Density is the only physical property that can be obtained directly from the model. To obtain the yield strength and tensile strength you would have to get the name of the material and then get the physical properties out of the XML .sldmat file. See the API helpl for PartDoc::GetMaterialPropertyName2. I would seriously doubt that this is possible in an equation. Are you talking about a SolidWorks equation, or using a VBA macro or external .exe program to perform calculations based on physical properties?
-handleman, CSWP (The new, easy test)