  A unified approach to the calculation of dispersive forces on ground-state
bodies and atoms is given. It is based on the ground-state Lorentz force
density acting on the charge and current densities attributed to the
polarization and magnetization in linearly, locally, and causally responding
media. The theory is applied to dielectric macro- and micro-objects, including
single atoms. Existing formulas valid for weakly polarizable matter are
generalized to allow also for strongly polarizable matter. In particular when
micro-objects can be regarded as single atoms, well-known formulas for the
Casimir-Polder force on atoms and the van der Waals interaction between atoms
are recovered. It is shown that the force acting on medium atoms--in contrast
to isolated atoms--is in general screened by the other medium atoms.
