Mass and weight

Discussion in 'Physics' started by logearav, Oct 4, 2011.

  1. logearav

    Thread Starter Member

    Aug 19, 2011
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    Revered members,
    Mass is the quantity of matter contained in the body and Weight is the force of attraction due to gravity. But why in food products or for that matter in any product , say a health drink, it has been mentioned Net Weight 300 gram.? Shouldn't it be Net Mass?
     
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Yes, you are right. However, weight is equal to mass times acceleration. Acceleration in this case is gravity, and since gravity is constant here on earth, "mass" and "weight" are sometimes used interchangeably. It is technically incorrect to say this, as mass and weight are not the same, but they do have the same value on earth.

    But technically, yes, food packaging should say Net Mass, not Net Weight.
     
  3. BillO

    Well-Known Member

    Nov 24, 2008
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    It could be that they actually weigh the product, rather than calculate it's mass.

    Food items are especially difficult to determine their mass as they are often not uniform enough in density to calculate this from some other measure, such as volume. However, it's a simple matter to fill a bag or box or mold on a scale and stop once it reaches a fixed weight.

    So, weight might not be such a bad choice for items packaged and sold on earth, and is therefore undoubtedly technically more correct.
     
  4. debjit625

    Well-Known Member

    Apr 17, 2010
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    NO, actually you answered your question ,in measurement of mass their is no influence of gravity, but in case of the products you are talking about are measured on planet earth and we have gravity so no way a product can be measured in Net Mass, unless space research programs are involved.
    You can say the unit used for weight is wrong i.e.. it should not be just gram, kilo- gram ...etc ,for example it should be like this 100 Kg.m/s^2 i.e.. 100 kilo-gram meter per second square, but we just use the unit for mass which is wrong.
     
  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    but who cares how heavy your box of cereal is? Wouldn't you rather know the mass (the actual amount) of cereal you've got?
     
  6. Blackbull

    Well-Known Member

    Jul 26, 2008
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    I recall my physics teacher, at school, saying that a mass at the poles would have a different weight at the equator, but that was 50 years ago!!!
     
  7. BillO

    Well-Known Member

    Nov 24, 2008
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    Well, I guess if I were somewhere else other than Earth. Even then, I could do the math and get the mass.
     
  8. someonesdad

    Senior Member

    Jul 7, 2009
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    Mass is measured all the time on Earth and it's not technically difficult to do with a mass balance as long as you have standards of mass -- no need to invoke any space research :eek:. It is in fact more difficult to measure weight accurately because you need to know the mass and the acceleration of gravity, both to better accuracy than you want to state the weight.

    Common usage (and people that don't know any better) express weight in mass units. The acceleration of gravity is implied in such a number if you want it to be a real force. Thus, some people will use a non-SI unit like "kgf", which means the force experienced in Earth's gravitational field by a mass of 1 kg. Since the acceleration of gravity varies depending where you are on the Earth, this nomenclature isn't used by people doing careful work (roughly better than half a percent) because it's ambiguous.

    Similar confusion exists over the term "atomic weight". Most working scientists just point the confusee to a basic physics text and get on with life. :p
     
  9. debjit625

    Well-Known Member

    Apr 17, 2010
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    Which instrument you are talking about that can measure mass more accurately on earth rather weight i.e.. (mass under gravity) can you give any source, if its so much available I would like to buy one not kidding...I hope it will not cost too much and If you think you will use standards of mass then sorry because that mass will be under the influence of gravity and please don't use math to get mass form weight...

    As per I know Weight = mass * acceleration due to gravity i.e.. W = mg and g is approx 9.81 m/s^2...
    Of course you can say a balance will not measure the perfect or accurate weight but it will measure weight rather mass on earth. In general I found different kinds of balance in markets or in shops for measuring products, they don't use any other type of instrument...

    Anyway I feel no measurement is accurate,as to some extent their will be errors...
     
  10. BillO

    Well-Known Member

    Nov 24, 2008
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    A simple balance scale.

    To wit:
    [​IMG]

    You just need one of these and a set of standard masses that will cover the measurement range.


