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Apparent Weight Concept Help

  1. Oct 24, 2006 #1
    I'm trying to understand the utility of the concept of apparent weight. I'm not trying to over throw centuries of science, but it just seems not to be very useful to me.

    Why is it useful to say 'the apparent weight in an accelerating elevator is more (less)' rather than to just say 'you weigh more (less) in an accelerating elevator'?

    I mean, if you are in an accelerating reference frame, and put something on a scale, well, it seems to me, what the scale says it what it weighs!

    Perhaps the usefulness of this will become clear as I learn more?

    Thanks,
    Dorothy
     
  2. jcsd
  3. Oct 24, 2006 #2

    Andrew Mason

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    The problem is that there are different definitions of weight! If only everyone would agree on a definition.

    We refer to orbiting astronauts as being weightless. But we say W = mg. Since g = GM/R^2, mg is definitely not 0 for the astronauts.

    I agree that weight should be the Normal force, which varies depending on the other forces acting. But that is not a universal definition. W=mg means that if vertical forces other than gravity are acting the Normal force is different than W. In this case, (ie where we define W=mg) N is the apparent weight.

    AM
     
  4. Oct 24, 2006 #3

    andrevdh

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    Pilots flying in high speed aircraft doing manuevers experience a lot of g forces. Same with astronauts when they are launched. Or the inhabitants in the space station are constantly falling towards the earth and are in a state of weightlessness. All these people will tell you that these effects are very real for them and have serious consequences for them (Too much gs could cause a pilot to pass out and crash. Weigtlessness have detrimental effects on physical - and psychological health).
     
  5. Oct 24, 2006 #4
    Well, to take the textbook definition, weight is the gravitational force that acts upon an object. When you don't change your height from the centre of the Earth too much, or your mass, it is always constant.

    Apparent weight is pretty much the measured weight that you obtain while measuring it using things like a Newton scale etc.
     
  6. Oct 24, 2006 #5
    I know what apparent weight is. What I don't understand is the utility of the concept. My textbook makes a big deal out of this, but I have yet to see it add anything but an unnecessary complication to things. Actually, I just think it is a stupid idea, to be totally truthful.

    I know I am at a very elementary level of physics, so I was wondering if it had some use at a more advanced level.
     
  7. Oct 24, 2006 #6

    OlderDan

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    You have already seen that people have different points of view on this, but I think there is a good reason for making a distinction between weight and apparent weight. I know you know the difference, but let's not forget that weight is not an inherent property of an object (in contrast to mass). Weight is the gravitational force and varies with distance from the earth (or whatever big blob of mass you are near). The "weightless" astronaut mentioned earlier is not called the "apparently weightless" astronaut becaue we are a bit sloppy and lazy about how we use words sometimes, but not only are these astronauts apparently weightless, they truly weigh less in space than they weigh on the earth. It is nice to have a short word reserved for identifying true weight. I would rather say that an astronaut weighs less in orbit than on earth than say the gravitational attraction between the earth and the astronaut is less in orbit than on the earth. Furthermore, as you well know, and others have observed, there are all kinds of changes that take place in an objects apparent weight that leave its true weight unaltered. If you are going to redefine apparent weight to weight to save on using a word, then you are going to have to redefine what is now called weight to preserve the distinction. If you want to change the name of blue to red then you need to come up with a new name for red.
     
  8. Oct 25, 2006 #7

    Andrew Mason

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    A balance scale will measure mass. This does not vary from place to place on the earth. But a spring scale will measure apparent weight and that will vary from place to place (as does weight=mg). There is no device available that measures actual weight (mg). So why not call the apparent weight, actual weight and call mg just mg? That is all DW is saying and I tend to agree with her.

    AM
     
  9. Oct 25, 2006 #8
    Just think of it as the word 'Weight' by itself has been specially reserved for the force that is acting. As in, it is the TRUE force of gravity that acts upon an object. In the case of a spring scale, we are actually measuring the Tension in the spring, which is almost equivalent to its weight, Newton's third law and all that.

    We add apparent because indeed, that is what is apparent to us. E.g. Apparently, that is the force of gravity that we read on the Newton Scale since it is equivalent to the tension. In truth, it is actually somewhat more different when you are near the equator than when you are near the South or North pole, not only because you are further away from the centre of the Earth, but also because of the centripetal force that gravity 'contributes' to in a sense.

    Anyway, I am not that sure about your textbook. It is not that important you know that much about it, but it is IMPORTANT that you can differentiate between apparent weight and weight. Once you have done that, you probably can store it in your head somewhere in case you need it and there is no need for you to pursue it that deeply.
     
  10. Oct 25, 2006 #9
    Just think of it as the word 'Weight' by itself has been specially reserved for the force that is acting. As in, it is the TRUE force of gravity that acts upon an object. In the case of a spring scale, we are actually measuring the Tension in the spring, which is almost equivalent to its weight, Newton's third law and all that.

    We add apparent because indeed, that is what is apparent to us. E.g. Apparently, that is the force of gravity that we read on the Newton Scale since it is equivalent to the tension. In truth, it is actually somewhat more different when you are near the equator than when you are near the South or North pole, not only because you are further away from the centre of the Earth, but also because of the centripetal force that gravity 'contributes' to in a sense.

    Anyway, I am not that sure about your textbook. It is not that important you know that much about it, but it is IMPORTANT that you can differentiate between apparent weight and weight. Once you have done that, you probably can store it in your head somewhere in case you need it and there is no need for you to pursue it that deeply.
     
  11. Oct 25, 2006 #10

    OlderDan

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    Perhaps there is no better argument for keeping the adjective apparent than the fact that we cannot measure what we have been forced to call true weight or actual weight in this discussion. We agree that there is such a thing, and we agree that when we look at our measuring devices we might very well not be measuring this thing. Whatever we call this thing, what we measure is its apparent value under a certain set of conditions, not the thing itself.

    We already have a name for the thing we are actually measuring. It is the normal force. Why don't we just call it the normal force instead of either weight or apparent weight? I think it is because of the physiological effects that we experience near the earth when the true weight and the normal force are out of the balance we have grown accustomed to because of where and how we spend most of time here on earth. If I am in the accelerating elevator, and the scale reads 1/6 of what I am accustomed to seeing it read, how do I feel? Is it not true that I would feel the same as I would if I were standing at rest on the surface of the moon where my true weight is 1/6 of what it is here on earth?

    I understand what the OP is saying, and that it is not based on a conceptual misunderstanding. It is merely a matter of preference for what we are going to call this thing we observe. I am merely stating my preference for doing it the way it has been done. You and the OP have a different preference, and you are certainly entitled to your preference, as I am to mine.
     
    Last edited: Oct 25, 2006
  12. Oct 25, 2006 #11
    Thanks to everyone for their thoughts on this topic. I appreciate it. Well, I thought the idea was not very useful, and I still lean in this direction. But some incredibly smart people obviously find it worthy. So for myself, I will keep an open mind on this subject.

    Thanks again to everyone here,
    Dorothy
     
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