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Curious door problem

  1. May 7, 2006 #1


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    There is something that has been bugging the hell out of me. We have a lecture hall at our university, it has 4 doors. At the front, there is a main door, theres one at the side, and 2 at the back facing opposite directions. Now on some days, this one back door is absolutely impossible to keep close (its little hinges thing doesnt work i guess). You can pull it closed as hard as you can and you can maybe close it (as in bring the door all the way in) but you can't keep it close. So during class, it's just kinda swinging a little bit. Now the insane part is that if someone opens the side door which is like 30 feet away, the back door closes completely by itself. When the side door closes however, it opens up again in the back. The lecture hall is fairly large so there can't be some pressure related phenomenon going on... i highly doubt they're connected...

    My theory: Ghosts.

    What is yours? What the heck is going on here? And better yet, why am i the only one finding this outrageously confusing in my class?
  2. jcsd
  3. May 7, 2006 #2
    Air pressure in the room is higher than the air pressure outside the room. When the side door opens, the higher air pressure rushes out, and sucks the back door closed. When the side door is closed, the higher air pressure along with the mis-aligned hinges help push the door open. Doors have large surface areas, so small drafts can have a large effect on them.
    Last edited: May 7, 2006
  4. May 7, 2006 #3


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    :grumpy: :grumpy: :mad: :mad: :yuck: :yuck:

  5. May 7, 2006 #4
    To draw in by establishing a partial vacuum: a cleaning device that sucks up dirt.
    To draw in by or as if by a current in a fluid.
    To draw or pull as if by suction:

    What's the big deal, sucker? :wink:
  6. May 7, 2006 #5


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    I was always taught never to talk about 'air sucked in', one should always talk as in atmospheric pressure is forcing air in or forcing the door closed. Perhaps it was drilled in a bit too hard by an over zealous high school physics tutor who use to have "Physics doesn't suck" :yuck::wink:

  7. May 7, 2006 #6
    I’ll go with rationalism over mysticism in this case and say Air Flow (Or Fluid Hydraulics).

    Air flow, during those “some days”, favors going though the room and out the side door with a less desirable option being the back door to exit. With all doors closed, both back and side are getting pressured to open from air leaks.

    On one of those “some days” brace open the side door just enough for flow to draw air from back and close back door. --- Then open the front door wide and see if the back does regain the airflow needed to open again.

    No Ghosts (or sucking)
  8. May 7, 2006 #7


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    Heh, sounds like my linear algebra classroom. One back door will never stay shut and the other will never open. Go figure.
  9. May 7, 2006 #8


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    Don't think in terms of static pressure, think in terms of dynamic pressure caused by a constant breeze flowing into the room.

    Air is coming into the room from an undetermined source - perhaps ventilation, perhaps cracks, whatever, doesn't matter.

    That door is the weakest for closing. The air pushes it open. If any other door is opened, the pressure on that door is immediately releived, and it closes.

    Pretty straightforward.

    In my house, the back door creaks open and closed just a little bit. From this it can be concluded that there is another door in the house open (usually upwind). If I close the other door, the back door stops creaking open and closed.
  10. May 7, 2006 #9
    I thought that's what I said until everyone started attacking me for using the word suck :grumpy:!
  11. May 8, 2006 #10


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    If the latch engages the strike in the frame when the door is shut, then it must be doing so only slightly. You can cure it by shimming the strike plate outward with a spacer.
    If it doesn't engage, it could still a problem of too much gap. Otherwise, the bolt might not be properly aligned. Make sure that the hinges are tight so the door isn't sagging. If they are, then look to where the bolt touches the strike. (If you can't see in there, put some lipstick or paint on the end of the bolt and close the door; it'll leave a contact mark.) The hole in the strike can then be enlarged with a Dremel or file until it's sufficient for the bolt to fall into.
    I know that this isn't an answer to your question, but the others have taken care of that. It's just a way to remedy the situation if it bothers you.
  12. May 8, 2006 #11
    Hi, sorry for the silly question.

    Does the back door open in direction to inside the room or in direction outside the room?
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