Weight vs. Mass: I know their NOT the same, but cannot comprehend why

[Diche Bach]

This is like physicists practicing to lawyer-talk . . . Can one of you guys clarify what is really the difference here?

In everyday usage, mass is sometimes referred to as "weight", the units of which may be pounds or kilograms (for instance, a person's weight may be stated as 75 kg). In scientific use, however, the term "weight" refers to the reaction force to any mechanical force on an object which acts to move it away from a natural path of free fall. No matter how strong the gravitational field, objects in free fall experience no forces of weight, and are thus said to be weightless.

The force known as “weight†is proportional to mass and acceleration in all situations where the mass is accelerated away from free fall. For example, when a body is at rest in a gravitational field (rather than in free fall), it must be accelerated by a force from a scale or the surface of a planetary body such as the Earth or the Moon. This force keeps the object from going into free fall. Weight is the opposing force in such circumstances, and is thus determined by the acceleration of free fall. On the surface of the Earth, for example, an object with a mass of 50 kilograms weighs 491 newtons, which means that 491 newtons is being applied to keep the object from going into free fall. By contrast, on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 newtons, because only 81.5 newtons is required to keep this object from going into a free fall on the moon. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g = 9.80665 m/s2 is the Earth's gravitational field, (expressed as the acceleration experienced by a free-falling object).

Actually this page on Mass versus weight is a bit better but I'd still like to hear what you guys say.

What I'm getting is: mass is what an object has when it is in freefall, i.e., how much inertia it has when gravity is not acting on it?

[Charzy]

Mass is constant for a given object. No matter where it is, its mass will stay the same.

Weight is mass multiplied by the gravitational strength acting on the object.

So, two identical objects with one being on the Earth and the other on the Moon will have equal mass, but the one on Earth will have a higher weight.

[Diche Bach]

Wow that was so much simpler than that garble on the first wiki page!

So basically, for macroscopic objects on Earth and not in freefall, mass and weight are synonymous?

[KOCOUR]

Well thats not easy to explain. But Shortest explanation would be, that:

Mass is constant that gives information of how big force i need to excert on object to change its state (like from still to moving), respectively how much will said object oppose the change.

And weight is variable that sais how much force you excert on the ground you staing on or rope you hanging on in given gravitational field.

The mass is allways the same at normal nonrelativistic velocity, but weight is changing in dependence on gravitational field that influences the object.

You can allso oversimplify and say that mass is : how much materiall is the object made of.

and weight is how heavy that materiall is in specific gravitional field.

[Awaras]

Wow that was so much simpler than that garble on the first wiki page!

So basically, for macroscopic objects on Earth and not in freefall, mass and weight are synonymous?

From the wiki page:

For all practical engineering purposes on Earth, mass in kilograms is converted to weight in newtons by multiplying by 9.80665 m/s2 (standard gravity).

Coloqually, we usually say that something weighs x kilograms but that is wrong, since the kilogram is a unit of mass, not of weight...

I think this is the greatest source of confusion.

Edited September 2, 2013 by Awaras

[Crush]

So basically, for macroscopic objects on Earth and not in freefall, mass and weight are synonymous?

Indeed.

An object with a mass of 20kg which rests on earth surface at sea-level and at the equator also has a weight of 20kg.

An object with the same mass of 20kg resting on on the moon has a weight of 3.3kg.

Weight is the force an object applies to the ground. A platform stable enough to carry a weight of up to 10kg would be able to carry this object on the moon, but would be crushed by it on earth.

But when it comes to moving an object (accelerating it), it's the mass which matters, not the weight. Accelerating a car with a mass of 1000kg to 10 m/s takes exactly the same amount of energy on moon and earth (when ignoring ground friction, which is relative to weight, and air-friction). A gun fired horizontally with a muzzle velocity of 300 m/s on earth will have the same muzzle velocity on the moon, even though the bullet weights less on the moon and will have the same destructive energy (speed² x mass) (again, ignoring air-friction).

Edited September 2, 2013 by Crush

[shand]

agreed Awaras.

the way i think of it is:

the mass is a property of a thing that effects how it is affected by other things. (acceleration is force/mass)

Weight is the property of a thing that effects how it effects things. (somethings weight is in newtons, force. so the weight is how much effort to raise/resist an object for example)

this obviously has flaws, but from a conceptual POV i find it helpful

[Stochasty]

An object with a mass of 20kg which rests on earth surface at sea-level and at the equator also has a weight of 20kg.

No!

Kilograms is strictly a unit of mass; the colloquial usage of kilograms as a measure of weight is a product of sloppy thinking and should be forgotten. The weight of a 20kg object resting on the surface of the Earth will be 182N.

