Yes it does, but in the videos, this opposite force is just not applied as it should, so the effect is less then its supposed to be.

Very cool development! I think the application for eles will have a positive impact on gameplay. Often the damage output for eles ends up being less than it could be due to its path being blocked by own (my skill issues) and enemy units. Maybe its negligible, but do the pikemen in video 2 impart an equal and opposite force to the ele that would slow it down? I could see this as a way to nerf it if people think its too hard to impede eles' movement.

Just downloaded you mod and tried it, it said in the mod.json file it was for 0.29.0, is that right? Then I changed it to 0.28.0, and I clicked start new match, and got this:

Gotta say I like the inertia in these two videos: https://gitea.wildfiregames.com/0ad/0ad/pulls/8957#issuecomment-140218  Especially the ships slowly drifting before stopping looks so much more natural than the abrupt stop now.

So for the explanation again, we could now plug any force that could contribute to unit motion and we just sum up the vectors. So it's almost free there, when calculating unit motion. Maybe excepted we still have to check for collisions. The rest of what makes the unit motion looks realistic is inertia, which is also just a ""force"" vector that carry unit velocity over the game turns. Calculating the forces themselves is generally making some dot products and sometimes maybe some trigonometry (kept as minimum for angle of impact etc) so also possible to be kept very cheap. For the current demo of the boat we use 1 trig function which seems reasonable.