# Object>>rigid body>calculate mass: mass estimates are bit out on shape created using Maths Mesh tool

I was hoping to use Blender to calculate the mass of shapes with curved surfaces. To start I took a 1m cube and performed a boolean difference with a parabolic curve (x=0 to 1m)that was extended horizontally for 1m and removed the upper part of the cube.

The volume remaining is 1/3 m^3 (by basic integration of x^2 -> x^3/3). However, using the "calculate mass" option for rigid body under Physics properties, using lead as a material for the density. the mass of the cut object was 30.6% of the mass of a 1m cube also of lead - as opposed to the 33% I expected and hoped for.

I am trying to figure out where the error has occurred. The parabolic cut looks fine and so I think there must be an inaccuracy in the way Blender estimates the volume of shapes with curved surfaces.

Please could you tell me how Blender estimates the volume of an object when using the calculate mass menu item.

I have constructed scales in Blender using rigid body simulation and hoped to show Archimedes Method of Mechanical Leverage where integrated shaped (straight line slope to parabolic to cubic and so on - can be shown to balance). However, to do this convincingly, I need Blender to have more accurate mass estimation than I get here.

• I tried using 3D Print Toolbox to calculate the volume but got 2.8m3 which is clearly wrong for a cut out from a 1m3 cube. Dec 17 '20 at 11:39
• My coclusion is that Blender calculates mass of regular shapes but not complex surfaces. I gotround this by making my object out of microcubes. Numerical integration ... Dec 19 '20 at 22:08
• Did you apply the Boolean? It appears physics ignores the modifier stack. Feb 24 at 20:31
• Thanks for your input. I ended up just stacking cubes in for next loop. With the cubes I got the mass exactly which was neat as my end product are scales. Perhaps the mass calculator cant handle non-linear shapes? You can see the result here half way down: en.wikipedia.org/wiki/The_Method_of_Mechanical_Theorems I'm not sure what you mean by physics and modifier stack and will look it up. Is there a good source? I find the Blender manual sketchy. Mar 1 at 9:32
• Go to the "Modifiers" tab for the object (a.k.a the stack), choose your Boolean, and hit 'Apply'. Mar 1 at 20:53