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I scaled the standard cube to be 2 x 1 x .5, created several copies of it and moved these around so that they should not topple over according to the laws of physics for the following reason:

  • The center of gravity (COG) of the top brick is at x = -1.349, the second brick extends from x = -1.449 to +0.551, hence should nicely carry the top brick.
  • The COG of the top two bricks together is at x = (-1.349 -0.449)/2 = -0.899, the third brick extends from -0.949 to +1.051, hence should nicely carry the top two bricks.
  • The COG of the top three bricks is at x = (-1.349 -0.449 +0.051)/3 = -0.582333..., the fourth brick extends from -.616 to +1.384, hence should nicely carry the top three bricks.
  • The COG of the top four bricks is at x = (-1.349 -0.449 +0.051 +0.384)/4 = -0.34075, the fifth brick extends from -.366 to +1.634, hence should nicely carry the top four bricks.
  • The COG of the top five bricks is at x = (-1.349 -0.449 +0.051 +0.384 +0.634)/5 = -0.1458, the sixth brick extends from -.166 to +1.834, hence should nicely carry the top five bricks.
  • The COG of all six bricks is at x = (-1.349 -0.449 +0.051 +0.384 +0.634 +0.834)/6 = +0.0175, the floor plane extends from x = 0 upward, hence should nicely carry the bricks.

Nevertheless, if I run the animation, the bricks start to topple and three of them fall down to negative infinity ...

Is this a problem of the blender physics engine that it cannot deal with such fragile balancing? Or did I do something wrong in the modelling?

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  • $\begingroup$ Apply scale to all objects and use a cube for your floor/collision instead of a plane. $\endgroup$ – Dontwalk Sep 20 '17 at 17:33
  • $\begingroup$ @Dontwalk Scale is applied (shows as 1.0000 for X, Y, and Z in the N pane). And the toppling also happens if I use the bottom brick as floor (by setting its physics to "Passive") $\endgroup$ – Hagen von Eitzen Sep 20 '17 at 17:58
  • $\begingroup$ I think the "top" brick is simply hanging too far out. If I delete it (top brick) and set the "Steps" to 400 the animation is perfectly stable. This may help blender.stackexchange.com/questions/1811/… $\endgroup$ – Dontwalk Sep 20 '17 at 18:48
  • $\begingroup$ @Dontwalk Of course the whole point is that the ensemble is barely stable (but according to the rules of classical physics it definitely is stable with sufficient safety margin for numerical imprecision) and removing the top brick defies the purpose. -- But the "Steps per Second" (with all bricks still there) really did it: 200 give only a short tremble in the beginning, 300 and more make it almost unnoticeable (on the other hand, 20 looks like an earthquake and one brick even falls through the floor). -- I suggest you turn the Steps-hint into an answer ... $\endgroup$ – Hagen von Eitzen Sep 20 '17 at 20:27
  • $\begingroup$ Thank you. But the way this forum works is that if the steps link solved it for you it's already been answered by that previous post and a Moderator should mark this post as a Duplicate. Happy Blendering :) $\endgroup$ – Dontwalk Sep 20 '17 at 20:32

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