Flow-induced moving boundary
 

How would one define a moving boundary that isn't moving but is able to move?

I am looking at a CFD analysis in which the the flow itself is the cause of the surface movement (reverse to defining constant moving boundary conditions).

Eventually, I will move to more complex geometries. However, I first want to consider a case such that a flow stream is directed toward a perpendicular flat plate. I need the flat plate to react according to the force created by the stream.