If you remember, even in HL 1 when you ran fast into a wall, you would bounce back a bit (and if you were running fast, you would slide a bit before stopping). It depends on how much energy Gordon can absorb, and how much friction he causes (remember in FPS, you act like a rolling ball, not a walking person). If there's enough friction holding Gordon in place, a small enough reactive force pushing you back when you run into a barell won't make you move at all (only the barrell will move). If the force of impact is larger then the friction, Gordon will move after impact. If he is set to absorb a lot of energy on impact, there will be a smaller reaction with the barell. If no energy is absorbed (which is easiest to calculate), there is conservation of energy as well as momentum, so Gordon will bounce back as though the barell and Gordon were both extremely hard metal balls of different masses.
If in CSS you get bounced back a lot without running fast into the objects, or the objects stay still as though they have tons of mass, then something is screwy with the physics. But if you just bounce back about the same amount as a barell bounces away when you run into it (since full barells have about the same mass, if not more), then it's just conservation of energy and momentum in action.
In SP physics, there's probably some extra code used to mimic real human interactions (not just like hard metal balls), where the friction is greater specifically for collisions, and collisions have energy losses (not like 2 metal balls, but like silly putty/ or plasticine depending on the object)
If in CSS you get bounced back a lot without running fast into the objects, or the objects stay still as though they have tons of mass, then something is screwy with the physics. But if you just bounce back about the same amount as a barell bounces away when you run into it (since full barells have about the same mass, if not more), then it's just conservation of energy and momentum in action.
In SP physics, there's probably some extra code used to mimic real human interactions (not just like hard metal balls), where the friction is greater specifically for collisions, and collisions have energy losses (not like 2 metal balls, but like silly putty/ or plasticine depending on the object)