Mass vs Mass as a Force (Weight)

[Dale]

SI spring scales are labeled in kg, not Newtons.

Yes, but this one is correct, if the spring scale is properly calibrated at its location.

 

[The Fez]

Yes, I agree. The mistaken belief that the pound is a unit of force comes directly from the engineering community. I have even had engineering textbooks that incorrectly asserted that as fact. I don’t know how a simple mistake in the engineering literature turned into the common view held by non-engineers.

It is not a mistake at all. In engineering, we distinguish between a pound force (lbf) and a pound mass (lbm). Confusing, maybe. But not a mistake.

 

[Dale]

It is not a mistake at all. In engineering, we distinguish between a pound force (lbf) and a pound mass (lbm). Confusing, maybe. But not a mistake.

It is not a mistake to use lbf as a unit of force and lbm as a unit of mass. It is a mistake to use lb as a unit of force.

 

[jtbell]

When I was in school in the 70's in the U.S., it was common for physics textbooks to preach that the pound was unequivocally a unit of force. I believe that Halliday and Resnick clung to this practice religiously at least into the 90's. However, I have no references to offer in support of this claim.

In my entire college-student and -teaching career, intro physics courses always used the pound as a unit of force. I don't have a copy of H&R handy, but I do have Serway & Vuille's College Physics (8th edition, 2009). In section 1.1, "Standards of Length, Mass and Time", it mentions the

[...]U.S. customary system, in which the units of length, mass and time are the foot, slug, and second.

In section 4.3, "Newton's Second Law", it states:

In the U.S. customary system, the unit of force is the pound.

Then follows a table of SI and U.S. customary units of mass, acceleration and force, which lists the slug as the U.S. customary unit of mass. These may be the only two places in the book that mention the slug. It doesn't even appear in the index.

 

[Herman Trivilino]

1lb force is the force exerted by gravity on 1 lb mass.

Yes, but how much gravity? The now antiquated kilogram-force was defined as the force exerted by gravity on one kilogram of mass. They specified the strength of gravity: 9.80665 m/s². Even though there is no place on Earth where $g$ has that constant value as it varies with time. It was introduced so that people could use units of mass to measure force.

No one ever established such a "standard value" for $g$ to be used to define the pound force. But that doesn't stop people from using the pound (a unit of mass) to measure force.

 

[jbriggs444]

Yes, but how much gravity? The now antiquated kilogram-force was defined as the force exerted by gravity on one kilogram of mass. They specified the strength of gravity: 9.80665 m/s². Even though there is no place on Earth where $g$ has that constant value as it varies with time. It was introduced so that people could use units of mass to measure force.

No one ever established such a "standard value" for $g$ to be used to define the pound force. But that doesn't stop people from using the pound (a unit of mass) to measure force.

Agreed.

This also applies similarly to the definition of the slug. You can't just multiply the pound mass by one standard gravity (in units of feet per second squared) without having a figure for standard gravity.

 

[gmax137]

Well, all this confusion arises because people speaking or writing can be sloppy: they mean pound-force (lbf) but they say "pound." Or they mean pound-mass (lbm), but they say "pound." That's just sloppy; careful engineers never say "pound" they always add "-force" or "-mass."

On the other hand, look at the poor Btu. How many Btu in a kilowatt-hour? Look it up, you will see 3412, 3412.14, 3413, 3415. Shouldn't it be a single value? Well, a Btu is the energy to raise one pound-mass of water one degree (Fahrenheit), but you can choose at what temperature to measure this energy. To raise the water from 33 to 34F, from 60 to 61F, from 68 to 69F... there are numerous "Btu" defined at different temperatures, for different reasons.

Compounding this in engineering circles is the notion of significant figures, and (related) doing calculations with slide rules. In slide rule land, there really is no difference between 3412 and 3413.

 

[JT Smith]

I always thought that lb without qualification meant lbf not lbm. It's only slightly interesting to discover that the default is the other way around. It's still potentially ambiguous and confusing.

