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Weight limits when splitting the weight between two points

  1. Dec 10, 2015 #1
    I have a hammock that is rated to 400 lbs.

    I want to use two neodymium magnets to
    suspend the hammock from two steel posts.

    Do I need two magnets rated 400 lbs each?
     
  2. jcsd
  3. Dec 10, 2015 #2
    The tension is going to be several times larger.
    Consider the angle at the attachment point.
     
  4. Dec 10, 2015 #3
    Ideally the hammock is stretched at almost 180 degrees between the two points.
    I suppose the question should have been what weight do I need?
     
  5. Dec 10, 2015 #4
  6. Dec 10, 2015 #5

    billy_joule

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    Science Advisor

    When the hammock rope is hanging vertically the tension at each support is half the weight of the hammock.
    When the hammock rope goes towards horizontal the tension at each support goes towards infinity.

    So in reality, no rope can ever hang horizontally, they form a catenary:
    https://en.wikipedia.org/wiki/Catenary
    (of course in many cases it's close enough to horizontal for many uses - eg a string line used in constructing houses)

    The tension at each support can be found via
    T= W / 2sinθ
    where θ is the angle below horizontal (ie vertical rope = 90o, horizontal rope = 0o )
    eg for 5 degrees:
    T = 400lb / 2*sin5 = 2,300lb

    IME of hammocks 5 degrees probably wouldn't be very comfortable.

    Why magnets anyway? Something like a carabiner would be much cheaper and easier to use (I'm assuming you're doing it for quick release).
     
  7. Dec 10, 2015 #6
    I have a storage unit in which I can make no structural modifications. I can not drill into the steel beams. I remembered that neodymium magnets could hold incredible weights and thought I could run an eye hook thru a disk magnet.

    What I meant by 180 degrees, when I hang the hammock between two trees, empty I tie it as tight as I can, then when I add weight it sags, yes.
     
  8. Dec 11, 2015 #7

    sophiecentaur

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    Gold Member

    Novel and worth having a go. Of course, if your magnet is strong enough to support your hammock, you might have a real problem prising it off the cabinet, once you have finished with the hammock. Would you want to leave the (expensive) magnet behind? I say 'expensive' because it would need to hold (without slipping) many times your own weight. That could be 50kN, if the sag is to be limited. I found this.
     
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