As a scenario let's take a box filled with air in empty space and no gravitational field around. As the box is opened,the air inside will rush outside and the box will move in opposite direction because of Newton's third law of motion but what is exactly pushing the box or where does the force coming from?
Air Force – Where Does the Force of Air Pushing on Something Come From?
forcesnewtonian-mechanics
Best Answer
Before the box is opened, air molecules are bouncing off of every surface of the box. Let's focus on the surface with the door (call it the "front" wall), and the opposite surface (the "back" wall). Molecules bouncing off the front wall exert a force on the box in the forward direction, but these forces are balanced by molecules bouncing off of the back wall.
When the door is opened, molecules simply escape through the door and no longer bounce off the front wall. Therefore the forces due to molecular collisions on the back wall are not compensated by collisions with the front wall, and there is a net backwards force on the box.