Plugins help Google Sketchup become a free substitute for more expensive cadd systems. Two plugins in particular, GetCentroid and Volume Calculator, can help an engineer make usable calculations based on 2D / 3D models.
Plugins are just text files with a .rb extension (Sketchup uses the Ruby programing language). You place them in the google sketchup plugin sub-directory folder before reopening sketchup.
Hint: Join Sketchucation.com and you will have access to an amazing amount of free plugins that can make just about any task easy. (Plus free advice and tutorials.)
The GetCentroid plugin by Alexander Schreyer, allows an engineer to find the area and centroid of 2D faces. For example if you have a T-beam homework set, you can sketch out the exact size of the face in Sketchup, on the XY axis.