Laboratory experiments were performed to study the impact of the relative particle protrusion P/D (P is the protrusion height and D is the diameter of the target particle) on the mechanism of entrainment of sediment particles from a spherical rough bed. The target particle to be entrained was instrumented with electronic sensors, which can measure the tri-axis linear acceleration, and consequently the inertial hydrodynamic forces, during the entrainment process. The velocity field was obtained using the two-dimensional Particle Image Velocimetry (PTV) technique and the velocity data were synchronised with the force data relative to the entrainment time. Experimental results show that the magnitudes of inertial drag force and lift force have a decreasing trend as particle protrusion increases. The ratio of inertial lift force to drag force reveal that drag force slightly dominates the entrainment process at P/D > 0.7 while lift force slightly dominates at P/D < 0.62. Also, the inertial drag and lift coefficients were computed by the force data and velocity data. The inertial drag coefficient was found to be independent of P/D when P/D < 0.62 but declined with increasing P/D for P/D > 0.62. Similarly, the variation of inertial lift coefficient with P/D reversed at P/D = 0.7. This variation of force coefficients with P/D is consistent with the independence of inertial forces with respect to P/D when P/D > 0.62-0.7. In summary, the inertial forces demonstrate that the impact of protrusion on the particle entrainment becomes less important when P/D > 0.62-0.7.