TurtleMeister]

There is no device that can directly measure gravity.

Isn't gravity being measured when the torsion balance is used to find the value of G?

 

[D H]

Emphasis mine:

There is no device that can directly measure gravity. Einstein's equivalence principle comes into play.

Isn't gravity being measured when the torsion balance is used to find the value of G?

Only indirectly, multiple times indirectly. What is being measured directly is the twist in a torsion ribbon. This direct measurement is transformed to a force thanks to prior calibrations of the mechanism (more measurements). This in turn is transformed to G by knowing the masses involved (more measurements) and knowing the distance between these masses (even more measurements). These multiple layers of indirection and the very precise measurements needed in each step exemplify why G is one of the least well-known (or just the least well-known, period) physical constants.

 

[russ_watters]

A spring scale can only directly measure mass, but it may have a button on it that makes it read in kg...

Ugh, so that was a typo, in case people didn't realize it: a spring scale can actually only measure weight. But by assumption of a certain g, it converts to mass. Sorry.

 

[sophiecentaur]

Whilst a spring balance only can measure a force - therefore weight, I would say that a true balance only really compares the forces produced by two masses which isn't mass, directly.
The whole topic is all a bit circular, really. You have force, distance, energy and time, which relate to mass. It's all a matter of convenience which is more fundamental. Where standards are concerned, I think that you should start with those quantities which are easiest to specify most accurately and reliably (time and length, for a start) and build on those for the other measurements.
Mass, in terms of 'numbers of carbon atoms', or somesuch would possibly be better than using standard, arbitrary, blocks of platinum but you've still got to implement a system which is better, in practice, than going over to Paris every so often and checking your sub-standard masses.

 

[Danger]

There is a problem when relying on inertia or gravitational influence to measure mass in as much that gravity is different depending on where it is measured and if you use this measurement to produce your test mass.

I rather suspected that my lack of writing discipline might result in misunderstanding, but I was being lazy. When I mentioned gravitational influence in the post that you quoted, I meant the influence of the mass upon something else, not the effect of gravity upon the mass. Sorry.

 

[JDługosz]

Non-scientists, and anyone not working on a physics or engineering problem, tend to be sloppy about units. Throughout history our intuition of mass and weight were of one thing, and early systems did not make a difference. If what a scale measures is bad, just look at a tire gauge that shows "kg/cm²" which is absurd, but someone's idea of "converting" pounds per square inch.

Until we have commerce off-earth, that will continue. Once a client on the moon argues with a supplier on the meaning of the price of a pound of nails, the laws will be rewritten with proper rigorous language.

If talking about such things, I'll be clear to avoid non-sequiturs such as "The space suit only weighs 25kg on the moon".

 

[D H]

Until we have commerce off-earth, that will continue. Once a client on the moon argues with a supplier on the meaning of the price of a pound of nails, the laws will be rewritten with proper rigorous language.

US law is quite rigorous about this. A pound of nails weighs the same at Point Barrow, American Samoa, the surface of the Moon, and on the ISS. Legally, weight is a synonym for mass in the US and the pound (without any qualifier) is a unit of mass.

 

[sophiecentaur]

An when I was at school, a pound weighed 32 poundals.
Thank God we went SI, subsequently.

 

[TurtleMeister]

There is no device that can directly measure gravity. Einstein's equivalence principle comes into play.

Isn't gravity being measured when the torsion balance is used to find the value of G?

Only indirectly, multiple times indirectly. What is being measured directly is the twist in a torsion ribbon. This direct measurement is transformed to a force thanks to prior calibrations of the mechanism (more measurements). This in turn is transformed to G by knowing the masses involved (more measurements) and knowing the distance between these masses (even more measurements). These multiple layers of indirection and the very precise measurements needed in each step exemplify why G is one of the least well-known (or just the least well-known, period) physical constants.

Thanks for the clarification. I understand and agree with what you are saying. There are many things that cannot be measured directly.