Existence of solutions for the equations modelling the motion of a rigid body in a viscous fluid

We introduce a concept of weak solution for a boundary value problem modell ing the interactive motion of a coupled system consisting in a rigid body i mmersed in a viscous fluid. The fluid, and the solid are contained in a fix ed open bounded set of R-3. The motion of the fluid is governed by the inco mpressible Navier-Stokes equations and the standard conservation's laws of linear, and angular momentum rules the dynamics of the rigid body. The time variation of the fluid's domain (due to the motion of the rigid body) is n ot known apriori, so we deal with a free boundary value problem. Our main t heorem asserts the existence of at least one weak solution for this problem . The result is global in time provided that the rigid body does not touch the boundary.