A boundary layer surface vortex sheet is utilized to investigate the development of the unsteady forces acting on a rotating and surging cylinder at a Reynolds number of 20 000. Planar particle image velocimetry and force balance measurements are employed to assess the flow field and force response. The boundary layer vortex sheet is categorized into non-circulatory and circulatory components. During acceleration the non-circulatory or added mass vortex sheet is successfully recovered from experimental data. This makes a decomposition of the forces, calculated only from particle image velocimetry measurements, possible. They compare well to force balance data. Additionally, the evolution of the boundary layer vortex sheet is assessed as the flow separates to form a vortical structure in the surrounding flow. When the influence of varying freestream velocity and rotation rate are removed, the surface vortex sheet at the location of separation varies little during vortex formation, even as the separation point moves along the surface of the cylinder.