Scientists vote on new way to measure a kilogram

[Speakpigeon]

Weight is a force, and we measure it in units of force. The SI derived unit for force is the Newton: 1 N = 1 kg x 1 ms^-2. An item's weight is the product of its mass multiplied by the acceleration due to gravity: W = mg (i.e. F = ma). Since g is approximately 9.81 ms^-2 at sea level, most scales do the conversion from weight to mass for us: m = W / 9.81 ms^-2.

The number we see in kilograms is not actually our weight; it's our mass; the fact that people refer to their mass as their "weight" is simply a misconception. I didn't learn that my "weight" was actually my calculated mass until high-school science class, and I still refer to my body mass as my "weight" because that's how most people think of it.

The prefix kilo (from the Greek word χίλιοι) literally means "thousand", so a kilogram will always be exactly one thousand grams. It's not arbitrary. Kilograms and grams both units of mass, in the same way that the metre and the kilometre are units of distance.

Sorry, but the number I saw on the scale this morning was 81.2 kilogrammes, a bit up from yesterday. I can assure you it's my weight, notwithstanding what you seem to have learnt at school. Scales show weight because they're dumb instruments and couldn't possibly tell your mass given the way they work. This is true whether the scale is mechanical or electronic. Sure, you could make scales smart as a priest to tell you mass, but they don't do that. I'm not going to explain. I think any sensible person, especially someone who did "high school", should be able to understand that by himself. Just think about it instead of repeating what you're less-than-smart high-school "science" teachers told you.

Of course, it's possible I just misunderstood what you meant if ever, God forbid, you somehow goofed up your explanation. So, any clarification necessary?
EB

 

[Speakpigeon]

The intrinsic property of a gram is it's weight and it's mass.
Happy now?

I am confused now lion. Am I buying potatoes by mass or weight? What is the difference between mass and weight?

I definitely don't want to pay for weight. Weighty things are hard to carry home and eating them in turn makes you gain weight. What you want is pay for mass. Mass is a rough indication of the quantity of nutriments in your potatoes and you're looking for something nutritious. So, definitely mass. Yet, scales, as I just explained in my previous post, don't tell mass. They all tell weight. Hey, life is just such a hassle, isn't it? Still, you will find that human beings are resourceful and can take advantage of a bad day. Somehow, they understand weighty potatoes are also nutritious. So, yeah, weight should do. Notice that we don't pay this weight we don't really want with anything we would really want to keep. We just give a very light bit of paper in exchange for weight. Not a bad deal, hey?
EB

 

[Speakpigeon]

It's the same as the definition of a kilogram - you vote whatever you want it to be.
998 grams, 997 grams, 1002 grams...take your (scientific) pick.

You think a kilometre could be 998 metres, 997 metres, 1002 metres?



Man, a kilogramme... Don' you know what "kilo" means here!

kilo-
a combining form used in the names of units of measure equal to one thousand of a given base unit: kiloliter; kilowatt.
from French, representing Greek chilioi a thousand

Sorry to be so pedantic about it. But it seems you need to learn some basic facts. Things apparently not in the Bible.
EB

 

[steve_bank]

The intrinsic property of a gram is it's weight and it's mass.
Happy now?

I am confused now lion. Am I buying potatoes by mass or weight? What is the difference between mass and weight?

The definition of mass and weight is arbitrary.
It's the same as the definition of a kilogram - you vote whatever you want it to be.
998 grams, 997 grams, 1002 grams...take your (scientific) pick.

Sorry, you fail your science merit badge test.

Somebody posted it. Mass does not change. Weight is farce in Newton's from one of Newton's Laws Force = Mass x Acceleration.

Gravitational acceleration id 9.8 meters/second^2

Force for 1 kg = 1kg x 9.8 = 9.8 Newton's

The spring and indicator on a simple spring scale is designed to indicate true mass in our 1g Earth environment. Coil springs have linear deflection with force.

Given your lack of knowledge and experience and the general combined knowledge of regulars on the science forum you might try asking for explanations before jumping to answers. It is all basic stuff, but you are not up to competing over it.

There is a series of 'for dummies' books that are actually good. A few years back I picked up Chemistry For Dummies for a quick review. Try Physics For Dummies. Or get a non calculus based intro physics text used at a two year college.

There is an old saying, god helps those that help themselves.

Hard to fly with eagles when you are cackling like a turkey, so to speak...

 

[Speakpigeon]

The spring and indicator on a simple spring scale is designed to indicate true mass in our 1g Earth environment. Coil springs have linear deflection with force.

Which shows scales are designed to show weight. That's what you can measure with a spring. The physical phenomenon measured is the weight. You can't measure mass directly using a scale.
EB

 

[bilby]

The spring and indicator on a simple spring scale is designed to indicate true mass in our 1g Earth environment. Coil springs have linear deflection with force.

