In this paper we present a highly expressive, robust, and easy-to-build systemthat provides force-feedback interaction for mobile computing devices (MCD).Our system, which we call Fortissimo (ff), utilizes standard built-inaccelerometer measurements in conjunction with generic foam padding that can beeasily placed under a device to render an expressive force-feedback performancesetup. Fortissimo allows for musically expressive user-interaction with addedforce-feedback which is integral for any musical controller --a feature that isabsent for touchscreen-centric MCDs. This paper details ff core concepts,hardware and software designs, and expressivity of musical features.