Btu is even worse in that in many instances it is used as a unit of power, implying Btu/hr.

Oh well...

 

[hutchphd]

Summary:: understanding mass and mass as a force(weight)

so how do we really measure mass?

I used to tell aspiring freshman physicists:
If you want to measure the weight of an object, lift it (from the surface of the earth)
If you want to know the mass, try to shake it.

.

 

[vanhees71]

I think today we should stick to the SI units in all teaching up to the point when we teach relativistic electrodynamics (a tautology, if you ask me, but not in the common textbook literature) in the theory course, where the SI is a nuissance, measuring the frame-dependent components of the Faraday tensor in different units ;-)).

 

[sophiecentaur]

It's still potentially ambiguous and confusing.

Moral: steer clear of Imperial Units and Sloppy Engineers; move to Europe(??) - for the units and not the Engineers, who are all as bad as each other.

Ask a company to deliver Three metres of concrete to your house and they will pour out 3m³ for you.

 

[jbriggs444]

Moral: steer clear of Imperial Units and Sloppy Engineers; move to Europe(??) - for the units and not the Engineers, who are all as bad as each other.

Ask a company to deliver Three metres of concrete to your house and they will pour out 3m³ for you.

A bargain, I tell you. Over here we only get cubic yards.

 

[BvU]

Look at it from the bright side: in all those underdeveloped countries that haven't been forced to adopt metrication, the scientists are a lot handier changing units, dealing with conversion constants and what have you.



$\$

 

[sophiecentaur]

A bargain, I tell you. Over here we only get cubic yards.

From what I hear, DIY materials and equipment are cheaper in US than here (UK). And everyone seems to have a bigger workshop too - that's really unfair!

 

[BvU]

So much about heavy mass versus slow mass. Thread closed.

Oops, I'm not a moderator !

$\$

 

[vanhees71]

Look at it from the bright side: in all those underdeveloped countries that haven't been forced to adopt metrication, the scientists are a lot handier changing units, dealing with conversion constants and what have you.



$\$

And then promising Mars probes crash, because NASA uses metric and programmers Imperial units (I think in this case m vs. ft)...

 

[jbriggs444]

And then promising Mars probes crash, because NASA uses metric and programmers Imperial units (I think in this case m vs. ft)...

Wiki says pound(force)-seconds versus Newton-seconds. Out by a factor of 4.45.

 

[rsk]

thats what I am trying to figure out. I am 104Kg but what does that mean? 1Kg is 1000g yes, but again, what does that actually mean. How many atoms is in 1Kg of me, or something? Do all objects that have a mass of 1Kg have the same number of atoms? if not, then how can 1Kg be constant? IE, 1Kg of water have same amount of atoms of 1Kg sand?

Ah, now you're straying dangerously close to chemistry.
Perhaps reading a little about molar mass, molecular mass, relative atomic mass and similar might help.

To a physicist, kg is mass and remains the same whether on Earth or the moon or in deep space.
Non-physicists often use the word weight where a physicist would use 'mass'.

 

[Digcoal]

Summary:: understanding mass and mass as a force(weight)

Been thinking about mass today and came up with a question. Why is that objects on this Earth describe its weight in Kg, g, mg, Mg? IE 2.2Lbs is 1Kg. So why do we name the 1Kg to be mass, but the 2.2Lbs we call weight. Both have to be a unit of force if there is a conversion factor between the two numbers. if 1Kg IS 2.2 lbs, then 1Kg is also considered weight because a 1.3Kg jar of honey is NOT 1.3Kg on the moon or anywhere else.

we know mass to be what we are made of, atoms, etc. so when and how decided it to be a unit of force?

so how do we know what really is. How do i know what my mass really is? Some say my mass is 104Kg, but that is purely based on a gravitational constant of 9.81.

so how do we really measure mass?

thanks

Grams is used as a WEIGHT measurement outside of the science community and in everyday life.

The problem arises when we used the same TERM to define MASS.

