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Question about weight/mass/force I can´t explain !

  1. May 4, 2009 #1
    Hi everyone ! I´m posting as I couldn´t find any information on the forum, or maybe I can´t find a way of searching.

    My question is based on a discussion me and my girlfriend had about "making yourself heavier", and we tried a little experiment: First I lifted her in my arms without any problems, and then I tried a second time while she "applied downwards force" or "tried to make herself heavier" and obviously was harder to lift. She claimed that the second time she tried to "anchor" down, but I insisted that was impossible since she could not add "weight" or increase the downwards force without acceleration.

    It´s kind of complicated to explain, but I´ll try to draw something to make more sense ! If anyone quite understood my point, all interpretations will be welcome ! Thanks !
     
  2. jcsd
  3. May 4, 2009 #2

    mgb_phys

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    She can't make her heavier without changing her mass (chocolate works for this)
    There are a couple of 'magic tricks' where you lift someone - then press you arms downward - then fail to lift them. It's all to do with muscle memory etc.
     
  4. May 4, 2009 #3
    Good Point there with the chocolate !

    I totally agree, you can´t make yourself just "heavier" but then the question remains... It was obvious it was way harder to lift her when she tried to "push down" and it wasn´t fatigue, cause we tried the "heavier" setup first, and the second time she "lightened up" and it was much easier then...
    So ?
     
  5. May 4, 2009 #4

    mgb_phys

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    What exactly was the setup ?

    A simple test is have the person stand on a pair of scales and 'push-down' assuming they can't push against some other fixed object (like the ceiling) they can't change the steady reading on the scales
     
  6. May 4, 2009 #5

    tiny-tim

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    Welcome to PF!

    Flowers work also! :biggrin:
    Hi Cvitanic! Welcome to PF! :smile:

    If she "tried to make herself heavier" by "anchoring down", and if you were holding her with both your arms, she was probably exerting a horizontal pressure pushing them apart, which would make harder work for you.

    Try again using just one hand! :wink:
     
  7. May 4, 2009 #6
    Re: Welcome to PF!

    Sounds about right... I´ll try your idea !
     
  8. May 12, 2009 #7
    this is difference between the living and non living things, non living things can exert force of only of ther weight but living things can add the force beside their weight by using internal energy because we eat food and have nutrition,
    this is the reason for what i have understood , were u asking same things ?, let me know
     
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