Frictional resistance develops from the interaction between the particle to its surroundings. Entrainment also has to overcome the resistance that occurs because of particle cohesive bonds. The main force reponsible for entrainment is fluid drag . Fluid drag causes the particle to move because of horizontal force and vertical lift. Within a medium of erosion, both of these forces are controlled by velocity.
Horizontal force occurs from the push of the agent against the particle.
If this push is sufficient to overcome friction and the resistance of cohesive bonds, the particle moves horizontally. The vertical lift is produced by turbulence or eddies within the flow that push the particle upward. Once the particle is lifted the only force resisting its transport is gravity as the forces of friction, slope angle, and cohesion are now non-existent. The particle can also be transported at velocities lower than the entrainment velocities because of the reduction in forces acting on it. Many hydrologists and geomorphologists require a mathematical model to predict levels of entrainment, especially in stream environments.