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Where is the point of application of the Acsimet force ?

  1. Aug 24, 2003 #1

    pkh

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    Do you know what the force Acsimet is ? Yes, certainly. Do you know Where the point of application of that force ? Yes, I think. But I don't :-(. Can you show me and explain your answer for me ? You can say it's the centre of gravity of the object... but WHY ?

    Thank you !
     
  2. jcsd
  3. Aug 24, 2003 #2

    HallsofIvy

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    I am embarassed to admit that I DON'T know what "acsimet" means, much less what an "acsimet force" is! I do know that, as far as straight line motion is concerned, wherever the actual forces apply to a mass, we can treat the TOTAL force as if it were applied to the "center of mass"- basically, that the definition of center of gravity.

    If forces are not "balanced" around the center of mass, the will be a rotation that a force applied to the center of gravity would not cause.
     
  4. Aug 24, 2003 #3

    pkh

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    Well, Acsimet force is the force applying on an object in a liquid (such as water, oil, etc). The intensity : F=V.Gamma
    where
    V-volume of the part in water of object
    Gamma-specific weight of the liquid

    In the case where the object in the lique is liberal (not linked to anything who is n't in the lique), I'm agree too with you about the reason why the force acsimet applying at the centre of gavity, if not the rotation is obvious.
    But there 're the others cases, you can click on this link for an example:
    [​IMG]
     
    Last edited by a moderator: Aug 25, 2003
  5. Aug 26, 2003 #4
    Last edited: Aug 26, 2003
  6. Aug 26, 2003 #5

    pkh

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    Well, I made a mistake with "Archimedes" . Thank you for the link :smile: .
     
  7. Aug 31, 2003 #6

    pkh

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    I read all but the object in the explaination is libre (it has no link with the other object which aren't in the water). Moreover, I don't understand the symbol of triangular shape. What does it mean, that mathematical symbol ?
     
  8. Aug 31, 2003 #7

    HallsofIvy

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    arcnets: Thank you VERY much for the clearing up the "Archimedes". It was clear that PKS's native language was not English (you should try to read my French!!) and that one stumped me.

    PKS: There are links at the site: the "dots" on the left (although unlabeled) are links to other "pages" of the document. I didn't see any triangular mathematical symbol. At the end, there is a triangle inside a disk which is clearly a "next page" link.
     
  9. Aug 31, 2003 #8

    pkh

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    Rehi ;-),
    Yes, you're right, I'm not english but I'm not french too (although I can use both of those 2 languages) :-).
    And the symbol which I don't understand, you can see here
    .[​IMG]
    I always wait your explains.
    P.S: My nickname is PKH, not PKS .
     
  10. Aug 31, 2003 #9
    That stands for the gradient. The partial derivative of the components of the vector p.
     
  11. Aug 31, 2003 #10

    HallsofIvy

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    PKH: sorry about getting your "name" wrong.

    meister is correct that the upside down triangle is the "gradient" (sometimes read "del"). His explanation is a little opaque. The gradient of a function of several variables is the vector whose x component is the derivative with respect to x, y component is the derivative with respect to y, etc.

    The gradient vector has the nice that it always points in the direction in which the function increases fastest and that it's length IS that fastest increase. Of course, at a point of maximum value the function will not increase so it has no direction of fastest increase (and since fastest decrease is opposite to the direction of fastest increase, at a minimum there is no direction of fastest increase) so the gradient must be 0. This is exactly the same as the more basic idea that at a critical point, the derivative is 0. In fact it is more correct to think of the gradient as THE derivative of a function of several variables than the partial derivatives.

    Since most physical laws can be expressed as minimizing or maximizing something, they typically involve the gradient.
     
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