This paper presents an unknown input observer for the estimation of external forces acting on mechanical systems from only acceleration measurements. To circumvent arising stability issues, a classical unknown input observer extended with an additional filter is proposed. The influence of the design parameter of the filter is analyzed. A guideline for the parameter tuning depending on predefined estimation goals is given. In contrast to existing methods, no prior knowledge about the unknown acting force is assumed in the design process. The performance of the presented concept is compared to a state-of-the-art approach in both simulation studies and on a experimental test setup.