 His early works in dynamics , the science of motion and mechanics were his 1590 Pisan De Motu ( On Motion ) and his circa 1600 Paduan Le <unk> ( Mechanics ) . The former was based on Aristotelian – Archimedean fluid dynamics and held that the speed of gravitational fall in a fluid medium was proportional to the excess of a body 's specific weight over that of the medium , whereby in a vacuum , bodies would fall with speeds in proportion to their specific weights . It also subscribed to the <unk> impetus dynamics in which impetus is self @-@ dissipating and free @-@ fall in a vacuum would have an essential terminal speed according to specific weight after an initial period of acceleration . 