Which shows scales are designed to show weight. That's what you can measure with a spring. The physical phenomenon measured is the weight. You can't measure mass directly using a scale.
EB

Spring and piezoelectric scales measure force. Pan and steelyard scales compare the force on two masses (one known, the other unknown) under a common acceleration, and thereby measure mass.

Most bathroom scales are the first kind. On the moon, they would give an incorrect value for mass. But some gyms and medical facilities use steelyard balances - and these would give correct measurements of body mass in any non-zero acceleration conditions (ignoring tidal forces - but if you are close enough to a black hole or collapsar for these to be significant, your diet is the least of your problems).

 

[bigfield]

Spring and piezoelectric scales measure force. Pan and steelyard scales compare the force on two masses (one known, the other unknown) under a common acceleration, and thereby measure mass.

Oops--I overlooked those.

 

[Lion IRC]

Mass does not change.

Well then why can't we keep using the eternally unchanging, grand platinum bar?

 

[skepticalbip]

Mass does not change.

Well then why can't we keep using the eternally unchanging, grand platinum bar?

I assume you mean the official metric mass standard cylinder that is kept the CGPM in France. Obviously, access to it is extremely limited and copies that are kept in other countries as reference masses have limited access too. Also they need to be shipped back to the CGPM periodically for re-certification. The proposed system of defining the Kilogram would better define the precise reference mass and make it easily reproducible anywhere (even on Mars if we ever colonize there) without need for access to the official standard at CGPM because it would no longer be the official standard. Also the current official standard being destroyed by vandalism or in a war wouldn't matter if it had been replaced by an easily replicated definition.

 

[Sarpedon]

What he meant to say was that mass doesn't change due to local gravity.

The mass of objects does in fact change due to (very slow) reactions to air and electromagnetic particles. The Grand Kilo has been around for long enough that there are have been very, very slight changes to it.

You are aware of course that the force of gravity varies depending on one's location on Earth?

 

[steve_bank]

Mass does not change.

Well then why can't we keep using the eternally unchanging, grand platinum bar?

The reported increase in mass in the standard is said to be due to contamination. And that leads to why a different standard has been sought for decades.National primary kg standards are compared to the Paris standard. Handling causes changes. The new standard can be created anywhere, and it appears fairly simple. Elegant is the word.

National time and length standards are currently relatively easy to create such that the second and meter around the world will be within known experimental uncertainties. Now the kg will join the other two.

The second used to be derived from the Sidereal Day, the time it takes for a star to come around in the sky. Big Ben rings non and people set their clocks. I believe they used to fire a cannon o synchronize ship clocks at anchor.

What in part spurred a time standard was the growth of railroads. Station clocks and schedules. In the USA a radio signal indicates time broadcast from Colorado. If you have a short wave receiver you can hear it.

Standards like science evolve.

 

[lpetrich]

Mass does not change.

Well then why can't we keep using the eternally unchanging, grand platinum bar?

A bar was used for the meter. It's a cylinder that was used for the kilogram.

That issue aside, there is a big problem. There is only one of that cylinder. That makes it very precious, and thus very hard to use as a reference. That is why a sizable number of secondary standards exist, because those ones may be risked more easily.

By comparison, the new standard can be implemented anywhere in the Universe.

 

[bilby]

What he meant to say was that mass doesn't change due to local gravity.

The mass of objects does in fact change due to (very slow) reactions to air and electromagnetic particles. The Grand Kilo has been around for long enough that there are have been very, very slight changes to it.

You are aware of course that the force of gravity varies depending on one's location on Earth?

Yup. It doesn't even pull in a consistent direction - The Earth is neither homogeneous nor a sphere. Tangentially (pun intended) this is why your GPS shows the 0 longitude meridian at a point about 120 metres from the Greenwich observatory - the time standard remains fixed at the original location, but the vector of gravitational attraction at that point doesn't pass through the geometric centre of the Earth, and GPS (WGS84) uses a parallel zero meridian which does (but which is not precisely parallel to the gravitational vector at the earth's surface).

More fool the English, for choosing an (arbitrary) zero line that lies close to the boundary between a large slab of continent and a large area of ocean, at their latitude.

The imperfections of the natural world make metrology much more fun, but much less intuitive, than they would be if everything were neat and perfect.

 

[lpetrich]

The second used to be derived from the Sidereal Day, the time it takes for a star to come around in the sky. Big Ben rings non and people set their clocks. I believe they used to fire a cannon o synchronize ship clocks at anchor.

Actually, it's the mean solar day, but the principle is correct. It's the mean solar day because the Sun's direction alternately speeds up and slows down over the year, as much as 15 minutes.  Equation of time has more.

What in part spurred a time standard was the growth of railroads. Station clocks and schedules. In the USA a radio signal indicates time broadcast from Colorado. If you have a short wave receiver you can hear it.

From  Time zone, the first standard-time broadcast ever was from Greenwich Observatory in Britain in the mid 19th cy. The broadcasting was done by telegraph, and is first big users were railroads. US railroads each had their own time standards for a while, with the US settling on an overall time standard in the late 19th cy.

