About Gravity
The Gravity command simulates the effect of a gravitational force on the motion of an assembly. You can define gravity if you have a Mechanism Dynamics license.
Click 
to access the Gravity dialog box and define the gravitational acceleration vector for your model. This vector simulates gravity, the fundamental physical force that pulls one rigid body toward another. Once defined, a single, uniform gravitational force is applied to the entire top-level assembly. Rigid bodies in your assembly, with the exception of the ground body, will move in the direction of the specified gravitational acceleration.
to access the Gravity dialog box and define the gravitational acceleration vector for your model. This vector simulates gravity, the fundamental physical force that pulls one rigid body toward another. Once defined, a single, uniform gravitational force is applied to the entire top-level assembly. Rigid bodies in your assembly, with the exception of the ground body, will move in the direction of the specified gravitational acceleration.Once gravity is defined, a WCS icon and an arrow showing the direction of the gravitational acceleration appear on your model. You can also use the Gravity dialog box to edit or remove gravity.
If you want gravity to be included in the calculations for a dynamic, static, or force balance analysis, select the Enable gravity check box on the External loads tab of the Analysis Definition dialog box. If you do not select this box, a gravitational force is not applied during the analysis.