In imperial measurements, pounds is a measurement of force/weight and slug is a measurement of mass.

This is why science uses Newtons for force/weight and grams for mass.

If there were as hard a push to use Newtons on scales instead of kilograms as there is to push for the use of metric over imperial measurements, your confusion would not occur. I am sympathetic to your plight because I had the same issue reconciling the two ideas a decade ago.

 

[jbriggs444]

Grams is used as a WEIGHT measurement outside of the science community and in everyday life.

If you are talking about bathroom scales, they are too imprecise for anyone to say whether they measure force or mass. The number that is presented could be pounds-force or it could be pounds-mass.

If you are talking about a scale that is legal for commerce, it measures mass. It is calibrated and certified to do so. If it presents a result in pounds those will be pounds mass. The result may or may not accurately reflect pounds force depending on local g.

 

[Digcoal]

If you are talking about bathroom scales, they are too imprecise for anyone to say whether they measure force or mass. The number that is presented could be pounds-force or it could be pounds-mass.

If you are talking about a scale that is legal for commerce, it measures mass. It is calibrated and certified to do so.

I failed to include "legal for commerce" with science as specific instances where kg is used for mass and not weight.

A bathroom scale is used to answer what question that people ask in everyday conversation?

"How much do you weigh?"

When you ship a package, what is asked?

"What is the weight of the package?"

The reason commerce uses mass is because it is the direct measurement used to calculate fuel costs for shipping. However, everyday usage is predominately weight. This is why the original question was asked, and why it took me a while to understand it myself. Mass and Weight are conflated because we have the same symbolic representation for two entirely different concepts.

It is the same as calling cats "cats" and birds "cats." Distinction without distinct terms leads to unnecessary confusion.

 

[Digcoal]

I used to tell aspiring freshman physicists:
If you want to measure the weight of an object, lift it (from the surface of the earth)
If you want to know the mass, try to shake it.

.

You can also determine the mass of an object using the original scales humans used long ago: a known mass on one side and the undetermined mass on the other. Just good old leverage and chunks of matter; no shaking required.

 

[hutchphd]

Swiftian Alternative:
Weight (in kg) will be repurposed as strictly a marketing term for cans of beans and fat reduction potions. It will not be used in any technical literature. The approved term will Force of Gravity in Newtons (at penalty of digit removal). We can and did, after all, demote Pluto.

 

[jbriggs444]

The reason commerce uses mass is because it is the direct measurement used to calculate fuel costs for shipping.

I would suggest a much more mundane reason: Because the buyer and seller need to agree on amounts of goods that will satisfy their agreements.

Historically, mass standards have proven convenient (carry reference masses around and calibrate/certify scales on site) and do not result in disagreements due to variations in local g.

 

[Digcoal]

While true, you see people get around this by using volume for things like ice cream to hide the fact that consumers get less product by fluffing the product up. Dove does this with their soap. Gas is more expensive during the day than at night because the same amount of gas takes up more volume when warm than when cold which is why gas is sold by volume and not mass even though the number of reactions is determined by number of molecules and not how much space those molecules occupy. I guarantee you that if and when people stop buying gas during the day, gas stations will update their sales price to reflect mass instead of volume.

They use mass when they are concerned about more and are privy to the distinction. Therefore, updating common parlance is important so that everyday consumers can also be privy to these tricks.

 

[jbriggs444]

They use mass when they are concerned about more and are privy to the distinction. Therefore, updating common parlance is important so that everyday consumers can also be privy to these tricks.

Not everything is an evil conspiracy. Mass is used because it is conveniently standardizable. A force standard be a poor substitute.

Substituting out a word such as "weight" won't change that.

 

[Digcoal]

"Not everything is an evil conspiracy." Great. Who are you talking to because I did not say that. lol

As for the rest of your response... welcome to the thread about why people are confused about mass and weight.

