(color online) (a) Pressure p dependence of the macro-friction coefficient μ for systems with N = 1024 particles. The solid line is a power-law fit to the data: μ_J − μ ∼ p^0.5 with μ_J = 0.0952 ± 0.0003 being the value at the unjamming transition. (b) Correlation between the macro-friction coefficient μ and the packing fraction gap Δφ between sheared and non-sheared systems. The dashed line is a linear fit.