I will then present the purpose: TGMM the relationship between the a vs. r and a vs. Mo. Not sure if we can do this on an actual sheet of rubber or not but we could use Tycho Brahe's data to find the relationship with r. If we can't do it on a rubber sheet then at least qualitatively we can reason that a varies directly as Mo and so we would get a is proportional to Mo/r^2.
Next I plan to have a class discussion to find the approximate mass of the earth (using Erastonthenes' R value for the Earth and assuming that the density of the earth is that of silicon dioxide) and finally the proportionality constant "G". Then I will either describe Cavendish's experiment or have the students perform the experiment to find the actual G and mass of the earth.