 

[steve_bank]

https://en.wikipedia.org/wiki/Sidereal_time#Sidereal_time_definition
https://en.wikipedia.org/wiki/Solar_time
https://en.wikipedia.org/wiki/Second

Ok. I always had sidereal day in my brain. Solar day makes more sense going back to the invention of latitude and longitude. Position is determined by angle of the sun above horizon at local noon.

Solar time is also represented by a sun dial clock. A practical daily clock.

Apparently sidereal time in astronomy has been replaced.

 

[Speakpigeon]

The spring and indicator on a simple spring scale is designed to indicate true mass in our 1g Earth environment. Coil springs have linear deflection with force.

Which shows scales are designed to show weight. That's what you can measure with a spring. The physical phenomenon measured is the weight. You can't measure mass directly using a scale.
EB

Spring and piezoelectric scales measure force. Pan and steelyard scales compare the force on two masses (one known, the other unknown) under a common acceleration, and thereby measure mass.

Most bathroom scales are the first kind. On the moon, they would give an incorrect value for mass. But some gyms and medical facilities use steelyard balances - and these would give correct measurements of body mass in any non-zero acceleration conditions (ignoring tidal forces - but if you are close enough to a black hole or collapsar for these to be significant, your diet is the least of your problems).

Excellent. Somebody at least here understands both English and science. Thanks.

For "Pan and steelyard scales", technically speaking they measure a weight by comparing it with the weight of a reference body. Mass is inferred from that, not measured.

Same thing if you determine the volume of a box by measuring the three sides. Volume is inferred from that. Would you say you are measuring the volume?

We can measure a volume of water by scooping out with a cup of known volume so that we effectively compare volumes. But this isn't comparable to pan and steelyard scales, where the comparison is done on the weight.

But Lion IRC is right, I'm just being pedantic.
EB

 

[Speakpigeon]

That issue aside, there is a big problem. There is only one of that cylinder. That makes it very precious

I hope they will keep it and show it in the Louvre museum. Perhaps next to the Joconde, so that we would now understand why she is smiling.

By comparison, the new standard can be implemented anywhere in the Universe.

Yeah, the old one, there is just one in the entire universe. Blimey!
EB

 

[lpetrich]

https://en.wikipedia.org/wiki/Sidereal_time#Sidereal_time_definition
https://en.wikipedia.org/wiki/Solar_time
https://en.wikipedia.org/wiki/Second

Ok. I always had sidereal day in my brain. Solar day makes more sense going back to the invention of latitude and longitude. Position is determined by angle of the sun above horizon at local noon.

Solar time is also represented by a sun dial clock. A practical daily clock.

The daytime/nighttime cycle is much more salient in human affairs than the motions of the stars, so that's why it's solar days instead of sidereal days that are the big time reference.

Apparently sidereal time in astronomy has been replaced.

Astronomers still use that for pointing their telescopes. However, it's not a primary time standard.

 

[bilby]

Spring and piezoelectric scales measure force. Pan and steelyard scales compare the force on two masses (one known, the other unknown) under a common acceleration, and thereby measure mass.

Most bathroom scales are the first kind. On the moon, they would give an incorrect value for mass. But some gyms and medical facilities use steelyard balances - and these would give correct measurements of body mass in any non-zero acceleration conditions (ignoring tidal forces - but if you are close enough to a black hole or collapsar for these to be significant, your diet is the least of your problems).

Excellent. Somebody at least here understands both English and science. Thanks.

For "Pan and steelyard scales", technically speaking they measure a weight by comparing it with the weight of a reference body. Mass is inferred from that, not measured.

All measurements are inferences.

Same thing if you determine the volume of a box by measuring the three sides. Volume is inferred from that. Would you say you are measuring the volume?

Yes, I would. Because you are.

We can measure a volume of water by scooping out with a cup of known volume so that we effectively compare volumes. But this isn't comparable to pan and steelyard scales, where the comparison is done on the weight.

Of course it is. These are exactly analogous.

But Lion IRC is right, I'm just being pedantic.
EB

Arithmetic is universal, and is a perfectly good way to make inferences, whether consciously or unconsciously.

 

[Speakpigeon]

All measurements are inferences.

I don't see how.

I measure the length of an object by looking at a tape measure or a ruler I put alongside it. It's direct objective experience. I don't see where would be the inference in that. I don't even need to believe these things are out there in the world. I just look at the figure written on the tape or ruler.

Same for measuring the weight. I look at the measure of a force.

What there is are assumptions. For example, that I'm not inebriated or otherwise intoxicated. That my vision works well enough. That the ruler or tape I use is a reliable measurement tool. And so on.

Measure by inference seems merely an extension of the original idea. It's just loose talk.

Obviously, most of what scientists describe as "measures" are of that sort. I wonder how much this is comforting to them to use this word.

Inferences are abstraction and as such are not observed out there in the world but are themselves inferred from observation. Maybe you think of scientific theories as "measures"? I certainly don't.
EB