- #1

But how can you define mass? I'm from Europe and we measure both mass and weight in kilograms.

So I wonder how you can decide your mass in kg if you aren't affected by any gravitational pull?

Hope you understood the question

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- Thread starter Maxwells Demon
- Start date

- #1

But how can you define mass? I'm from Europe and we measure both mass and weight in kilograms.

So I wonder how you can decide your mass in kg if you aren't affected by any gravitational pull?

Hope you understood the question

- #2

ranger

Gold Member

- 1,685

- 2

Mass is the measure of the inertia (resistance to acceleration) of an object or the amount of matter it contains. We can measure mass using an inertial balance or a triple beam balance.

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- #3

Do i get this right..?: Even if you're weightless (without any gravitational pull) you have to put a force on a mass to accelerate it, right?

please explain so i can get a deeper understanding of mass

- #4

billiards

- 771

- 15

Do i get this right..?: Even if you're weightless (without any gravitational pull) you have to put a force on a mass to accelerate it, right?

please explain so i can get a deeper understanding of mass

First of all no, strictly speaking your weight is not in units of kg, in fact in physics your weight is a force and as such it has units of Newtons N.

There's a very simple law, I think it's Newton's 2nd law, you've probably come across it. It basically states:

F=ma

The way I think about it is your weight is a force F, gravity provides an acceleration a, if you want you can call the acceleration g to help you remember that it is due to gravity.

F=mg

...or if you like...

weight=mg

...rearrange...

mass=weight/g

your mass is your weight divided by the acceleration due to gravity.

- #5

Mentz114

- 5,432

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...Do i get this right..?: Even if you're weightless (without any gravitational pull) you have to put a force on a mass to accelerate it, right?

Very emphatically yes. Newton's laws give

acceleration = force/mass

The more massy a thing is, the harder you have to push.

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- #6

The more massy a thing is, the harder you have to push.

alright, so if an object is floating around in space, weightless, and the object has a high mass I may not be able to push it away from me, accelerate it, even though its weightless. Because I can't put enough force on the high mass object to accelerate it??

- #7

russ_watters

Mentor

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- #8

Russ, ahh okay...! now I understand, thank you.. I also push against a bit of friction of course?

- #9

cristo

Staff Emeritus

Science Advisor

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I'm from Europe and we measure both mass and weight in kilograms.

Really? I don't think you do; people probably use the terms mass and weight interchangeably, when they shouldn't!

- #10

rcgldr

Homework Helper

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Both the large mass and you accelerate away from each other, but if the mass is large, you accelerate more than the mass.alright, so if an object is floating around in space, weightless, and the object has a high mass I may not be able to push it away from me, accelerate it, even though its weightless. Because I can't put enough force on the high mass object to accelerate it??

Regarding a specific value for mass, I'm not sure if there's an extremely accurate definition. A kilogram of mass is defined by one actual instance of an object defined to be a kilogram. A scale would be used to create duplicates.

I'm not sure if there's an atomic definition for a specifc mass that correlates well with a kilogram (eg, how many protons really equal 1 kg of mass?).

- #11

of course we also measure mass in other units, but we mostly use kg also in school..

- #12

f3nr15

- 22

- 0

Mass is a constant quantity regardless of your location.

Weight is the gravitational force on mass ..

W = m

Mass is in Newtons (N)

Weight is in kilograms (kg)

and g is in ms

- #13

- #14

f3nr15

- 22

- 0

Oh ok I thought the platinum-iridium bar kept at 0

Later the metre was to be measured from the wavelength of light emitted by an excited kypton (-86 I think ?) isotope.

Then later the metre was defined to be the distance light travels in 1/299 792 458th of a second as you say ...

- #15

The meter was first defined by an artefact, a meter stick, and this is still kept in Paris along with the kg-prototype... whether the meter stick is defined by the wavelength of light emitted by an excited krypton isotope or not, I don't know..

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- #16

rcgldr

Homework Helper

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A meter was redefined to be based on wavelengths of light. The yard stick standard was dumped, and an inch was re-defined to be exactly 2.54 cm. Time is defined by the decay rate of some radio-active isotope. Only mass is still stuck being defined as some piece of metal, instead of so many molecules of a certain element. There are fairly accurate approximations for how many molecules of elements or compounds have 1 kg of mass, but this is not the standard.The meter was first defined by an artefact, a meter stick, and this is still kept in Paris along with the kg-prototype... whether the meter stick is defined by the wavelength of light emitted by an excited krypton isotope or not, I don't know..

- #17

- #18

arunma

- 927

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But how can you define mass? I'm from Europe and we measure both mass and weight in kilograms.

So I wonder how you can decide your mass in kg if you aren't affected by any gravitational pull?

Hope you understood the question

This is certainly a good question. I think the others have already provided excellent responses. It's important to remember that there are

However, I just want to add that one of my freshman physics professors, who is very accomplished in high energy physics, was once asked by my fellow student to explain what mass is. Her response: "I don't know what mass is." I guess that the "true nature" of mass is a bit more of a metaphysical question than a scientific one.

I'm not sure if there's an atomic definition for a specifc mass that correlates well with a kilogram (eg, how many protons really equal 1 kg of mass?).

There are approximately one thousand moles (one mole is [tex]1.602 \times 10^23[/tex]) of protons in a kilogram. This is because the periodic table's atomic weights, which are measured in amus (atomic mass units), also give the molar masses of the elements in grams per mole. It so happens that the molar mass of hydrogen (basically the same thing as a proton) is about 1 g/mol.

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