    Alternatively, one of these, called a Beam Balance. It has weights attached. Again, you need to get one that covers the range of interest.
    [​IMG]
     
  11. debjit625

    Well-Known Member

    Apr 17, 2010
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    Yes,you can use that but you have to take it to outer space,as if you use that scale on earth gravity will play its role..
     
  12. BillO

    Well-Known Member

    Nov 24, 2008
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    A balance needs some gravity, or cceleration to work, but the amount will not influence the measurement or accuracy. The only place they will not work is in zero gravity.

    A balance weighs a known mass against a test mass. So, it will do exactly the same job here or on the moon or on Jupiter. A gram will measure as a gram.
     
  13. someonesdad

    Senior Member

    Jul 7, 2009
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    debjit625, I don't mean this disrespectfully, but I'm a bit surprised at your apparent unfamiliarity with balances. They have been used for thousands of years and are well-developed. The highlight was probably the precision analytical balances that chemists used to measure relatively small masses. With care, IIRC, you could work to five significant figures or better. I figured nearly everyone who took a science class in late grammar school or high school would have come across a balance such as the class Harvard trip balance -- of which the good ones could stand up to nuclear explosions easily and almost always under teenagers' use.

    Ultimately, I can think of a few objections to using a balance for measuring mass:

    1. It might somehow depend on the Earth's gravitational field.
    2. The "gravitational mass" that it measures might be different from the "inertial mass" that you want.
    3. You might not be in a gravitational field.

    The first objection is overruled by the fact that Earth's gravitational field doesn't vary significantly over the dimension of the typical scale (say, a meter or less in a direction perpendicular to the line of action of gravity). The second objection is taken care of by virtue of the experiments that have demonstrated the equality of gravitational and inertial mass. Coincidentally, I just looked that up about a week ago and found that the experiments show equality to around 12 or slightly more significant figures. I can't help with objection #3. :p

    BillO's pictures show an Ohaus triple beam balance. I've got a similar model made by Ohaus (measures to 2610 g and resolves to 0.1 g). I used similar balances in school when I was a kid. I wouldn't part with it for the world, nor would I buy an electronic balance. If you take care of them, these mechanical balances will work indefinitely. Some day if I get some extra toy money, I'll supplement it with an Ohaus Cent-i-gram balance, another excellent design that gives 0.01 g resolution. Of course, that's the problem with too many hobbies: too many needed toys and not enough money...
     
  14. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Your standard balance scale here on earth is calibrated for earth's gravity (I assume we're talking about an electronic balance, not one of those old-style pivot ones). If you put an electronic balance on the moon, it will measure much less mass than it would on earth. Balances on earth rely completely on the force of gravity.
     
  15. BillO

    Well-Known Member

    Nov 24, 2008
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    Then its not a balance scale.

    Balance scales work by balancing a known mass with a test mass. If it does not work this way, then it is not a balance scale. If it does work this way, then it is gravity magnitude independent.

    Yes, a balance scale needs a force (gravity or acceleration) to work, but the result is not dependent on the magnitude of that force because it acts equally on the test mass and the known mass. using either of the two scales I showed below, the results would be exactly the same on the moon or Jupiter as they would be on earth

    Be careful, as the term 'balance' has been incorrectly applied to analytical scales, which are actually strain cell, or strain gauge scales. I own one, and it definitely measures weight, not mass.
     
  16. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    This is what is commonly referred to as a "balance" nowadays. I agree, it's a bit different from the original idea, but it's what it is called:

    [​IMG]

    I agree with this part. That was my point. Most "balances" use gravity (hence weight) of the object to find its mass. I was saying that it does not actually measure mass, and that it should be measured some other way.
     
  17. Georacer

    Moderator

    Nov 25, 2009
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    Since we have the ability, we should call things as they are. Balance is one thing and scale is another. We avoid the confusion that way.
     
  18. BillO

    Well-Known Member

    Nov 24, 2008
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    100% agreed.
     
  19. DerStrom8

    Well-Known Member

    Feb 20, 2011
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  20. BillO

    Well-Known Member

    Nov 24, 2008
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    Not a reflection on you DerStrom8, and no such intention from my part.

    The practice is all too common, in too many areas.
     
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