Star formation is among the most complex processes studied by astronomers to date. It is characterised by a multi-scale interplay between gravity, turbulence, magnetic field, galactic shear as well as ironising radiation. The key to understand star formation is to pin down the physical mechanics at work. I focus on the followings:
-- the interaction between galactic disk and molecular cloud, through shear. This mechanism can shut down star formation completely under certainty conditions
-- the interaction between turbulence and gravity, which controls cloud fragmentation.
-- the interaction between stars and gas, through gravity (molecular contrails).