Rigid Domain
Add the Rigid Domain node and select one or more geometrical objects to make them a rigid body. Rigid Domain is a material model, with only one material property: the mass density. It can be used for
By default, an Initial Values node is added (see Initial Values (Rigid Domain)).
You can add functionality to the rigid domain through the following subnodes:
Fixed Constraint (Rigid Domain) to fully constrain the rigid domain.
Prescribed Displacement/Rotation to prescribe the displacement of individual degrees of freedom.
Applied Force (Rigid Domain) to apply a force in given point.
Mass and Moment of Inertia (Rigid Domain) to add extra mass and moment of inertia in a given point.
Spring Foundation (Rigid Domain) to add a translational or rotational spring or damper in a given point.
The Rigid Domain node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).
Shell Properties

This section is only present when Rigid Domain is used in the Layered Shell interface. See the documentation for the Rigid Domain node in the Layered Shell chapter.
Density
The default Density ρ is taken From material. In this case the material assignment for the domain supplies the mass density. For User defined enter another value or expression.
If any material in the model has a temperature dependent mass density, and From material is selected, the Volume reference temperature list will appear in the Model Input section. As a default, the value of Tref is obtained from a Common model input. You can also select User defined to enter a value or expression for the reference temperature locally.
Default Model Inputs and Model Input in the COMSOL Multiphysics Reference Manual.
Center of Rotation
Select a Center of Rotation Center of mass, Centroid of selected entities, or User defined. The center of rotation affects how displacements are interpreted, and is also used as the default in various subnodes.
For Center of mass, the center of rotation is taken as the center of mass of the rigid domain.
For Centroid of selected entities select an Entity levelBoundary, Edge, or Point. The available choices depend on physics interface and geometrical dimension. The center of rotation is located at the centroid of the selected entities, which do not need to be related to rigid domain itself. As a special case, you can select a single point, and thus use that point as center of rotation.
For User defined, enter the Global coordinates of center of rotation, Xc, in the table.
Select the Offset check box to add an optional offset vector to the definition of the center of rotation. Enter values for the offset vector Xoffset.
The center of rotation used is the sum of the vector obtained from any of the input methods and the offset vector.
Constraint Settings
When a rigid domain shares a boundary with a flexible material, all nodes on that boundary are constrained to move as a rigid body. As a default these constraints are implemented as pointwise constraints. If you want to use a weak constraint formulation, select Use weak constraints for rigid-flexible connection.
Modeling Rigid Bodies: Application Library path Structural_Mechanics_Module/Connectors_and_Mechanisms/rigid_domain
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics selected:
Physics tab with Shell or Layered Shell selected:
Physics tab with Beam selected: