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Weighing yourself on a solid surface vs carpet

  1. Aug 18, 2012 #1
    Recently I started thinking about it, because apparently you get different results. At first, I thought you would weigh less on a carpet (according to the scale), because the carpet supports some of your and the scale's weigh. But thinking deeper into it, I actually figured it shouldnt make a difference, since the Earth's gravitational force is same in both cases and it doesnt really matter if the scale is supported entirely by the solid surface or the solid surface + the carpet, because they have to balance the same force anyway in order for the scale to remain still/not move.

    So gaining interest in the subject, I started googling it, and to my surprise I found statements to both that "the scale shows you weigh more on a carpet" and "the scale shows you weigh less on a carpet than on a solid floor."
    I also came across this interesting explanation, yet it only explains one version: http://www.newscientist.com/article/dn2462-people-weigh-less-on-a-hard-surface.html

    So Im a tad confused here. Does the result depend on the scale itself and the way it's designed/manufactured? (Because I can think of a couple of minor details which could give both results in scales.) Or is there a better explanation out there which supports one statement and crushes the other?

    Thanks in advance
     
  2. jcsd
  3. Aug 18, 2012 #2
    Why don't you try it and tell us what your results are?
     
  4. Aug 18, 2012 #3

    HallsofIvy

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    I like Skeptic2's response but with a null result, you might argue that you scale is not accurate enough to measure the difference.

    Fawk3s, have you learned that "F= ma"? As long as you have 0 acceleration (I started to say "motionless" but actual speed or velocity doesn't matter), the scale will record the same weight whether it is on a hard surface, or carpet, or a spring or whatever. The key is "0 acceleration". IF, as a result of placing the scale on a carpet (or spring or whatever) you had, momentarily, non-zero acceleration, then, at that instant, the scale would be off by "ma" where a is the acceleration.
     
    Last edited: Aug 18, 2012
  5. Aug 18, 2012 #4
    The only difference the carpet makes is that the scale can shift around much easier on carpet, and this can potentially throw off the measurement quite a bit.
     
  6. Aug 18, 2012 #5

    mfb

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    Two other possible effects I found:
    - On a carpet, the scale might be tilted a bit, which increases internal friction as it is not designed for forces horizontal to its surface.
    - You are always moving a bit, which leads to a variable force on the scale. On a solid surface, this directly changes the force the scale sees. On a carpet, you get some damping. Now, depending on the measurement concept, the scale might react differently in those cases.

    That is an important statement.
     
  7. Aug 18, 2012 #6
    I actually thought of that as well, and thats why I said I can see how it can both show that you are heavier and lighter. Im glad you pointed that out. :smile:
    And I guess that statement pretty much does seal the deal indeed.

    Thank you for the answers.
     
  8. Mar 19, 2014 #7
    Mechanical scales have several levers inside to transfer your weight to the dial. When you put one of these scales on a carpet, the feet of the scale sink down, and pressure is applied to the bottom of the scale. This pressure bows the bottom of the scale, which then moves the anchor points of the internal pieces, and changes the weight readout. Depending on the design of the scale, this can change your weight in either direction.
     
  9. Mar 19, 2014 #8
    One thing to understand is that scales cannot possibly measure your weight directly. A scale can only measure the force it exerts on you (normal force), but if the scale is on flat ground, and you do not accelerate when you are standing on it, then you can ASSUME that the normal force is equal to the weight regardless of the surface the scale sits on. You do raise a good point, that perhaps the scale isn't perfectly level on carpet - in which case the normal force would not be quite equal to the force of gravity, but there isn't anything inherently present in carpet that would make a scale measure a lesser force.
     
  10. Mar 20, 2014 #9

    QuantumPion

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    If you put the scale on a mattress or trampoline and stand on it, I suspect the scale would indicate a much lower weight. This is because the force of your weight is distributed over a larger surface area than is covered by the scale. A scale on carpet would be the same effect but less pronounced. A scale on a hard surface works because the entire force of your weight is directed on to the scale.

    Imagine sitting in a chair and putting the scale under one leg. The scale will only measure a portion of your weight. If the legs are even (e.g. using books to support the other chair legs to the same level as the scale) the scale would measure 1/4 of your weight.
     
  11. Mar 20, 2014 #10

    A.T.

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    If you only touch the scale, all your weight is supported by the scale, no matter how hard the surface is. Whether the scale still correctly measures that weight, is an engineering / design issue.
     
  12. Mar 20, 2014 #11

    CWatters

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    Correct.

    It's an engineering problem really, not a physics problem.
     
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