when researchers find a reasonably complete fossil of a dinosaurthey can determine the mass and weight of the living one,how can this be achived(Archimedes principle)?

Achimedes principle would not help in this case. Estimating the mass and weight of a 'living' being from bones is something that a forensic scientist (or in this case paleoforensic scientist) would do. They use bone length measurements, regression analysis, functional anatomy, and data from a national forensic skeletal data bank of standard population measurements to determine the approximate height of an individual.