  Massive gravity, galileon and braneworld models that modify gravity to
explain cosmic acceleration utilize the nonlinear field interactions of the
Vainshtein mechanism to screen fifth forces in high density regimes. These
source-dependent interactions cause apparent equivalence principle violations.
In the weakly-screened regime violations can be especially prominent since the
fifth forces are at near full strength. Since they can also be calculated
perturbatively, we derive analytic solutions for illustrative cases: the motion
of massive objects in compensated shells and voids and infall toward halos that
are spherically symmetric. Using numerical techniques we show that these
solutions are valid until the characteristic scale becomes comparable to the
Vainshtein radius. We find a relative acceleration of more massive objects
toward the center of a void and a reduction of the infall acceleration that
increases with the mass ratio of the halos which can in principle be used to
test the Vainshtein screening mechanism.
