  In this work we study the coupled system of partial and ordinary differential
equations describing the interaction between a compressible isentropic viscous
fluid and a rigid body moving freely inside the fluid. In particular the
position and velocity of the rigid body in the fluid are unknown and the motion
of the rigid body is driven by the normal stress forces of the fluid acting on
the boundary of the body. We prove that the strong solution, which is known to
exist under certain smallness assumptions, is unique in the class of weak
solutions to the problem. The proof relies on a correct definition of the
relative energy, to use this tool we then have to introduce a change of
coordinates to transform the strong solution to the domain of the weak solution
in order to use it as a test function in the relative energy inequality.
Estimating all arising terms we prove that the weak solution has to coincide
with the transformed strong solution and finally that the transformation has to
be in fact an identity.
