  We use the high spatial and spectral resolution of the PAWS CO(1-0) survey of
the inner 9 kpc of the iconic spiral galaxy M51 to examine the effect of gas
streaming motions on the star-forming properties of individual GMCs. We compare
our view of gas flows in M51 -- which arise due to departures from axi-symmetry
in the gravitational potential (i.e. the nuclear bar and spiral arms) -- with
the global pattern of star formation as traced by Halpha and 24\mu m emission.
We find that the dynamical environment of GMCs strongly affects their ability
to form stars, in the sense that GMCs situated in regions with large streaming
motions can be stabilized, while similarly massive GMCs in regions without
streaming go on to efficiently form stars. We argue that this is the result of
reduced surface pressure felt by clouds embedded in an ambient medium
undergoing large streaming motions, which prevents collapse. Indeed, the
variation in gas depletion time expected based on the observed streaming
motions throughout the disk of M51 quantitatively agrees with the variation in
observed gas depletion time scale. The example of M51 shows that streaming
motions, triggered by gravitational instabilities in the form of bars and
spiral arms, can alter the star formation law; this can explain the variation
in gas depletion time among galaxies with different masses and morphologies. In
particular, we can explain the long gas depletion times in spiral galaxies
compared to dwarf galaxies and starbursts. We suggest that adding a dynamical
pressure term to the canonical free-fall time produces a single star formation
law that can be applied to all star-forming regions and galaxies, across cosmic
time.
