  We consider the model describing the vertical motion of a ball falling with
constant acceleration on a wall and elastically reflected. The wall is supposed
to move in the vertical direction according to a given periodic function $f$.
We show that a modification of a method of Angenent based on sub and super
solutions can be applied in order to detect chaotic dynamics. Using the theory
of exact symplectic twist maps of the cylinder one can prove the result under
"natural" conditions on the function $f$.
