The powder motion induced by the gas flow has been identified as one of the critical phenomena in laser powder bed fusion processes that significantly affects the build quality. However, the gas dynamics and its induced driving forces for the powder motions have not been well quantified. A numerical model is developed to investigate such powder-gas interactions. With a combination of computational fluid dynamics and particle tracking techniques, the model is capable of simulating the transient gas flow field surrounding the powder and the forces exerted on powder surfaces. The interaction between metal powders and a free jet is investigated with the current model. In the simulation results, the entrainment and the ejection motions of powders with respect to the free jet can be predicted. It is found that the driving forces of these motions are majorly contributed by the pressure differences in the gas flow surrounding the powder, and the powders can also interact with the jet to significantly alter the flow field. Quantities which are difficult to measure by experiments are quantified by the simulations, such as the velocity and pressure field in the gas, as well as the subjected forces and torques of powders. Such quantitative information provides insights to the mechanisms of the powder-gas interaction in laser powder bed fusion processes.