[jason1990]

Suppose that initially the perfume bottle is in the corner of the room. Then initially, the center of mass of the perfume molecules is also in the corner of the room. When the perfume bottle is opened and the particles begin diffusing, the center of mass will move towards the center of the room, as the particles distribute themselves uniformly. So a net force is acting on the particles. This net force is the difference between the forces applied by walls near the bottle and walls far from the bottle. Walls near the bottle will initially receive more collisions and apply more force to the system of particles. If there were no walls (or the bottle was initially in the center of the room), then the particles would diffuse evenly in all directions, the center of mass would not move, and there would be no net force. I suspect, however, that this observation is not relevant to your original question. If it's not, then sorry for interrupting your lively thread. [img]/images/graemlins/smile.gif[/img]