For 10 points, name this quantity measured in kilograms.
At the top of the valence band, electrons have a negative value for the "effective" form of this quantity. In an experiment done by Eotvos and repeated by Dicke, two types of this quantity were shown to be equivalent. The luminosity of a main-sequence star is approximately this quantity raised to the 3.5 power. The difference in this quantity for a nucleus versus the component nucleons is called its namesake "defect." The base unit in the SI system for this quantity is defined by a cylinder made mostly of platinum. According to Newton's second law, force equals the product of acceleration and this quantity. For 10 points, name this quantity measured in kilograms. 
At the top of the valence band, electrons have a negative value for the "effective" form of this quantity.
mass [do NOT accept "weight"] 