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The relative importance of gravity, turbulence, and magnetic pressure in molecular clouds determines how quickly a cloud will evolve, and how efficiently it will form stars. Different star formation theories predict very different energy balance as a function of spatial scales, from quasi-equilibrium state on all scales, to a switch from turbulent to gravity-driven evolution from large to smaller scales. Here, I present a recent study that aims at computing the virial ratio of 27 infrared dark clouds, from tenths of parsec scales using the N2H+(1-0) optically thin dense gas tracer, to tens of parsec scales using 13CO(1-0). This evolution of the virial ratio as a function of scales shows common trends between clouds, reaching high values (~5) on tens of parsec scales, then decreasing down to sub-viral values on parsec scales, to finally increase to virial ratio values of ~1 at sub-parsec scales. We discuss these results in the context of the dynamical evolution of star-forming clouds in the Galaxy.