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Define the differance between Mass and Weight

  1. Feb 4, 2007 #1
    Defining the differance between Mass and Weight. It's something that I've not been able to grasp with the info I have. Can someone direct me to a clear explanation. From: firstly a laymans interpretation and then some references to the math involved to varify the basic principal differances involved?

    Thanks
    Robin07
     
  2. jcsd
  3. Feb 4, 2007 #2
    Use google. I'm not great with words, but I think weight, is just the mass multiplied by gravity.
     
  4. Feb 4, 2007 #3

    ranger

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    You can think of mass as the amount of matter an object has. Mass does not change as the force of gravity or even the shape of the object changes. If you have a sheet of paper and you crumble it up, the mass before and after are the same.

    Now weight is a property that mass has when in a gravitational field. In other words, weight is the [downward] force exerted upon an object [mass] by gravity. Weight has a tendency to be variable depending on your location. To be more specific, becuase the force of gravity varies slightly depending on where you are on earth, your weight will change while the mass remains constant.

    W = mg
    m - mass of the object
    g - force of gravity
     
  5. Feb 4, 2007 #4

    DaveC426913

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    Weight is wholly dependent on gravity. A loose nut in a space station or an interstellar probe - they both have mass, even though neither have any weight.
     
  6. Feb 4, 2007 #5

    russ_watters

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    Consider pushing a car vs pushing a person on a bike. Why is it harder to push the car?

    Since we reside on Earth, it is easy to couple weight and mass, but if you put that car and that bike on the moon, it will be just as difficult to push the car as it is on Earth.
     
  7. Feb 5, 2007 #6

    D H

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    The gravitational force in low earth orbit is nearly that on the earth's surface. A loose nut in the space station weighs about 95% of its weight on the surface of the earth. So why do we say that the space station is a zero-gee environment?

    There are two kinds of weight: 'actual weight', the force on an object due to gravytiy, and 'apparent weight', the total force on an object less gravity. The loose nut in the space station has a significant actual weight (the gravitational force is 95% of that on the Earth's surface) but a negligible apparent weight (gravity is the only force acting on the nut). For an object stationary on the Earth's surface, the actual weight and apparent weight vectors are nearly equal in magnitude but point in nearly opposite directions. The actual weight vector points toward the center of the Earth while the 'apparent weight' vector points along the local vertical.

    Actual weight is a useful concept for aeronautic engineers, who use weight to represent the force of gravity acting on an airplane. Actual weight is, however, merely a mathematical construct, as there is no way to measure it directly. Your bathroom scale, the accelerometer on the airplane, and the otoliths in your inner ear all measure apparent weight.
     
  8. Feb 5, 2007 #7

    russ_watters

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    I've never heard of the two different definitions, and since it is generally obvious what someone is talking about, I don't really see the need.
     
    Last edited: Feb 5, 2007
  9. Feb 5, 2007 #8

    D H

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    It is not generally obvious. People often refer to the space station as a weightless environment. With weight defined as the force due to gravity, objects in the space station are far from weightless.

    When people refer to the space station as a weightless environment, they are talking about apparent weight. When aircraft designers talk about weight as a force acting on the place, they are talking about actual weight.

    This is a physics forum. Loosy-goosy language is fine for non-technical applications. It is not fine for science.
     
  10. Feb 5, 2007 #9

    russ_watters

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    When is it useful to define the environment inside a space station as not being weightless and how is it not obvious what someone means when they say 'an astronaut in the space station feels weightless'?
     
  11. Feb 5, 2007 #10

    FredGarvin

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    Well then I suggest you re-read the post you quoted. Dave said "A" space station or interstellar probe. I see no mention of the ISS except in your posts.
     
  12. Feb 5, 2007 #11

    D H

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    The OP asked for the difference between weight and mass. Post #3 (Ranger) defined weight as the downward force due to gravity. Post #4 (DaveC) talked about a loose nut in a space station as being weightless. We have two people giving contradictory answers because of the use of loose terminology.

    This confusion between apparent and actual weight has popped up many times in this forum and elsewhere. Using a more specific terminology removes the confusion. Now what is wrong with that? BTW, the terminology isn't mine.
     
  13. Feb 6, 2007 #12

    DaveC426913

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    Well... D_H is right.

    In Earth orbit, you are under strong, competing forces - gravity and inertia (the fictitious 'centrifugal force'). You would be apparently weightless - exactly the same as in a free-falling elevator.

    OTOH, way out on the edge of the solar system, the loose nut would be a different kind of near-weightlessness - it would be experiencing a very small amount of gravity and a very small amount of inertia.

    In both cases though, the mass is constant.
     
  14. Feb 6, 2007 #13

    NoTime

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    Err...
    Mass is a Measure of the Amount of Inertia.
    Inertia is an objects resistance to acceleration.

    The confusion comes in because Mass is normally determined by weighing an object.
    Works fine with the constant (for most purposes) acceleration of gravity on the surface of the Earth.

    However, Weight is a measure of acceleration.
    Mass is the value of Inertia, an intrinsic property of an object, wherever it is.

    So, as a practical matter you say Mass if you are referring to an objects property and Weight if you are referring to how it is being accelerated.
     
  15. Feb 7, 2007 #14
    My apologies for not being more involved but this is not to say that I haven't read your posts. I'll re-read the post this comming week end and repond with my clearer understanding thanks to you all.

    Again My apologies
    Robin07
     
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