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The Rubber Band Theory of Bow Strings

  1. Oct 25, 2016 #1
    When I was 6 or 7 I decided to make a bow and arrow one day. I had never seen or handled one in real life, but it was intuitively obvious to me from TV that they way to do this was to tie the ends of a stretched rubber band to the ends of a stick.

    This would have worked, after a fashion, except that the rubber band broke when I nocked the arrow and drew back. I asked my father to solve this problem, but he said that this wasn't the way Indians made their bows. He said they had no rubber bands and that the bow strings actually didn't stretch at all: it was the springiness of the bow that supplied the 'elastic' function.

    Now, it's true I had noticed that the bows in movies bent when the string was drawn back, but I had taken that to be an irrelevant flaw in the sticks they used: too flimsy, and was sure the 'spring' that impelled the arrow was the elastic string, which had to be some sort of very strong rubber band that Indians knew how to make. In fact, I thought an improved bow could be made by using a stout stick that didn't bend at all so that all the springiness was relegated to the operative rubber string.

    That latter supposition was almost true: it may not be improved, but a none-the-less effective arrow launcher can actually be made from a stout stick and strong rubber band, like a bungee cord.

    Regardless and however, the point of the story was my cultural inside-the-box thinking had lead me astray: being aware of rubber bands, I immediately conceived of the bow and arrow as working by rubber bands, and couldn't conceive of what I had seen in the movies as working by any other means. Having only ever seen them on TV and in movies, never having touched one in real life, but being aware in real life of rubber bands, having actually handled them, I came to the conclusion bow strings had to be strong rubber bands.

    So, I'm wondering how prevalent this misconception may be among small kids. Did you also assume bow strings were rubber bands when you were little?

    And, if you never thought about bows and arrows as a kid, feel free to post any misconception you arrived at all on your own about how anything worked.
     
  2. jcsd
  3. Oct 25, 2016 #2

    RonL

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    I wasn't aware of any misconceptions until I joined PF, now I seem to have lost count ?:) :biggrin:
     
  4. Oct 25, 2016 #3

    collinsmark

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    I'm trying to remember if I had the same misconception as a child. I think at some point I did, but it's hard to remember. If I did have that misconception that bow strings were stretchy, then it must have been cleared up pretty early. Still, something tells me that I probably did think that at some point early on.

    ----

    A bit on bow physics for those interested:

    Loosely speaking, no good can come from having a stretchable bow string. A bow string with significant stretch to it brings no advantages to the table yet introduces several disadvantages.

    The entire point of a bow -- its only purpose in life -- is to transfer potential energy (stored somewhere) into the kinetic energy of an arrow. Any kinetic energy that doesn't make it into the arrow, such as the kinetic energy of the string and vibrations in the bow's limbs and riser is wasted. As a matter of fact, it's more than just wasted, it actually is detrimental to the bow. These vibrations caused by residual kinetic energy are a primary cause of string wear and wear on the bow's limbs. As a matter of fact, if you were to "dry fire" a bow, i.e., release the string from full draw with no arrow nocked, it usually results in irreparable damage to the bow.

    The string is the part of the bow that has the greatest motion. Sure, the bow's limbs move a little bit, but not very much and not very fast. The string, at least initially, is where most of the unwanted, leftover kinetic energy is. You can reduce the string's kinetic energy by reducing the mass of the string. Of course, you want the string to be strong too, so it won't break (so it cant be too thin), but no thicker than it needs to be. In short, you want the strings to be strong and light. The stronger and lighter the better.

    And it's this reason in part, that you don't want the string itself to be the storage medium for the potential energy. Storing the potential energy in the string would require that the string be more massive for practical reasons and might compromise the string's strength. This would reduce the efficiency of the bow. More-so, if some part of the bow would break, I would prefer the part that stores the potential energy to be away from my face (at full draw, the string typically touches the archer's cheek).

    Consistency and repeatability are everything in archery. Having an elastic, stretchable string adds another variable into the mix, a variable that might change with time as the bow or string ages, or may even change with temperature. Also, the bow is typically "tuned" to have given, known tension on the string for a particular draw length, and it is desirable that it not change over time. For this reason, bow strings are designed to have a little stretch as reasonably possible. This often involves "pre-stretching" the bow string as part of the manufacturing process before the string ever makes it onto the bow.

    So for these reasons, the strings are kept simple: strong, light, and simple. The potential energy is stored in the bow's limbs. The limbs are away from the archer's face, and are easily controllable in such a way that favors consistency and repeatability.
     
  5. Oct 26, 2016 #4

    OmCheeto

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    A bit off topic, but, the following made me feel a bit less self-critical.



    I now have a bad addiction, to "Sixty Symbols".

    ps. Your "Toy" thread, really got me going, regarding; "I have no idea how the world 'really' works".
    Which, I think, is a good thing.
     
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