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[ansysquestions2995]

 Can somebody explain the logic behind this statement below the image?Why will it be under constrained?what does under constrained mean exactly in mathematical and physical terms?

 

[peteroznewman]

A 2D body has 3 degrees of freedom (DOF): X, Y and Rotation about Z.  You can't have a Static Structural solution without constraining those three DOF.

If the body has X = 0 along the edge parallel to the Y axis, that removes two of the three DOF, but the body is free to slide up and down in the Y direction.  The red constraint adds Y = 0 to constrain that DOF.

A 3D body has 6 DOF so needs six constraints.