The right way to think about the difference between mass and weight is this: mass is the amount of "stuff" an object has. When that "stuff" interacts with gravity, that interaction will produce a gravitational force equal to the product of the mass and the strength of the gravitational field. This force is what we call weight.

Remember Newton: force equals mass times acceleration. Weight is a force; it's value is equal to the mass of the object times the local gravitational acceleration (9.81 m/s^2, for the surface of the Earth). In other words, mass tells you how fast an object will accelerate when a given force is applied; weight tells you how much force you need to apply to counteract gravity.

The two are not in any way interchangeable; it is one of the travesties of the imperial unit system that we use weight rather than mass as the standard unit of measure.

Edited September 2, 2013 by Stochasty

[Crush]

No!

Kilograms is strictly a unit of mass. The weight of a 20kg object resting on the surface of the Earth will be 182N.

Among physicists, but laymen usually measure weight in kg.

So far, I haven't read a diet guide "Lose 100 Newton in 10 days".

[Awaras]

Among physicists, but laymen usually measure weight in kg.

So far, I haven't read a diet guide "Lose 100 Newton in 10 days".

Well, that's basically a coloquial oversimplification... When you say 'this thing weighs 1 kg' you are actually saying 'this thing weighs exactly the same as this standard weight with the mass of 1 kg'.

Edited September 2, 2013 by Awaras

[Stochasty]

You cross posted with my edit; sorry.

"But laymen..." should never be considered a valid excuse for an abuse of units. Laymen also use the term weight when they mean mass and vice versa. It's sloppy thinking, and it's wrong; it should not be presented as valid.

(For instance, you could lose weight by going to the moon, but that won't help your obesity problem; what you really want to lose is mass, so "losing 10kg" would be correct, if only by the accident of being doubly incorrect.)

Edited September 2, 2013 by Stochasty

[Diche Bach]

Among physicists, but laymen usually measure weight in kg.

So far, I haven't read a diet guide "Lose 100 Newton in 10 days".

Very interesting that the confusion is really so deeply ingrained in popular terminology. I coulda swore that a kg was a unit weight, but now I know better!

[lajoswinkler]

Weight is the force objects with mass excert upon something (a scale perpendicular to the vector of gravitational force, for example) when the object is in the gravitational field.

Less powerful gravitational field, less weight. Weight's units are newtons. So you can say that the average weight of a human body is a bit less than 700 newtons.

The fact many people use weight as a synonim for mass is purely a linguistic issue. They are not actual synonims.

Mass is an intrinsic property of some objects. Its units are kilograms. In gravitational fields it is visible as weight, in accelerated (linear or circular, doesn't matter which vector we're talking about) systems is visible as inertia. That's why you can stand on Gilly and hold an elephant with your small finger, but you'll have a very bad time if the animal is moving and you have to stop it, or if you want to move it and it's resting.

In relativistic systems (moving at a significant fraction of c) it shows up with several interesting phenomenons, but let's not complicate things.

Here's an analogous example. Imagine you're in the middle of nowhere, in space. Nothing is around you. Pure vacuum. You aren't orbiting anything because the space is just a void. You've got two spherical objects, let's say two glass balls, an insulated spring scale and some insulated apparatus for holding one ball and the spring, facing each other, not letting them move around.

One ball is negatively charged, another is positive. Doesn't matter how much. You put one ball into the apparatus, and you fix the second on the scale's hook and you fix the scale on the apparatus. The balls attract each other and the spring scale registers a pull and a number.

In this analogous case, weight would be the electrical force registered on the scale and mass would be the charge.

(That's where the analogy stops, though. Charges can be positive and negative, whether mass is always positive. Electrical and gravitational force are very different things.)

F = m * a

9.81 N = 1 kg * 9.91 m/s^2 (a is not the same on every spot on Earth, but 9.81 m/s^2 is the most used and remembered value)

I always say that something has a mass of x kilograms. I never use weight. I guess that's because of the education, but I'm glad it's like that. It's the correct name.

Edited September 2, 2013 by lajoswinkler

[shand]

actually the problem here ISNT the misuse of units. its the misuse of TERMS.

when the diet says "lose x kg of weight" they really mean losing xkg, what they DONT mean is weight. they mean mass. (otherwise they'd tell you to climb a hill)

think of ALL coloquial uses of the term "weight" (or weighs) and how many of them really mean mass. An aircraft weighs this much - oh really, tarmac or cruising altitude? this oxygen cylinder weighs this much - at the bottom of the ocean? sealevel? top of mount everest?

The only way you can define the "weight" of a thing that changes location is by refering to the mass.

the problem is mass is really hard to measure (unless you know the gravitational force at your exact location)

[Stochasty]

actually the problem here ISNT the misuse of units. its the misuse of TERMS.

