I'm having hard time understanding the concept of forces action on particles in SPH. For example, in "classical" fluid mechanics, shear force shows how much one layer of fluid is dragged down by another. In SPH, on the other hand, I can't quite visualize the same concept.
In one of the lectures of Monaghan on Youtube, he mentions that instead of calling momentum equation he prefers calling the acceleration equation. So, does it mean regardless of the force(surface or body), we are interested in the rate of momentum loss and it does not matter if the force is a surface or body?