  We suggest a lecture on Newtonian gravity, discussing how the use of the
scientific method allows to rule out some of the models for the shape of Earth
leaving a spherical Earth as the only possibility, in agreement with empirical
observation. In particular, we suggest to focus on a physical quantity, namely
the gravitational acceleration, that can be both computed theoretically and
measured in experiments. Rather than insisting on the numerical code for the
evaluation of the gravitational acceleration -- that can be computed anyway in
any numerical analysis course -- we want to focus on the formal setting of the
problem, emphasizing the importance of having a mathematical formulation
capable of quantitative predictions, present the result and discuss it both
qualitatively and quantitatively, comparing it with daily experience. We let
the student notice that assuming a non-spherical Earth would result in a
parallel component of the gravitational acceleration, in disagreement with
measurement; moreover, in case we consider a disk-shaped Earth we show that
this parallel component becomes more and more important as the boundaries of
Earth are approached by an observer. Finally, we make quantitative predictions
on how this abnormal component of the gravitational force would affect the free
fall motion. We think that the topics discussed here can be presented both in
high school and university classes of general physics, as well as in
introductory classes about the scientific method.
