Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Julia on 14/10/2009 12:30:04
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Julia asked the Naked Scientists:
I've recently started taking archery classes.
One of the first things we're told is to never "dry fire" the bow - that is, pull the string all the way back and let go with no arrow loaded. Apparently, the bow can actually shatter if you do this. I don't really understand why.
My instructor explained that ordinarily all that force is transferred to the arrow - but it seems like the amount of energy to propel that tiny little arrow would be quite small compared to what would be required to shatter a bow.
Can you help with this?
What do you think?
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I think releasing the bow without an arrow to absorb the stored energy would result in it being adsorbed by the bow its self flexing it in the reverse direction to that for which it is designed.
I am put in mind of the effect of cutting off the current in an unterminated inductor which causes a high voltage oscillation which could damage the insulation, automobile ignition coils are fitted with an internal spark gap to avoid this happening.
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Julia asked the Naked Scientists:
I've recently started taking archery classes.
One of the first things we're told is to never "dry fire" the bow - that is, pull the string all the way back and let go with no arrow loaded. Apparently, the bow can actually shatter if you do this. I don't really understand why.
My instructor explained that ordinarily all that force is transferred to the arrow - but it seems like the amount of energy to propel that tiny little arrow would be quite small compared to what would be required to shatter a bow.
It depends where and for how much time you deliver this energy. If delivered in a small region and for a very short time, you can have a pressure high enough to shatter any material.
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Hypothesis ...
The arrow acts as a gauge: its length effectively limiting the distance the bow can be drawn.
Without an arrow ("dry fire") it would be possible to draw the bow back further than when loaded with an arrow.
If drawn back further this would create greater forces in the bow so making it more likely to shatter than when loaded.
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As syhprum says, the energy has to go somewhere. When the bow fires an arrow, it accelerates the arrow very rapidly, so, even although the arrow does not have a lot of mass, it absorbs a large amount of energy. That energy transfer slows the rate at with the bow returns to its unextended state. Without an arrow to accelerate, the energy is all applied to the bow, so it returns to its unextended state very rapidly without any restraint UNTIL the bowstring goes taught. When that happens the bow decelerates very quickly indeed. The effect is similar to whacking one end of the bow against a brick wall, so it breaks.
Least, that's my theory! [;D]
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I'm with lightarrow and Geezer. It is evident from the speed/distance that the arrow flies that the bow has transferred significant energy to it.
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My guess. Oscillating vibrations perhaps even reinforcing themselves depending on the mass and length of your bow, the tension and material of that bow and string.
And an evil sorcerer behind the bush?
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My guess. Oscillating vibrations perhaps even reinforcing themselves depending on the mass and length of your bow, the tension and material of that bow and string.
And an evil sorcerer behind the bush?
I think you may be overlooking the fact that the bowstring is rather inelastic. When the bow is returning to its "unloaded" state, the bowstring is slack, then, the bowstring suddenly goes taught and prevents further motion of the bow. The acceleration (negative in this case) is very great and sufficient to snap the bow. I suppose it's also possible for the bowstring to snap instead.
When the bowstring is accelerating an arrow, the bow returns much more slowly because the bowstring is always in tension.
A bow is a spring. Springs don't tend to break due to oscillation, but they can break if they receive a great enough "shock load"
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From a random unsubstantiated archery site a modern recurve bow is 80% efficient. Therefore if you take away the arrow you have 5 times as much energy to be absorbed by the bow, which it can't and it can fail.
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For no substantiated reason other than it's obvious that I'm right, I don't think the bow would be damaged if we were to fully extend the bow then cut the bowstring. The bow would certainly oscillate quite a bit (it might be hard to hold on to it!) but I don't think the bow would would break.
I believe it's the enormous deceleration that's produced when the bowstring suddenly prevents the bow from returning to it's rest state that does the damage.
[diagram=535_0]
In this version, the bowstring is pulled back, then cut. The extent of recovery of the bow is limited by two blocks of concrete instead of the bowstring. When the ends of the bow whack into the concrete blocks at high speed, the bow breaks (wonder if the baby falls too?). Whether it's two concrete blocks, or an inelastic bowstring, the result will be the same.
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Sounds good to me OG