So, let's say we have a spacecraft in deep space. It activates its rocket engines, to keep it simple. The engine reaction channels exhaust through the aft nozzle, right? How does that chemical reaction actually induce a change in the velocity of the spacecraft? The particles have to push against the geometry of the spacecraft to provide a force. Do they do that and how when their direction vector clearly goes away from the spacecraft?
And here's something else. A pressurized spacecraft opens up an airlock which decompresses the entire deck, would that create a small thrust for the spacecraft and how? Is it the contact hull - air as it is accelerating outwards?