  We describe a partial filament eruption on 11 December 2011 which
demonstrates that the inclusion of mass is an important next step for
understanding solar eruptions. Observations from the \textit{Solar Terrestrial
Relations Observatory Behind} (STEREO-B) and the \textit{Solar Dynamics
Observatory} (SDO) spacecraft were used to remove line-of-sight projection
effects in filament motion and correlate the effect of plasma dynamics with the
evolution of the filament height. Flux cancellation and nearby flux emergence
are shown to have played a role in increasing the height of the filament prior
to eruption. The two viewpoints allow the quantitative estimation of a large
mass-unloading, the subsequent radial expansion, and the eruption of the
filament to be investigated. A 1.8 to 4.1 lower-limit ratio between
gravitational and magnetic tension forces was found. We therefore conclude that
following the loss-of-equilibrium of the flux rope, the radial expansion of the
flux rope was restrained by the filamentary material until 70\% of the mass had
evacuated the structure through mass-unloading.
