Why do ajar windows open wide when you close the door?
You’ve likely noticed an interesting phenomenon that an ajar window tends to open even wider when you quickly close the door of a room you’re in. In this post, fluid mechanics delivers the explanation for why this happens!
Let’s start with State 1: your window is ajar and the door is in a position to soon be closed. Let’s also assume that for a moment you’re still pondering what did you want to bring from the fridge, so the air in your room has a good chance of mixing with the air from outside your house. This means that both inside and outside your room there’s pretty much the same amount of air molecules per unit volume. It’s equivalent to saying that the air in your room, as well as in the rest of your house, has an atmospheric pressure – the same pressure as outside your house.
State 1: The air in your room has atmospheric pressure.
Now you begin closing the door. If you do that fast enough, the air molecules in front of your door (marked in the picture below) are pushed outside of your room – into your corridor, let’s say.
State 2: We start closing the door.
Your room now contains a little bit less air molecules per unit volume compared to State 1. The first response to this, is for the air that’s already inside the room to decompress. That is, to fill-in the gap left by closing the door:
State 3: The door is closed. Your room now contains slightly less air molecules per unit volume.
As a result of this decompression, the air molecules in your room are arranged less tightly compared to State 1. This is equivalent to saying that the pressure in your room has dropped - it’s slightly lower than the atmospheric pressure outside the window. Now a slightly higher atmospheric pressure outside pushes the window surface from the outside, by trying to enter the room with decreased pressure. On the other hand, the decreased pressure in your room is not able to balance this pushing force. As a result, the window experiences a net force in the direction of opening the window even wider:
State 4: The larger pressure outside your house pushes on the window causing it to open even wider.
Surely this effect won’t last very long! The air from outside your house will quickly mix up with the air inside your room and the pressures come into balance again. The situation is back to State 1. All until the moment when you come back from the kitchen and displace the air in your room again.
Another interesting phenomenon is that when you open the door, the window has a tendency to close. You can make an analogous reasoning to explain this!
The extra bit:
If you can’t seem to obtain this effect in your room, there are a couple of possible reasons:
Reason #1: The window is already wide open. With the window already wide open, the air from outside your house that rushes into your room to balance the pressures will flow more or less parallel to the window’s surface. Hence, its action does not exert a force on the window in the direction perpendicular to the window pane.
Reason #2: You’re closing the door very slowly. In this case, the air in your room also decompresses very slowly. The pressures inside your room and outside your house will have enough time to slowly balance without pushing on the window hard enough.
Reason #3: Your door is leaky. If your door has any openings, or if they’re cut at the bottom to match the doorstep, you won’t manage to push enough air outside your room. Any air that is being pushed out will immediately come back through any openings in your door. You need to pick a different room!
Reason #4: It’s a window on an International Space Station. In this case, the air is only present inside the room you’re in and not outside your room. Hence, the air from the inside wants to escape the ajar window, regardless of the state of the door. This is because of a high air pressure inside compared to vacuum with zero pressure outside. In fact, the air escaping from the room will have a tendency to shut the ajar window rather than open if further. This is because the air inside your room pushes on the window from the inside. And you should appreciate it – your window is trying to save your life! 🛰️
📄 This post is an English translation of my old blog post.