  We present numerical simulations of the passage of clumpy gas through a
galactic spiral shock, the subsequent formation of giant molecular clouds
(GMCs) and the triggering of star formation. The spiral shock forms dense
clouds while dissipating kinetic energy, producing regions that are locally
gravitationally bound and collapse to form stars. In addition to triggering the
star formation process, the clumpy gas passing through the shock naturally
generates the observed velocity dispersion size relation of molecular clouds.
In this scenario, the internal motions of GMCs need not be turbulent in nature.
The coupling of the clouds' internal kinematics to their externally triggered
formation removes the need for the clouds to be self-gravitating. Globally
unbound molecular clouds provides a simple explanation of the low efficiency of
star formation. While dense regions in the shock become bound and collapse to
form stars, the majority of the gas disperses as it leaves the spiral arm.
