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I've started learning hydro static force system right now. I have problem understanding one thing. In the picture I'm looking for the forces on the black lines: A part is making the purple force above and the yellow upper force. A+B are making the "main" green force. A is also making the red force, which i cant understand why. if someone can please explain me why does force A causing it and why in that shape. My confusion

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I'm looking at your picture and I do not understand what all those colorful lines represent. For a start, I see no force as there is no vector... –  Pygmalion May 27 '12 at 7:48
    
Hi, each colored square or tri represent the distribution of the force. and each colored square or tri can be represent as resultant force (one vector) . I understand why the pink tri is there - because as you go down the pressure increase and at the upper point the pressure is zero. I dont understand why the red tri is like that, I just know it should be there. –  idan May 27 '12 at 8:00
    
Can you maybe upgrade your figure a bit, by drawing it as pressure, and indicate directions of increasing pressure? Why would the force by A decrease? Use that the pressure at the same horizontal distance is constant –  Bernhard May 27 '12 at 8:55
    
I updated the picture. now when I look at it, I understand that the RED force must be exist because its not possible that there is no pressure at point P. but I just cant figure how it calculated geometrically –  idan May 27 '12 at 9:23
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The green line shows what the pressure would be if there were no water in the region A, but as you add water to A it will shift the green line to the left. I don't understand why the red line is as you've drawn it. Your drawing suggests that the water in region A causes no pressure increase at the bottom of the tank but does cause a pressure increase at the top of region B. –  John Rennie May 27 '12 at 9:30

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