  We prove existence and multiplicity of periodic motions for the forced 2-body
problem under conditions of topological character. In the different cases, the
lower bounds obtained for the number of solutions are related to the winding
number of a curve in the plane, the homology of a space in $\R^3$, the knot
type of a curve and the link type of a set of curves. Also, the results are
applied to the restricted $n$-body problem.