 

[cmb]

Summary:: understanding mass and mass as a force(weight)

Been thinking about mass today and came up with a question. Why is that objects on this Earth describe its weight in Kg, g, mg, Mg? IE 2.2Lbs is 1Kg. So why do we name the 1Kg to be mass, but the 2.2Lbs we call weight. Both have to be a unit of force if there is a conversion factor between the two numbers. if 1Kg IS 2.2 lbs, then 1Kg is also considered weight because a 1.3Kg jar of honey is NOT 1.3Kg on the moon or anywhere else.

we know mass to be what we are made of, atoms, etc. so when and how decided it to be a unit of force?

so how do we know what really is. How do i know what my mass really is? Some say my mass is 104Kg, but that is purely based on a gravitational constant of 9.81.

so how do we really measure mass?

thanks

kg is a unit of mass.

lb is a unit of weight

lbf is a unit of force

Let me know if this might help;-

1. If you take a 1kg lab test mass and a scales that has a kg readout, then, assuming no errors and correctly made/calibrated, it'll read '1kg' for your 1kg test mass whilst here on Earth.

2. If you take a lb lab test weight and a scales that has a lb readout, then, assuming no errors and correctly made/calibrated, it'll read '1lb' for your 1lb test weight whilst here on Earth.

3. If you take the same 1kg lab test mass and the same scales, it'll read about '0.17kg' for your 1kg test mass on the Moon.
The reason is that your SCALES are no longer reading kg. Your SCALES are now wrong, but you still have a 1kg MASS.

4. If you take the same lb lab test weight and the same scales that has a lb readout, then it'll read about '0.17lb' for your 1lb test weight on the Moon.
The reason is that your WEIGHT is no longer 1lb.Your SCALES are right, you just don't have a 1lb weight any more.

HTH!

 

[Quester]

I started reading this thread because I recently became somewhat befuddled while working on a physics problem of my own.

When I went through physics courses back in the dark ages, the terms lbf and lbm did not exist. The terms used were lb for weight, which was regarded as a force due to the acceleration of gravity applied to 1 slug (the term used for mass). When I started seeing the lbf and lbm terms, I merely substituted them for the earlier terms.

However, my consternation came about when I first encountered the term "kg" used to refer to both "mass" and weight. I see examples all over the web of people asserting that 1kg of mass weighs 1 kg, which I cannot reconcile. I know that 1kg of mass removed thousands of miles from the nearest significant center of gravity will not weigh 1kg. The term "Newton" makes sense to me (when it is used) as it distinguishes between mass and force. When the term "kg" is used for both weight and mass, it can become confusing.

Perhaps I am still confused. Why are scales calibrated in kilograms (or grams, etc.) and not Newtons? Doesn't 1kg of mass weigh about 9.8 Newtons?

 

[Digcoal]

I started reading this thread because I recently became somewhat befuddled while working on a physics problem of my own.

When I went through physics courses back in the dark ages, the terms lbf and lbm did not exist. The terms used were lb for weight, which was regarded as a force due to the acceleration of gravity applied to 1 slug (the term used for mass). When I started seeing the lbf and lbm terms, I merely substituted them for the earlier terms.

However, my consternation came about when I first encountered the term "kg" used to refer to both "mass" and weight. I see examples all over the web of people asserting that 1kg of mass weighs 1 kg, which I cannot reconcile. I know that 1kg of mass removed thousands of miles from the nearest significant center of gravity will not weigh 1kg. The term "Newton" makes sense to me (when it is used) as it distinguishes between mass and force. When the term "kg" is used for both weight and mass, it can become confusing.

Perhaps I am still confused. Why are scales calibrated in kilograms (or grams, etc.) and not Newtons? Doesn't 1kg of mass weigh about 9.8 Newtons?

It is because scales measure opposing forces (Newtons) and are calibrated to report calculated masses (kg). The confusion is further exacerbated by common parlance involving asking for the WEIGHT of something and providing the calculated MASS given by a scale.

Kilograms is a measurement of mass. Newtons is a measurement of force, gravitational or otherwise.

I was in the same boat as you, at one point, so do not feel alone in this.