  We present a theoretical scheme based on su(2) algebra to investigate the
influence of homogeneous gravitational field on the quantum nondemolition
measurement of atomic momentum in dispersive Jaynes-Cummings model. In the
dispersive Jaynes-Cummings model, when detuning is large and the atomic motion
is in a propagating light wave, we consider a two-level atom with quantized
cavity-field in the presence of a homogeneous gravitational field. We derive an
effective Hamiltonian describing the dispersive atom-field interaction in the
presence of gravitational field. We can see gravitational influence both on the
momentum filter and momentum distribution. Moreover, gravitational field
decreases both tooth spacing of momentum and the width of teeth of momentum.
