for example: if i want to get the centre or mass in the middle i have to draw a contraweigt inside the part file and then use goalseek to get the centre of mass in the middle, the contraweigt should be another part in an assembly but then i can't choose peer variables to get it to change with goalseek. so i draw it as 1 part but it should be 2 parts.
How about making the part as a multi-body? When solved you can then "publish" the individual parts.
The Add Body command is on the Home tab→Solids group in both ordered and synchronous environments.
A new part or sheet metal file contains a single design body. In a file with a single design body, the Design Bodies collector does not appear in PathFinder. If you want to use the multi-body modeling method, use the Add Body command to create a multi-body file.
You can use the Add Body command in a new file to assign a name to the initial design body. The default name for the initial solid body is Design Body_1. As you add solid bodies to a file, the Add Body dialog box contains the active design body name.