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Suction related to opening a door quickly

  1. Mar 22, 2008 #1
    Say you go into a building where you open one door walk a few feet and open another door. I have noticed that if you open the first door fast the other door obviously opens due to suction? Basically are you doing the work it requires to open two doors or basically the summation of the work of opening both doors?
  2. jcsd
  3. Mar 23, 2008 #2


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    Staff: Mentor

    The work one does in opening the door is simply the force applied by one integrated over the distance applied. That work has nothing to do with the second door.

    When opens a door rapidly, one is both moving the mass of the door and applying momentum to the air in front of the door. That momentum applied to the air briefly displaces the air and reduces the pressure behind it (suction). The reduced air pressure in the chamber between two doors allows the higher pressure on the back side of the second door to push it open.
  4. Mar 23, 2008 #3
    That is based on a force impulse. The siphon is continuous. It would be analogous to hundreds of doors opening one by one with only one push. One way to explain this scenario, is each door has potential energy within a trigger release spring. All it will take is a small pressure difference to release this spring energy. That will make the pressure pulse for the door, etc.,

    This affect is common to chemical reactions. The reactants, before they can lower energy need to climb an activation energy hill. The spring is the difference between starting and final products. The activation energy is supplied by the final affect, with the result one gets more energy out than needed to activate the process. The current theory goes from start to finish directly. But nature uses an activation hump, with the energy to overcome the hump supplied by the final affect. That assumes one is trying to get a chain reaction for lowering potential, sort of like a siphon.
  5. Mar 23, 2008 #4
    So then the second door that swung open would have the same amount of work as it closes. thanks you guys.
  6. Mar 23, 2008 #5
    Yes exactly, you are indeed opening both doors instead of one. but its not suction.

    It's just that the object (air) carrying the force from one door to the other is noticeably compressible. It's identical to having both doors connected by a spring.

    The speed dependance is obviously due to air escaping easily past the opening you made in the fist door only if you open it slowly enough. Open it quick and the air acts like a spring before it can escape.
  7. Mar 24, 2008 #6
    Ahhhh, I feel like I understand it a lot better. Thanks to all.
  8. Mar 25, 2008 #7
    Oops, the mechanism could be suction, its just when I visualised it I assumed you'd pushed the first door rather than pulled on it.
    But it all works out the same really, in my opinion you are simply opening both doors at the same time so it requires twice the amount of effort to both doors quickly than it does to open one door slowly.
    i.e. twice the amount of energy is needed if the air is trapped, because you're actually opening both at once.
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