  The geodesic deviation equation, describing the relative accelerations of
nearby particles, and the Raychaudhury equation, giving the evolution of the
kinematical quantities associated with deformations (expansion, shear and
rotation) are considered in the framework of modified theories of gravity with
an arbitrary curvature-matter coupling, by taking into account the effects of
the extra force. As a physical application of the geodesic deviation equation
the modifications of the tidal forces due to the supplementary curvature-matter
coupling are obtained in the weak field approximation. The tidal motion of test
particles is directly influenced not only by the gradient of the extra force,
which is basically determined by the gradient of the Ricci scalar, but also by
an explicit coupling between the velocity and the Riemann curvature tensor. As
a specific example, the expression of the Roche limit (the orbital distance at
which a satellite will begin to be tidally torn apart by the body it is
orbiting) is also obtained for this class of models.
