Modeling is provided in three-dimensional simulation. Proxies are used as virtual rigid bodies to create (22) reusable part files. A custom physics object is defined (22) in the part file using the proxy to allow for control of object behavior, such as control of the rate of spinning of a spindle of a motor. By overriding (28) the part file with the proxy, links to up or owner part files may be made. Changes to physics properties in the different instances of proxies may be made without requiring or resulting in the same change to the other instances. The proxy acts as a virtual rigid body that may or may not move during simulation. The proxy body may be replaced with a rigid body from the assembly when both reference the same geometry object.