Applied Forces and Motions

Forces and moments can be present in the system. There are action-only forces which are applied to one point, and action-reaction forces which are applied to two points. Force components can be a constant value, a curve, an expression, or a user-written subroutine.

A special force is gravity. Acceleration is applied to a body and from mass and acceleration, the gravitational force is computed.

Motion is a scalar constraint to the system. Displacement, velocity, and acceleration-type motions are possible. The motion must depend only on time and not on any other measures in the model that could change during the simulation. In other words, at every time step with only time as the independent parameter, the solver should be able to evaluate the expression completely without using any other information about the model. For example, the motion cannot depend upon displacement or velocity or acceleration between two points in the model.

Motion can be specified as a constant value, a curve, an expression, or user-written subroutine. Motion is either defined as motion between two points or joint motion. When a motion is applied on a joint, one joint degree of freedom is controlled as a function of time. When a motion is applied between two points, movement along a user-specified direction is controlled as a function of time.

Forces are always defined at grid points, and can be applied to one grid point (action-only) or two (action-reaction). The bulk data entry MBFRC defines constant force; the entry MBFRCC defines force by a curve; and the entry MBFRCE defines a force by equation.

Moments are always defined at grid points, and can be applied to one grid point (action-only) or two (action-reaction). The bulk data entry MBMNT defines constant moment; the entry MBMNTC defines moment by a curve; and the entry MBMNTE defines a moment by equation.

GRAV defines the gravity acceleration.

The entry MLOAD can be used to derive force and moment set combinations.

Motion can be defined as grid point motion or as joint motion. Grid point motion can be applied to one grid point or two (relative motion). The bulk data entry MOTNG defines constant motion; the entry MOTNGC defines motion by a curve, and MOTNGE defines motion by an equation. Joint motion can be applied to translational or revolute joints only. The bulk data entry MOTNJ defines constant motion; the entry MOTNJC defines motion by a curve, and MOTNJE defines motion by an equation. The entry MOTION can be used to derive motion set combinations.