# The Naked Scientists Forum

### Author Topic: What happens when you fire a bow?  (Read 5885 times)

#### Steve Hooper

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##### What happens when you fire a bow?
« on: 19/10/2009 18:40:53 »
Steve Hooper  asked the Naked Scientists:
Chris,
I am an archery coach and a question has arisen and two different opinions quoted as answers and I was hoping you may be able to help.

When a recurve bow (the type used in the Olympics) is held at full draw the bow handle is pressing against the archers outstretched left palm (assuming a right-handed archer) and the string, with arrow connected to it, is pulled by the fingers on the right hand, the string being drawn across the body.

The question is what happens at the point of release?

I believe that when the string is released the relatively small mass of the string begins to move and overcome the static condition of the arrow (the arrow is only resting on the bow in the other hand and not fixed to it). The arrow accelerates as it is being pushed by the string and the mass of the string and bow limbs (basically the leaf-springs either side) begin to move forward (adding to the mass of the arrow and string). At this point the opposite reaction would be for the handle of the bow to press back into the archers left hand before recoiling away from the
palm.

Draw weight on the string-fingers due to the flex of the bow-limbs would be in the order of 15 to 25 kg (16.3kg in the attached photos) and the mass of the bow handle and associated static paraphernalia would be in the order of 5kg.

I believe the string would travel around 25cm before the bow handle appears to move forward.

I have enclosed some stop-frame images from an archer that shoots at high level (my Son/England team) to show the different stages of the release.

Any light you can shed would be appreciated.

Regards,

Steve Hooper
What do you think?

« Last Edit: 19/10/2009 19:17:16 by BenV »

#### lightarrow

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##### What happens when you fire a bow?
« Reply #1 on: 19/10/2009 19:55:56 »
Until you release the string, the force of your palm, directed forward, is obviously equal and opposite to the force you make on the string (let's say 25 kg). When you release the string, the bow limbs and the arrow go forward and because this could happen, you have to give the system (bow + arrow) a force directed forward, otherwise the bow would go back (the system center of mass stays still in absence of forces); this force is made by your pushing palm; when the arrow has abandoned the string, however, the force you make with your palm becomes suddenly* too big, and the bow goes ahead.
At least, this is my theory...

*actually, things are more smooth, the force of your palm becomes more and more bigger than the one required to move the system, even because of your left arm's inertia.
« Last Edit: 19/10/2009 20:00:25 by lightarrow »

#### Geezer

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##### What happens when you fire a bow?
« Reply #2 on: 20/10/2009 03:55:47 »
Ahem! Er, well, actually I think the bow is being pulled away from the archer.

The ends of the bow reach a considerable velocity while the arrow is being accelerated, therefore they have a substantial amount kinetic energy. But when the bowstring forms a straight line between the ends of the bow, the motion of the ends is suddenly arrested and the bow is no longer dynamic. Now the entire bow has the kinetic energy, so the archer has to arrest the bow to prevent it moving away from him.

If that is true, the pull effect should commence when the bowstring goes straight. Wonder if we can observe that in the photos?

I took another look at the pix, and it looks like the bow might be pressing against the archers fingers more than his palm in the last frame, but it's hard to say for sure.
« Last Edit: 20/10/2009 04:01:38 by Geezer »

#### lightarrow

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##### What happens when you fire a bow?
« Reply #3 on: 20/10/2009 12:21:10 »
No, no pulling at all, I don't agree.

#### Geezer

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##### What happens when you fire a bow?
« Reply #4 on: 20/10/2009 16:19:46 »
Didn't think you would!

So, where did the energy go?

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##### What happens when you fire a bow?
« Reply #5 on: 20/10/2009 17:14:40 »
I also do not think the bow will pull away from the archer. Not significantly anyway, I suppose the string and the ends of the bow could carry some momentum.
Quote
So, where did the energy go?

When drawn, elastic energy is stored in the bow. At this point, there is a force both on the arrow (although the right hand stops it from moving) and the left hand (every action has an equal and opposite reaction). When released, forces are still the same however because no work is done on the archers hand (Work = force * distance) no energy is spent on the hand, so it goes to the arrow.

From the look of the animation, the forward movement seems to be from his wrist relaxing after the sudden release of the backward pressure on his hand.
« Last Edit: 20/10/2009 17:43:49 by Madidus_Scientia »

#### lightarrow

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##### What happens when you fire a bow?
« Reply #6 on: 20/10/2009 20:37:02 »
Didn't think you would!

So, where did the energy go?
After the arrow has been thrown, the bow keeps moving ahead because of the palm's push; of course, at the end, you have to hold the bow with your hand and arm, to avoid it goes forward, but this is just at the end.

#### Geezer

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##### What happens when you fire a bow?
« Reply #7 on: 20/10/2009 20:47:04 »
I mean, what happened to the kinetic energy that was in the ends of the bow when the ends of the bow were prevented from moving further by the bowstring?

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##### What happens when you fire a bow?
« Reply #8 on: 21/10/2009 05:54:58 »

#### lightarrow

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##### What happens when you fire a bow?
« Reply #9 on: 21/10/2009 11:09:42 »
I mean, what happened to the kinetic energy that was in the ends of the bow when the ends of the bow were prevented from moving further by the bowstring?
I'm not so sure of all this effect by the bowstring; if this were the case, the string would be subdue to very high strenght after every launch; I think it's more the elasticity of the bow itself who stops the bowlimbs and so the kinetic energy would go in elastic deformation of the bow, and then into heat; but even if it were as you say, the kinetic energy would go in the elastic deformation of the string; probably it really goes there, but just in a smaller percent.

#### Geezer

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##### What happens when you fire a bow?
« Reply #10 on: 21/10/2009 16:14:20 »
I don't think so. In that case the deceleration produces sufficient force to snap the bow. In this case the deceleration is greatly reduced because the bow is transferring a lot of energy into the arrow.

We might get a clue if we could see what a crossbow does when fired because it eliminates any possible wrist action.
« Last Edit: 22/10/2009 05:29:02 by Geezer »

#### yor_on

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##### What happens when you fire a bow?
« Reply #11 on: 22/10/2009 22:34:47 »
A crossbow has a recoil just as a firearm, I have a scar on my eye (sight) to prove that. And so has a bow, there is a action and a reaction to both. But isn't the direction of that reaction directed forward/downward when it comes to ordinary bows?

#### Geezer

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##### What happens when you fire a bow?
« Reply #12 on: 23/10/2009 03:04:22 »
A crossbow has a recoil just as a firearm, I have a scar on my eye (sight) to prove that. And so has a bow, there is a action and a reaction to both. But isn't the direction of that reaction directed forward/downward when it comes to ordinary bows?
You are correct. There must be a reaction against the person that fires the crossbow and I would think that must also be true with an archery bow.

Perhaps my crossbow suggestion was not such a great idea!

#### litespeed

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##### What happens when you fire a bow?
« Reply #13 on: 01/11/2009 23:45:09 »
The Archer must resist the oposite and equal reaction caused by the release of spring energy. In other words, like an artillery gun, his forarm will be pressed back towards him as the arrow gains velocity. The less he gives, the greater force will be transferred to the arrow.

However, archery contests are not about total energy transferred to the Arrow, except of course in distance contests. In stead, it is about the consistency of velocity as the arrow leaves the string.  This means an accomodation between action and reaction that is endlessly repeatable.

I would acquire a velocity meter, like the gun shooters use. And work towards an absolute standard velocity. Then work on aim.

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##### What happens when you fire a bow?
« Reply #13 on: 01/11/2009 23:45:09 »