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Hi all - newbie question - Forces and static equilibrium

  1. Oct 2, 2009 #1
    first timer here and non english spoken so I might get few mistakes but but...
    Question is about static equilibrium and about with clock and against clock principe. When figuring out momentum til a point first we define if its positive or negative. I understand that its our choice to define that as we find it best suited but how do we do that ? To use forces X and Y components when calculating forces we watch at their axes. If a force goes from higher to lower Y its negative, same for X. Here when calculating moemntum its not like that. This is basics I know but i really struggle to understand how to define (positive or negative ) forces when doing momentum...

    Help will be highly appritiated...
     
  2. jcsd
  3. Oct 2, 2009 #2
    any help here ?
     
  4. Oct 4, 2009 #3

    nvn

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    For rotation in the xy plane, the moment vector, M, will be directed along the z axis, usually according to the right-hand rule. For the right-hand rule, for a counterclockwise moment, the moment vector will point in the positive z direction, and therefore, the moment is positive. For a clockwise moment, the moment vector will point in the negative z direction, and therefore, the moment is negative.
     
  5. Oct 4, 2009 #4
    This is the best answer I've got so far. Thanks nvn, but can I ask you to go little deepr into basics... What I strugle to understand is how do I define right hand rule forces that makes momentum and defining them as positive or negative. Like for instance on this picture that I found usefull.

    http://www.coolschool.ca/lor/PH12/unit1/U01L07/stat_eq2.gif [Broken]

    This article states that they are defined as clockwise or anticlock but HOW DO YOU DEFINE that. I understand very much of the following math operations but depending on + or - of a given force the result is almost always incorret in my calculations. This is becouse I dont understand how do you define + or - ... if you or anyone else would explain to me how is this done. I guess for many its selfexplained but I really strugle to understand...
     
    Last edited by a moderator: May 4, 2017
  6. Oct 4, 2009 #5

    Doc Al

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    Do you understand the terminology "clockwise" and "anticlockwise"? (I'm trying to understand what part confuses you.) In case you do not, look here: http://en.wikipedia.org/wiki/Clockwise" [Broken]
     
    Last edited by a moderator: May 4, 2017
  7. Oct 4, 2009 #6
    yes I do understand that part :)
     
  8. Oct 4, 2009 #7

    nvn

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    MisterOL: Place your right hand at the pivot point, in this case, point A. Curl your four fingers in the direction of the applied moment, as shown here. The direction your thumb is now pointing is the direction of the moment vector. Now reread post 3 to determine whether this moment vector is positive or negative.
     
  9. Oct 4, 2009 #8

    Doc Al

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    OK, so what part don't you understand? Note that giving clockwise torques a negative sign and anticlockwise torques a positive sign is just a convention.
     
  10. Oct 4, 2009 #9
    I have been reading many of your posts on this forum even the one with right hand torque rule to try to figure it out but with no help. Where is the clock located if we use cw and ccw terms ? is it 00-06 hours showing on clock same as force going in those directions hence clockwise and hence negative and 06-12 ccw and hence positive ?
     
  11. Oct 4, 2009 #10

    Doc Al

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    I'm not sure I understand what you're asking here.

    When trying to determine if the torque produced by some force is clockwise or counterclockwise, do the following. Imagine that the force in question is the only force acting. If there was a rod extending from the pivot point to the point of application of the force, ask yourself if the force would make the rod turn clockwise or counterclockwise. Then give the proper sign to the corresponding torque.

    Does that make sense?
     
  12. Oct 4, 2009 #11
    it probably would if I knew what rod and pivot means...
    Is pivot Momentum of a point that we want to use in equasion ?
     
  13. Oct 4, 2009 #12

    Doc Al

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    By "rod" I just mean an imaginary stick or rod; by "pivot" I mean the axis about which you are finding the torque. (You need an axis to define the torque due to a force.)
     
  14. Oct 4, 2009 #13

    nvn

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    MisterOL: Place the clock face on your paper, as if you are normally reading a clock. The clock arms rotate in the clockwise direction as time elapses, rotating around an axis perpendicular to the clock face. Where this perpendicular pivot axis intersects the clock face is called the pivot point. Counterclockwise would be the reverse rotation of the clock arms, if time could go backwards. See if the applied force would cause a clockwise or counterclockwise rotation about this pivot axis. Now place your right hand at the pivot point. Curl your four fingers in the rotation direction of the applied moment. Your thumb is now pointing in the direction of the moment vector. Now reread posts 7 and 3 to determine whether this moment vector is positive or negative.
     
  15. Oct 4, 2009 #14
    thanks nvn for the posts... they really got me on doing some aerobics :) basic logic say if tumb is pointing up curling of applied moment is ccw hence positive ?
     
  16. Oct 4, 2009 #15

    nvn

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    Yes, your sentence in post 14 is correct.
    Yes, that is correct, MisterOL. This is the right-hand rule.
    No; 06 to 00 hours, with the clock arms moving backwards in time, is counterclockwise, and hence positive.
     
  17. Oct 4, 2009 #16
    mek2.png
     
    Last edited: Oct 5, 2009
  18. Oct 5, 2009 #17

    nvn

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    MisterOL: If you want help with post 16, I first need you to do several things.

    (1) First, quit using .jpg for graphics; .jpg is only for photographs and is ruining your text and graphics; .png is for graphics. When you start creating a graphics file, before you draw on it, try saving it as "16 Color Bitmap" (or maybe "256 Color Bitmap"), and then save it as .png. Now draw your graphics and text.

    (2) Upload your .png file as a PF (physicsforums) attachment, not as an imageshack file. Click "Manage Attachments" to upload your file.

    (3) (deleted)

    (4) (deleted)

    (5) Your support reaction forces are missing on the whole structure. Draw the support reaction forces on the whole structure free-body diagram.

    (6) Your vector arrowheads are missing. Any time you draw a support reaction force or member force, or moment, the vector must have an arrowhead.

    (7) Change the direction of the arrowhead on your moment equation summation symbol, and put a plus sign inside the semicircle. I.e., change it to standard right-hand rule for now, not left-hand rule.

    (8) Draw coordinate system x and y axes somewhere on your first diagram.

    (9) Do not change your dimensions or applied loads. They are currently great.

    (10) Update the equations in your diagram.
     
    Last edited: Oct 5, 2009
  19. Oct 5, 2009 #18
    ok I asked someone more expirienced to help me with this problem and he defined it in a really simple metter. I will leave this picture as example if anyone encounters same kind of problems :) Thanks to doc and nvn who sent my toughts in right direction... Actually doc explined it in one of his other posts. On top picture called whole Draw F1 continuing ( this example horisontal direction ) until it is passed point ( pivot ) B then draw 90 degrees from B to this line. IF that line is imagined as clock hand and B is center of the clock ask your self is the direction of the force pushing clock forward or backwards.

    Is this correct my mentors ?
     
  20. Oct 5, 2009 #19
    ok nvn did png upload :) 4 is not horisontal roller. Its the same as A. did support forces directly on main pic with brown color since image will be to large...
     
  21. Oct 5, 2009 #20

    nvn

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    Hi, MisterOL. You forgot to do items 7, 8, and 10. And for item 5, you forgot to label your support reaction forces on your whole structure free-body diagram. Please do these items.
     
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