This sketch computes the center of mass for an area between two curves. The formulas used are related to those here: http://archives.math.utk.edu/visual.calculus/5/work.2/index.html Can you explain the formulas for the x and y moments of inertia? Why are they different? Check the definitions for M, Mx and My by clicking them in the algebra view. Why the differences from the usual formulas? Do you agree with the author's calculations? Is there a shape you could make in the sketch to check? Is it possible to make a shape for which the center of mass is outside the object?

We'll be using these ideas to make mobiles in the style of Alexander Calder. See http://mathhombre.blogspot.com/2009/06/mobile-math.html for reference.