Difference between weight and mass?

DaleSpam

Quote by Mr. Einstein

I would definitely have to disagree with you. The kg is a dimensionless unit. And my description of the use of a scale was accurate.

No, the kg has dimensions of mass, it is not dimensionless. See the official definition at: http://www.bipm.org/en/si/si_brochur.../kilogram.html

As well as the Wiki entry here: http://en.wikipedia.org/wiki/Kilogram

jedishrfu

Folks we are getting sidetracked here. The discussion is about mass vs weight not about whether mass as measured in whatever units of mass is dimensionless or not.

DaleSpam

Quote by Mr. Einstein

A known quantity on matter is compared to an unknown quantity. And the ratio of these two quantities {or weights) is what is referred to as mass.

This part is where you go wrong. When you write m = 5.0 kg what you are saying is that the dimensionless ratio between the mass of the object and the mass of the international prototype kilogram is 5.0. I.e. m/kg = 5.0 (dimensionless). Because the kg is dimensionful (dimensions of mass) and because 5.0 is dimensionless, in order to make that equality work m must also be a dimensionful quantity (with dimensions of mass).

DaleSpam

Quote by jedishrfu

Folks we are getting sidetracked here. The discussion is about mass vs weight not about whether mass as measured in whatever units of mass is dimensionless or not.

Understood. However, when misinformation is posted it needs to be corrected. Plus, I think that your post 3 took care of the OP's question quite well, I don't see that anything can be added.

jedishrfu

Quote by DaleSpam

Understood. However, when misinformation is posted it needs to be corrected. Plus, I think that your post 3 took care of the OP's question quite well, I don't see that anything can be added.

Agreed and thanks.

GideonRey

Correct me if I'm wrong, but this is the way I learned mass vs. weight.

Weight is the measurement of a gravitational force on a mass (I.e. A lead ball weighs 200 lbs on Earth, but only 125 lbs on an imaginary planet I just made up.). Whereas mass is the actual "quantity" of an object (I.e. Two lead balls with the same mass, will have the exact same amount of lead atoms.)

Buckleymanor

Two lead balls with the same mass, will have the exact same amount of lead atoms.)

You could have one of the lead balls with one less atom but warmer and it could still have the same mass as the other lead ball.

Buckleymanor

Two lead balls with the same mass, will have the exact same amount of lead atoms.)

Or you could have two amounts of the same gas both with the same amount of atoms but one warmer than the other.
The warmer one haveing more mass but lighter if placed within the other.

dextercioby

Quote by Buckleymanor

You could have one of the lead balls with one less atom but warmer and it could still have the same mass as the other lead ball.

Let's not get sidetracked by special relativity, shall we?

I think the OP's question has been answered: weight is a force given by Newton's universal attraction law or any simplification of it. The mass in Newtonian (Galilean) physics has the same meaning it does in chemistry: the quantity of substance in a certain physical body. A discussion of CGS vs MKS vs UK/US units is a totally different thing.

Buckleymanor

Let's not get sidetracked by special relativity, shall we?

It was not the intention to be sidetracked just the observation that mass can change with temperature and position.

CatWhisperer

I think the easiest way to think about mass is that it will be the same wherever you are, whereas your 'weight' on Earth is dependent on Earth's gravity, and would change if you were on the Moon or Mars. It so happens that you weigh yourself in kilograms on Earth the result DOES equal your mass, but the two concepts are different.

Lay-persons talk about weight in kg which is what is confusing, as kg is actually a unit of mass, and you would need to calculate your force in Newtons (the product of your mass and the Earth's gravitational acceleration of 9.8m/s^2) to discover your weight on Earth (in Newtons).

TheAbsoluTurk

The interesting thing to me is that while the Europeans routinely view Americans as less educated, they never state their weights in newtons.

CatWhisperer

That's because you can't buy a scale that measures in Newtons. It's scientific convention vs. lay-person speak. It happens. Just like with slugs vs. lbs in the imperial system, or interchanging the word 'speed' with 'velocity'.

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jedishrfu	Folks we are getting sidetracked here. The discussion is about mass vs weight not about whether mass as measured in whatever units of mass is dimensionless or not.

GideonRey	Correct me if I'm wrong, but this is the way I learned mass vs. weight. Weight is the measurement of a gravitational force on a mass (I.e. A lead ball weighs 200 lbs on Earth, but only 125 lbs on an imaginary planet I just made up.). Whereas mass is the actual "quantity" of an object (I.e. Two lead balls with the same mass, will have the exact same amount of lead atoms.)

TheAbsoluTurk	The interesting thing to me is that while the Europeans routinely view Americans as less educated, they never state their weights in newtons.

CatWhisperer	That's because you can't buy a scale that measures in Newtons. It's scientific convention vs. lay-person speak. It happens. Just like with slugs vs. lbs in the imperial system, or interchanging the word 'speed' with 'velocity'.