in reply to [OT] Forces.

In the end, it is the position of the body in space that is important, not its rotation, nor the exact position of any point on it

Then, you can just ignore the circle and apply the forces directly in the point A.

If the only thing that matters to you is the final position (it is not clear from your post), then you can avoid all the calculations. The stable position happens when AB is aligned with F. Though in the absence of any friction, what you have there is a pendulum.

If the force changes with the position, the logic remains the same, though you will have to find the point A where F(A) and AB align.

Replies are listed 'Best First'.
Re^2: [OT] Forces.
by ExReg (Priest) on Feb 14, 2016 at 17:13 UTC

    Without knowing the masses and the moments involved, I don't know that you can ignore the circle. A lot of force can get swallowed up in just increasing angular momentum. The rotation around A could be more significant than that around B.

[reply]
      Without knowing the masses and the moments involved,

      The model can return the area of the body, and from that its volume can be approximated, which combined with the densities can allow an reasonably accurate mass to be determined.

      It comes out at 0.75 Kg.

      However, this is a 2D finite element analysis, so the model is an infinitely thin radial (cross-axial) slice of the mechanism, which leaves me questioning the relevance of the total mass? Or whether there is any useful subdivision of it that can be used?

      The model can return a figure labeled as: "R^2 (ie. Moment of Inertia / density )", and further defined as: "Integral of (x^2 + y^2)".

      Which for the body in question gives a value of:9.88017e-8 m^5.

      But, at this point, I have no idea what if any relevance or use that has in what I am trying to calculate?

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

        You could use linear density (mass per unit length) if you want the mass. You only need the mass if you want accurate calculation of the speed in real units. If you just want the motion around the spindle, you can set the mass=1 arbitrary unit and integrate the magnitude of F_perp as you go around to determine the relative speed at any given point.

[reply]
Re^2: [OT] Forces.
by BrowserUk (Patriarch) on Feb 15, 2016 at 08:30 UTC
    Then, you can just ignore the circle and apply the forces directly in the point A.

    What I was trying to indicate was that I was aware of the small discrepancy in the final position of the point F; and that because the angles involved are chosen to be very small at each step of the iterations; the affect of that discrepancy is minimal for each step.

    However, the forces that will result from the next iteration of the stress tensor, are affected by the rotation of the body from the previous step, hence it cannot be ignored completely.

    It is clear that those small errors will accumulate through the iterations; and if once the model is run -- it'll take many days -- if the accumulated error is too large, which will be obvious once I can see the final position of the body -- it should end up back where it started once the assembly has made a full revolution around B which is my terminating condition -- then I will have a handle on what kind of additional calculation (or possibly a fudge factor) I need to incorporate at each step to alleviate the error accumulation.

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
      Then knowing the resulting force is not enough you also need to know the moment.

      The library you are using to integrate the tensor may also provide you some function to calculate its moment around some given point.

[reply]
        The library you are using to integrate the tensor may also provide you some function to calculate its moment around some given point.

        It does -- see Re^3: [OT] Forces. -- but at this point I am unsure how to make use of it?

        All I know about the value returned is this quote (the only one) from the manual:

        Rˆ2 (i.e. Moment of Inertia/density) This integral is used to determine the moment of inertial of the selected blocks. To obtain moment of inertia, the result of this integral must be multiplied by the density of the selected region. For 2D planar problems, the moment of inertia about the z-axis (i.e. about x=0,y=0)

        In terms of my diagram, x=0, y=0 is the point B.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
        In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]

In Section Seekers of Perl Wisdom