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I've noticed that if I shut my door when the window is open in a room, the door will tend to shut faster. If I shut the door when the window is closed with a normal force it will not fully close as if the air is cushioning it. I think when the window is open the door doesn't stop or slow down at all, it just slams.

I was wondering if this was to do with the pressure of the room or something. My hypothesis was that there is more room for the air to escape or there is some sort of convection? Just to note it is definitely not wind because I checked.

Here is a graphical simulation of the phenomenon: enter image description here

Can somebody explain what is going on?

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Your assumption that pressure is the reason is correct. Take a water bottle and try to blow air into it like a balloon. Doesn't take much for it to resist you does it? Now poke a hole in the bottom and try again. Now the hole (window) allows the pressure buildup to dissipate much more quickly. –  Brandon Enright Jan 16 at 22:19
    
Maybe a related phenomenon I observed is when you slam a door shut of a small enclosed room, like a restroom, the door slows down just before it hits the door post and then "bounces" back out a little. And at the end, depending on how hard I slammed the door, the door moves back in a little, but not as far as the position from which the door initially "bounced" outwards. I would guess that the air rushed out when "closing the first time" has some inertia, which causes a a lower pressure in the room before it has slowed the air flow down, pulling the door back in. –  fibonatic Jan 16 at 23:22
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@fibonatic you can also let a thin heavy sheet of glass fall against another sheet of smooth glass like closing a book. Right before it hits, the trapped air slows it down so much that it makes a wooshing sound and comes to a rest very gently. –  Brandon Enright Jan 16 at 23:33

4 Answers 4

up vote 4 down vote accepted

As the door nears the door frame there reaches a point where the door, for a moment, effectively seals off the air in the room from the air outside the room. This only happens for a moment, since most doors aren't 100% air proof. When this happens, as the door continues to close it decompresses the air inside the room, because the volume of the room increases as the door continues to close but the amount of air inside the room doesn't change because the room is briefly sealed off from outside the room. Thus there is an air pressure difference across the door, with the greater pressure coming from outside the room. This greater pressure slows the door down right before it closes.

On the other hand, with a window open air is let into the room and so even though the volume of the room increases as the door closes the air pressure from outside the window pushes air into the room to keep the air pressure inside the room about the same as outside. No pressure air pressure difference is found across the door and thus it does not slow down.

It is also possible that a fan or something inside the building could be creating a lower air pressure inside the building and thus there is a small air flow from the window into the building, which would push the door to close faster one the above-described brief sealing of the room happens, thus increasing the volume of the "slam!".

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I think you have the pressure and airflow backwards for a door shutting on a room. –  Brandon Enright Jan 17 at 1:10
    
I'm thinking of a door shutting on a hall, so that the door opens into the room and the window is in the room as well. –  Joshua Jan 17 at 1:11

When the window is closed, there is no place for the air in the room to go once it's passed through the doorframe. So little air will pass the doorframe and won't act on the door. Picture an electric circuit with a wire dangling, leading to nowhere.

Opening the door reconnects the wire, so to speak, and allows air flow. The flow may not be very noticeable, but the door has a large surface area and experiences a relatively large force from a relatively small PSI once it starts to get in the way of the flow.

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According to me, if you try to close the door with the window open it will get closed as usually. Here the air from outside your room enter your room through the window. If you try to close the door with your window closed no air will enter your room from outside, hence the air that was in your room expand(negligible)(rare) or a low pressure will be created (normally). This low pressure tends to pull your door back . But it'll not be pulled completely because as soon as the low pressure pulls the door, the air from outside enter your room through your door and the low pressure will be stabilised. I think you've noticed that this will happen only if you close the door suddenly and you might not have noticed this phenomenon if you close the door slowly.

Another phenomenon that I've observed is, when you open you open your door suddenly with your window closed (BUT NOT LOCKED) your window will be opened suddenly due to high pressure created inside your room.

Forgive me if I've spoken wrongly, since I'm still a learner. And please correct me if I'm wrong.

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It depends on the situation on both sides of the door. But the answer is related to pressure differences - effectively the air is blowing the door shut just like when you have a draft - but it is not strong enough to slam the door by itself.

Sometimes you'll find you barely tap the door and it slams shut - this is due to the difference between kinetic and static friction. Your tap provided just enough energy to overcome the static friction and the air does the rest.

You will need careful tests to be able to tell, in detail, what exactly is going on. Like I say, vary the situation on both sides of the door.

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