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Thread: Why is vacuum suction so darn powerful?

  1. #1 Why is vacuum suction so darn powerful? 
    Forum Cosmic Wizard icewendigo's Avatar
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    I made this pot of soup and put a lid on it after it was done. Later I try to lift the lid, expecting some suction effect but was nonetheless surprised how hard it was to remove it.
    I realize that water vapors ended up creating a area of low pressure below the lid, but darn I dont understand how that pressure
    difference in air that doesnt weigh much on the outside results in something that that feels like its 10 pounds / 4 kg.
    Now Im thinking of cat burglars with suction cups and have a hard time understanding how 4 hand sized suction cup can hold the weight of an adult?


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  3. #2  
    Administrator KALSTER's Avatar
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    Think more about the repulsion forces between molecules than the suction power of vacuum. It is the combined effort of gravity and electrostatic forces pushing the suction cups to the window surface, rather than the vacuum sucking it.


    Last edited by KALSTER; April 13th, 2012 at 08:11 AM.
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  4. #3  
    Brassica oleracea Strange's Avatar
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    At 15 pounds per square inch, a sucker with a radius of under 2 inches would easily support my weight.
    ei incumbit probatio qui dicit, non qui negat
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  5. #4  
    Comet Dust Collector Moderator
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    Its really not suction, it's the 14.7 pounds per square inch of atmosphere above the lid of the pot. So if it's 6" in diameter, force is about 411 pounds. I can do this in metric if you are a metric person...I am bi thermal and bi standard

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  6. #5  
    Administrator KALSTER's Avatar
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    Question is, why bother with imperial in the first place? Metric is easier to use as a rule of thumb: roughly 1kg/ square cm.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  7. #6  
    Comet Dust Collector Moderator
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    I agree, but most americans are so stupid As an astronomer, I do everything in metric; it's a real PITA to translate it for the US audience.
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  8. #7  
    Administrator KALSTER's Avatar
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    Quote Originally Posted by MeteorWayne View Post
    I agree, but most americans are so stupid As an astronomer, I do everything in metric; it's a real PITA to translate it for the US audience.
    I imagine it would. I hate it when I watch an interesting American produced documentary and the narrator starts talking about foot-pounds of torque, gallons and Fahrenheit.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
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  9. #8  
    Forum Cosmic Wizard icewendigo's Avatar
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    1kg/cm2 of pressure? But if I put my hand flat on a desk or a book, I dont feel x kg of air mass pushing down on my hand or on the book, I feel nothing as if there was no air. Why would air push more on a lid than on a book? How does that work?
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  10. #9  
    Administrator KALSTER's Avatar
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    As you hold your hand normally, the air pressure means there is an equal amount of pressure on every facet of your hand. Your hand on a book does not seal properly, so air can rush around and push back. Also, your hand itself is under an internal pressure equal to the air pressure.
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  11. #10  
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    And that's a good thing, or your hand would be crushed!
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  12. #11  
    Forum Cosmic Wizard icewendigo's Avatar
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    But what I dont get is there's no air between the book and the desk on which it is resting, and I have no trouble lifting a book that's on a desk, so it cant be the air pressure between the book and the desk since theres no air (at least not enough to provide much pressure). If it was just pressure there would be pressure on the book and it would be hard to lift until you get it half a millimeter off the desk, but there's no resistance at all.


    If you put a wet plate upside down (or something) it becomes hard to pickup as if there was suction but there is air inside or water which should be more dense than water, so with the book on a desk and wet plate its counter intuitive (to me in any case) that its just air pressure at work with the lid.
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  13. #12  
    Comet Dust Collector Moderator
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    Unless you seal it out, air goes everywhere.
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  14. #13  
    ***** Participant Write4U's Avatar
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    Which can be demonstrated when you place two glass plates on top of each other. The smoothness of each surface will force air from between the plates (creating a vacuum). Try that and you'll see that it becomes more difficult to seperate them. Sometimes you can even push the plates around, but not lift them. The air pressure from above and below will force the two plates together.
    Last edited by Write4U; April 13th, 2012 at 10:47 PM.
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  15. #14  
    Forum Professor jrmonroe's Avatar
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    Quote Originally Posted by icewendigo View Post
    Why is suction so darn powerful?
    In reality, suction has absolutely nothing to do with what you're experiencing. You're actually working against the ~100 km (~62 miles) of atmosphere above us. Try taking off the lid from an evacuated jar at the bottom of the ocean. Aha! Then try opening the same jar on the moon. Piece of cake.
    Grief is the price we pay for love. (CM Parkes) Our postillion has been struck by lightning. (Unknown) War is always the choice of the chosen who will not have to fight. (Bono) The years tell much what the days never knew. (RW Emerson) Reality is not always probable, or likely. (JL Borges)
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  16. #15  
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    Maybe try it fysically. Go swimming or shower then to rest and dry lay down on a glassplate or tilefloor. Then try stand up.
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