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A good analogy for the force on a 2 bolt anchor

  1. Jul 28, 2011 #1


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    So I do a wee bit of climbing, and an important thing to take into account when building anchors is the angles formed by the cord used to connect the pieces to the master point, where the rope will be attached.

    So to keep things simple here is the basic method - you tye a carabiner to the middle of a piece of cord, then tye each end of the cord to a bolt.

    Now most climbers know that you should keep the angle in your cord less then 60*, and anything over 120 is a disaster.

    Recently a couple of friends who have very little or no understanding of vectors have asked me why this is, and if I could come up with a good analogy. My explaination of force as a vector etc. helped a little, but I just couldn't think of a good analogy at all. The best thing I could come up with was to say that each bolt wants the master point to be directly below it so tries to pull it over. If there is one bolt then the master point moves over and hangs below it, but if there are two then the other bolt is also pulling, so the bolts kind pull on each other via the masterpoint and that is where the extra force comes from. Weak, right?

    So I guess this ridiculous and long winded post is me asking if anyone has a good analogy to help someone who didn't take physics in high school to understand what is going on in his anchor.

    I hope this made sense...
  2. jcsd
  3. Jul 28, 2011 #2
  4. Aug 3, 2011 #3
    No analogy needed. Get some thin thread and tie a 2 pieces between two fixed objects, say between two legs of a table. Leave enough slack in the string so that a finger in the middle of the string makes the good angle 60 degrees and the other string makes the bad angle greater then 120 degrees. Now push with the finger in the middle until the string breaks. The required forces should be noticeably different. The string making the bad angle will be easier to break. The bad angle multiplies the climbers weight force on the anchors and the webbing running between the bolts. The extra force on parts that may save your life is a bad thing?
  5. Aug 4, 2011 #4


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    Huh, I could swear that I posted a response to your earlier comment showing a set up, but you seem to get it.

    My friends know that it happens and know that the forces are increased but they really want to know why it happens.
  6. Aug 4, 2011 #5
    Then you have to teach them Newton's second law and how a vector force can be resolved into its components. Then follow up with my example above.

    Good luck!
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