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Mass vs weight

  1. Mar 2, 2007 #1
    I know that weight is how much you weigh when you are affected by a certain kind of gravitational pull.

    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. jcsd
  3. Mar 2, 2007 #2


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    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.
    Last edited: Mar 2, 2007
  4. Mar 2, 2007 #3
    Ranger, could you explain that "deeper"?

    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
  5. Mar 2, 2007 #4
    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:


    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.


    ...or if you like....




    your mass is your weight divided by the acceleration due to gravity.
  6. Mar 2, 2007 #5


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    Very emphatically yes. Newton's laws give

    acceleration = force/mass

    The more massy a thing is, the harder you have to push.
    Last edited: Mar 2, 2007
  7. Mar 3, 2007 #6
    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??
  8. Mar 3, 2007 #7


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    Consider how much effort it takes to push a car. You are not pushing against the car's weight (unless you are pushing it uphill), just against its inertia. And that inertia is the same whether it is on the earth, on the moon, or floating in space, so you'll have the same difficulty accelerating it.
  9. Mar 3, 2007 #8
    Russ, ahh okay...! now I understand, thank you.. I also push against a bit of friction of course?
  10. Mar 3, 2007 #9


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    Really? I don't think you do; people probably use the terms mass and weight interchangeably, when they shouldn't!
  11. Mar 3, 2007 #10


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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.

    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?).
  12. Mar 3, 2007 #11
    of course we also measure mass in other units, but we mostly use kg also in school..
  13. Mar 3, 2007 #12
    How I define weight and mass.

    Mass is a constant quantity regardless of your location.
    Weight is the gravitational force on mass ..

    W = mg

    Mass is in Newtons (N)
    Weight is in kilograms (kg)
    and g is in ms-2 or Nkg-1 (I think)
  14. Mar 3, 2007 #13
    a kg is defined by an artefact kept in paris made out of platinum. This is defined as exactly 1 kg. The meter used to be such an artefact too, but it was defined by Ole Rømer, I think, and is the distance light goes in vacuum in 1/299 792 458th of a second
  15. Mar 3, 2007 #14
    Oh ok I thought the platinum-iridium bar kept at 00C in Paris was supposed to be exactly one metre in length and copies of it was distributed across the world and was to measure distance not mass.
    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 ...
  16. Mar 3, 2007 #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..
    Last edited by a moderator: Mar 3, 2007
  17. Mar 3, 2007 #16


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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.
  18. Mar 3, 2007 #17
    yeah.. Do you have a good way to define mass yourself? you know, a way you would like it to be defined?
  19. Mar 3, 2007 #18
    This is certainly a good question. I think the others have already provided excellent responses. It's important to remember that there are two kinds of mass (that is to say, mass has two different physical effects). Inertial mass is defined as resistence to force. Gravitational mass is a property that causes an object to impart more gravitational force on other objects, in much the same way that electric charge causes electric force. To date, there is no experiment that has deduced any difference between inertial and gravitational mass. As far as gravity is concerned, the fact that mass creates force and resists force is actually the reason why all objects fall at the same acceleration.

    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.

    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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