I'd say it's both. Especially on the American side of the pond; not only do we tend to say weight when we mean mass, we also measure in pounds. Compounded stupidity there.

[shand]

I was going to ask what pounds where - i'd never thought about it. arent they just an old equivilent to kg (as in, rather then N)?

EDIT: Wiki shows it as a unit of mass - so much like kg. So don't know how that means "espeically" in america?

[Stochasty]

Nope. Pounds are a unit of force. (Unless you are talking about pound-mass, which is an even more confusing hack added on to the unit system once engineers started to realize that the difference between mass and weight mattered; the imperial system doesn't even have an independent unit for mass.)

EDIT: The current, formal definition of the pound is as a unit of mass, but that is neither how it is typically used (outside of science, which uses metric at any rate) nor its original meaning. The pound is traditionally a measure of weight; pound-mass was only invented as a concept once it became clear that weight was a bad standard.

Edited September 2, 2013 by Stochasty

[Charzy]

There's two big systems of measurement. Imperial and Metric.

Metric is kilograms and centimeters and meters, and everything is based on tens.

Imperial has pounds and inches and yards, and everything is based on whatever the hell the creator thought made a good number. Officially, only America and one other country (UK?) use Imperial, because it is vastly inferior to Metric.

[lajoswinkler]

Pounds are now units of mass, just like kilograms, but they're part of imperial system, whether kilograms belong to international system of units (SI).

Almost every country uses metric system because it's easy to use and consistent with other units. Newtons are defined as kilograms multiplied by metres and divided by the squared seconds.

USA is currently under metrification, but it's sluggish. SI units are teached in schools, but as I've heard, most people don't really understand them because they're abstract to them, as everything around them is measured in imperial units.

I'll never understand why is one of the most developed nations in the world so vehemently opposes the complete metrification. It's the only way to go, and they choose to go slow. People will never learn it unless metric system is forced through the media. For example weathermen, they should use km/h, not mph. Road to complete metrification is not an easy one. There are steps you have to take, but they're worth it. UK and Canada did a great job.

You can see here the current status.

http://en.wikipedia.org/wiki/File:Metrication_by_year_map.svg

USA, Liberia and Burma are still not official.

Edited September 2, 2013 by lajoswinkler

[shand]

Charzy: we use metric in the UK for practically everything except distance (and even then only for road distances). so practically the only imperial units i have any concept of are miles and yards.

i knew they were different, and how. i just wasnt aware what pounds were measuring. and it sounds like pounds doesnt even know half the time!

To be fair, i think it IS a measure of weight, considering you have pounds thrust - which makes it a completely useless unit which would be much better off not existing

EDIT: follow up question why isnt there a metric time unit if its so useful?

Edited September 2, 2013 by shand

[Stochasty]

EDIT: follow up question why isnt there a metric time unit if its so useful?

There is: seconds. Milli-, micro-, nano-, pico-, etc. are already part of the standard vernacular. Kilo-, Mega-, Giga-, etc. are used less often but are no less valid. (They will likely come into more common usage once humanity moves beyond the Earth; minutes, hours, and days are less useful when you aren't standing on a spinning ball of rock, and are counter-productive when you are standing on the wrong spinning ball of rock.)

[Mr Shifty]

We in the US continue to use the Imperial system only colloquially, i.e. in every-day life. Scientists and engineers these days almost all use metric (and we certainly prefer to use metric even when forced to use Imperial.) Part of this is just <ahem> inertia. Even when I work in kg and km, I'm always converting back to miles and pounds in my head because they're what I understand intuitively. It's like speaking another language.

Pounds, btw, are a unit of mass. When talking about force, you'll invariably see lbf (pounds-force) used, which are each the amount of force gravity exerts on one pound of mass at Earth's sea level = 32.174049 lb*ft/s^2

[Mr Shifty]

There is: seconds. Milli-, micro-, nano-, pico-, etc. are already part of the standard vernacular. Kilo-, Mega-, Giga-, etc. are used less often but are no less valid. (They will likely come into more common usage once humanity moves beyond the Earth; minutes, hours, and days are less useful when you aren't standing on a spinning ball of rock, and are counter-productive when you are standing on the wrong spinning ball of rock.)

Read Vernor Vinge's A Deepness in the Sky. The characters there all use metric time units based on seconds: ksecs, Msecs, etc. I had to come up with a quick-and-dirty conversion in my head: 1 ksec is about 15 minutes (actually 16.7 minutes.) 1 Msec is about 2 weeks (actually 11.6 days.) A year is about 30 Msecs, etc.

[Monkeh]

Thrust to Mass ratio anyone?

[Holo]

Kilograms are a measure of both mass and weight. Weight kilograms are equal to mass kilograms multiplied by the local gravity.