Prescribed Acceleration
The Prescribed Acceleration node adds a boundary or domain condition, where the acceleration is prescribed in one or more directions. The prescribed acceleration condition is applicable for Time Dependent and Frequency Domain studies. It is possible to prescribe a acceleration in one direction, leaving the solid free in the other directions. The Prescribed Acceleration node is a constraint and overrides any other constraint on the same selection.
The Prescribed Acceleration node is only available with some COMSOL products (see http://www.comsol.com/products/specifications/).
Shell Properties

Interface Selection

Coordinate System Selection
Select a coordinate system in which the given acceleration components will be interpreted.
Prescribed Acceleration
Select one or all of the Prescribed in x direction, Prescribed in y direction, and for 3D components, Prescribed in z direction check boxes. Then enter a value or expression for ax, ay, and for 3D components, az. For 2D axisymmetric components, select one or both of the Prescribed in r direction and Prescribed in z direction check boxes. Then enter a value or expression for ar and az.
In a stationary study, this node can either be ignored or treated as a constraint (similar to a Prescribed Displacement node with zero displacement). To control this, select an option from the Displacement in stationary study list — Free or Constrained.
You can add a Harmonic Perturbation subnode for specifying a harmonic variation of the values of the prescribed displacements in a frequency domain analysis of perturbation type.
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics selected:
Physics tab with  Layered Shell selected:
Physics tab with Membrane selected:
Physics tab with Truss selected: