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Double-Bagging - is there are real advantage for sharp contents?

  1. Jan 12, 2008 #1
    First off, let's leave all common sense aside. Common sense is going to derail the insanity that is this question =P

    I just went to buy some printer paper from Wal-Mart (not because I support Wal-Mart, but because it's a 30 second walk from where I work) and, foregoing my usual self-checkout as all of those lanes were preoccupied, I opted for the speedy 20 Items or Less checkout service. As I swiped my card and entered my pin number, the cashier was so nice as to double-bag my bulky printer paper, informing me that the corners are very sharp and will tear straight through one bag under the weight of the stack of paper. So I said thank you, got my $20 cash back and was on my way out. But I'm a sort of curious guy, so I started thinking... And here's my question to all you physicists (wanna-be physicists welcome too!). Now, I'm going to word this a little weird, but I think it will better illustrate my question.

    So let's say I'm in the market for a Katana (of COURSE my weapon of choice is the tonfas, but this is for illustrative purposes only) and I venture into my local Wal-Mart, because let's face it, their prices on Katanas are unbeatable. So I'm browsing through their fine sword collection and stumble across a beautiful one with a red handle, and it's really shiny to boot. It even matches my ceremonial kimono, and all at the low low price of $29.99! I ask the inately inept Wal-Mart employee if there are any more, and much to my chagrin I am informed it is the last one, and they have lost the box, but they are willing to part with their display model which I am repeatedly assured is just as sharp as any new one. So, being the impulse shopper that I am, I accept and head to the cash register. I hand the sword over to the cashier, she rings it up and as I swipe my card and start to put in my pin number she tells me she's going to be so courteous as to double-bag it, because the tip is very sharp and would puncture just a single bag easily. I'm not sure why she's elected to bag it like this; maybe that whole "hand scissors handle-out" thing. I would have been happy just to carry it, but whatever, I'll let her do her job. So now I have this sword double-bagged in Wal-Mart plastic bags, point down. My question is:

    Why is it less likely to puncture two bags than it would be to puncture just one? Wouldn't it just puncture the first one and then the second and fall through? Isn't there some infinitesimal amount of space between bag 1 and bag 2? Obviously double-bagging a gallon of milk is different; the weight distribution is comparitively vast, and it does not threaten to puncture the bag but instead to split the seam. So why would double-bagging sharp objects help? I mean obviously it does help, but why? And if we're going to get hung up on the elasticity of the plastic bags, why would it work with paper bags also, supposing I am eco-friendly?
    Last edited: Jan 12, 2008
  2. jcsd
  3. Jan 12, 2008 #2


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    Welcome to PF, Soliloqy.
    First off, you should hang out in GD and shoot for Funniest Member 2008. Great post.
    In my experience, double-bagging inherently sharp objects does no good at all, and for the very reason that you mentioned. A semi-sharp object such as the paper package seems to behave differently.
    My guess (not necessarily correct) is that the force upon the contact point changes when the inner bag tears. The bulk of the weight is still supported by the remains of the inner bag, so the pointy part exerts less pressure upon the outer one.
  4. Jan 12, 2008 #3
    No think about it this way:
    1 Bag can support say, 3 pounds before stretching and thus weakening the bag allowing something with a sharp point of contact to puncture easier.
    2 Bags could support 6 lbs and so on, thus will not stretch and make it weaker.
    I believe that is the point behind it, its not about the puncture, but the puncture due to weakening of the bag due to the weight.
  5. Jan 12, 2008 #4


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    I disagree. The weight-bearing capacity of the bags is more of a square or even logorithmic function. (I don't know what the hell it is, but it isn't linear.)
    Something like a sword, though, will slice through any number of bags with no regard to the ones that come first.
  6. Jan 12, 2008 #5


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    sure it is.

    if it's not a piercing or cutting agent, the weight bearing capacity is roughly linear. the stretch for both bags is about the same, the stretch-force function is roughly Hooke's law (until things stretch nastily and your bag is about to tear), and the forces add up. three bags oughta how about three times the load for a roughly consistent stretch of the bags.

    when there's a cutting agent, i dunno how to model it.
  7. Jan 12, 2008 #6


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    Thanks for bringing that up, RBJ. I agree that the load capacity of multiple intact bags is probably linear or close-to. My reference, which I should have expressed more clearly, was to multiple bags supporting an item that has proven capable of puncturing through one bag, but no longer has its full weight on the pointy part.
    I might still be wrong, but my past experience and 'gut sense' suggest to me that I'm not. You know, though, that I want the facts even if they conflict with my thoughts, so please help me figure out how the 'system' for this works. (I couldn't figure out any way to present that previous sentence that didn't sound sarcastic, so I'm just going to tell you straight out that I meant it literally. If I'm wrong, I want to know it and to know why. :smile:)
    Last edited: Jan 12, 2008
  8. Jan 12, 2008 #7
    bring a cloth bag to the store


    Is it easier to cut a single thin sheet of paper, or multiple sheets, or a very thick sheet? (with 'name your cutter')
    Last edited: Jan 12, 2008
  9. Jan 12, 2008 #8


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    A single bag with a small tear in it is still 90% supporting its contents - at least momentarily. If you then support this object (which is now an object with a outer lining that closely matches the second bag it's carried in, but only has a small hole with a small pointy bit) the second bag is much less likely to tear.
  10. Jan 13, 2008 #9
    I still think its meant to prevent ANY tears by lessening the likeliness of deformation due to weight, therefor increasing the ability to tear. (Like what Dave mentioned)

    I WILL agree though that 2 bags won't keep a hattori hanzo point from piercing any more than one in a realistic sense, but if you had 20 some bags then yes I think there would be enough support even at the sharp point to possibly prevent piercing. Thanks to the decreased pressure for each bag at that point. There must be some cutoff weight/sharpness/bags.
  11. Jan 13, 2008 #10
    Thanks guys, your answers are very informative, but I was looking for a more scientific answer, rather than just a common sense approach.
  12. Jan 13, 2008 #11
    2 > 1
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