A simple model for star formation based on supernova (SN) feedback and gravitational heating via the collapse of perturbations in gravitationally unstable disks reproduces Schmidt-Kennicutt relation between rate (SFR) per unit area, $\Sigma_{SFR}$, gas surface density, $\Sigma_g$, remarkably well. The velocity dispersion, $\sigma_g$, is derived self-consistently conjunction with $\Sigma_{SFR}$ ...