  The equation of motion for test particles in $f(R)$ modified theories of
gravity is derived. By considering an explicit coupling between an arbitrary
function of the scalar curvature, $R$, and the Lagrangian density of matter, it
is shown that an extra force arises. This extra force is orthogonal to the
four-velocity and the corresponding acceleration law is obtained in the weak
field limit. Connections with MOND and with the Pioneer anomaly are further
discussed.
