MadSci Network: Physics

Re: How come when you open a window the door is pulled in?

Date: Fri May 8 13:21:06 1998
Posted By: Everett Rubel, Other (pls. specify below), Master in Physics, Instruction Set, Inc.
Area of science: Physics
ID: 893455027.Ph

How come when you open a window the door is pulled in? 

Like if the door is slightly a jar it'll close completely and if the 
door is open and you close it it'll slam even though you didn't slam 

I think what you are seeing is the effect of differences in air pressure.  
This is just a guess I am making, so you should think about checking to see 
if these ideas are reasonable or not.  I would check myself, but I don't 
know of any building close by where this sort of thing happens.  I have 
seen it before though.
Here is my reasoned guess:
Many buildings have forced ventilation.  This means that there are big fans 
that push  heated or cooled air through ducts into the different rooms in 
the building.   For a particular room, the air is forced in.  If there was 
no outlet, the pressure in the room would increase until the pressure in 
the room matched the pressure in the duct, and the air flow into the room 
would stop.  For good ventilation, the air should flow through the room, so 
a place for air to go out is needed.  Then when some air is pushed into the 
room, the same amount of air is pushed out of the room.  The outlet has 
some resistance to the air flow, so the air pressure in the room is 
slightly higher than it would be without the ventilation system.
So far we have a room with a slight excess in air pressure compared to the 
outside air.  Here I make an assumption that the door you are talking about 
is to some other interior room or hallway.  If it is a door to the outside 
then we need to think of another argument.  
Let us say there are other rooms and hallways next to the room we are 
thinking about.  These should have the same air pressure as the room.  When 
you open a door between two rooms with the same air pressure, there should 
be no air movement from one room to the other.
Now let us think about opening a window in the room.  Now there is another 
way for air in the room to get out of the room.  That means it is easier 
for the air to get out.  Even though the ventilation system is pumping air 
in, the air is escaping even faster, and so the pressure in the room drops 
with the window open.
Now the pressure in the one room is lower than the pressure in the other 
room.  The air in the room with higher pressure creates a larger force on 
the door between the rooms than the air in the room with lower pressure.   
This is like a shoving contest between two people with different shoving 
ability.   The result is a force that pushes the door toward the room with 
lower pressure.
Here are some numbers to get an idea of what sort of force we are dealing 
with.  Suppose the difference in pressure is about  1000 Pascals ( Newtons 
per square meter).  This is about 1% of the total atmospheric pressure.  It 
is about the pressure difference between the top and bottom of a hill 100 
meters high. If the door has an area of 1 square meter, then the force on 
it will then be 1000 Newtons.   This is about the same force you would feel 
if two of your friends were to stand on you.  Just a small difference in 
pressure can create a  large force on the door.

With the proper room set up, you could try an experiment.  If you have two 
rooms with a door that slams as you describe, try opening windows in both 
rooms at the same time.  See if the door slams differently or slams at all.

You should ask around and see if any of your peers knows anyone who works 
in the field of heating, ventilation, and air conditioning ( HVAC engineer/ 
technician).  Such a person would have a ton of practical experience with 
this sort of situation.



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