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.Comment: Revtex4 file, 5 pages. Version to appear in Physical Review