To Enable Gravity
Select the Enable gravity check box on the External loads tab of the Analysis Definition dialog box when calculating DOF or running a dynamic, static, or force balance analysis. The enabled gravity effect is slightly different depending upon the type of analysis you run.
Dynamic and static analyses:
• A default value of 1 is assigned to a rigid body with density in a mechanism assembly unless you set another value. Click 
Mass Properties in Mechanism Design or set the density in Assembly mode.
Mass Properties in Mechanism Design or set the density in Assembly mode.• If your mechanism includes rigid bodies for which you have not assigned mass, you will not be able to run a dynamic or static analysis. This includes rigid bodies and subassemblies comprised entirely of datum curves or surface features, as well as massless, volumetric bodies.
• If you do not select the Enable gravity check box, gravity will be zero, regardless of the values specified in the Gravity dialog box.
Force balance analysis:
• Clear the Enable gravity check box to assume a mass of 1 for rigid bodies with no mass, and then run a force balance analysis without assigning mass to all rigid bodies.
• If you select the Enable gravity check box, you must specify a mass value for all the rigid bodies in your mechanism before you can run a force balance analysis.