Working Model is an engineering simulationsoftware product by Design Simulation Technologies. Virtual mechanical components, such as springs, ropes, and motors are combined with objects in a 2Dworking space. After the software is run, the program will simulate the interaction of the model's parts and can also graph the movement and force on any element in the project. It is useful for basic physics simulations, and can be quite a powerful dynamic geometric analytical tool, once you learn it. One drawback is that the basic commands like those for pulley and rope simulations appear to be limited to the 'string through a hole' type, but such limitations are quickly overtaken using the internal formula and scripting features of the software. WM was developed from a program called Interactive Physics, the fundamental difference between the two is that WM will read.dxf files which can be converted to 'objects' inside WM, and the way back, therefore being the conversion of Interactive Physics to the engineering and industrial world. The 3D version of Working Model is developed with the commercial name SimWise Motion also by Design Simulation Technologies, and works as a stand alone solution or an add-on for CAD products like Autodesk Inventor, SolidEdge, SolidWorks, CATIA, Alibre Design, Pro/E. Additional integration with Nastran FEA solver leads to what is called a "4D" solution with fem analysis at each integration step of the kinetodynamical simulation. In the last years, inclusion of part optimization and fatigue analysis still running on cheap personal computers lead even students and small companies to benefit of virtual prototyping as demonstrated by several reported cases by Lista Studio. WM was originally introduced in the early 1990s as a boxed software product from Knowledge Revolution in San Francisco, for Macintosh. Knowledge Revolution was acquired by MSC Software in Santa Ana, CA, and WM was than resold to Design Simulation Technologies, Inc. of Canton, Michigan. WM has a retail price of ~US$2,500 and free evaluation versions are available at workingmodel.com and design-simulation.com. Similar, but not as analytical, 2D physics sandbox simulators are: Physion], and Phun.