  We give a Hamiltonian analysis of the asymptotically flat spherically
symmetric system of gravity coupled to a scalar field. This 1+1 dimensional
field theory may be viewed as the "standard model" for studying black hole
physics. Our analysis is adapted to the flat slice Painleve-Gullstrand
coordinates. We give a Hamiltonian action principle for this system, which
yields an asymptotic mass formula. We then perform a time gauge fixing that
gives a Hamiltonian as the integral of a local density. The Hamiltonian takes a
relatively simple form compared to earlier work in Schwarzschild gauge, and
therefore provides a setting amenable to full quantisation.
