Weird crossbow physics question

[Caddyshack]

In one of my weird day dream moments I wondered if a cross bow could work on a zombie at point blank range.

I.e. if the tip of the bolt was already touching the zombie head when you pulled the trigger or does the bolt need to accelerate along the length of the bow.

I get that all the energy is already stored in the bow but maybe the metal of the bow needs to go fully back to straight before all the energy has been released?

If the bolt was against a brick wall would it push the bow back towards the person firing it?

[youngsyr]

There's a fixed amount of energy stored in the bow part of the cross bow. Once the string is released it will transfer into the bolt and propel the bolt into the zombie's head.

If there's enough force to penetrate at range, there's enough force to penetrate point blank.

If you fired it point blank into an immovable object like a solid brick wall, the force would be pushed into the wall, which wouldn't move and would push back against whatever was holding the cross bow, I.e. your hand/shoulder.

[Caddyshack]

youngsyr said:

There's a fixed amount of energy stored in the bow part of the cross bow. Once the string is released it will transfer into the bolt and ghrough the bolt into the zombie's head.

If there's enough force to oenetrate at range, there's enough force to penetrate point blank.

If you fired it point blank into an immovable object like a solid brick wall, the force would be pushed into the wall, which wouldn't move and would push back against whatever was holding the cross bow, I.e. your hand/shoulder.

Thanks, so do you think the bolt is doing the same feet per second in the first 1cm of travel on the rail of the bow as it is when the flights pass the end of the rail (fully left the bow) as I wonder if the bolt accelerates as the energy might not be fully released until the curve of the bow has fully gone back to un cocked….so then energy is released in proportion to the bow returning to un cocked state?

Does the bolt gather energy as the string releases and the curve returns to normal therefore the bolt needs room to receive all the energy?

Edited by Caddyshack on Tuesday 8th November 21:44

[youngsyr]

Caddyshack said:

youngsyr said:

There's a fixed amount of energy stored in the bow part of the cross bow. Once the string is released it will transfer into the bolt and ghrough the bolt into the zombie's head.

If there's enough force to oenetrate at range, there's enough force to penetrate point blank.

If you fired it point blank into an immovable object like a solid brick wall, the force would be pushed into the wall, which wouldn't move and would push back against whatever was holding the cross bow, I.e. your hand/shoulder.

Thanks, so do you think the bolt is doing the same feet per second in the first 1cm of travel on the rail of the bow as it is when the flights pass the end of the rail (fully left the bow) as I wonder if the bolt accelerates as the energy might not be fully released until the curve of the bow has fully gone back to un cocked….so then energy is released in proportion to the bow returning to un cocked state?

The bolt's speed is irrelevant/ a red herring.

The bolt needs a certain amount of energy to penetrate the zombie's head. The bow either has enough energy stored to penetrate or it doesn't. The bow will transfer all its energy to the bolt regardless of how far the bolt travels.

[Kawasicki]

The bolt has peak kinetic energy as it leaves the string, when acceleration of the bolt due to the string is zero

The force of the string on the bolt when fully tensioned is the same (or close to) the force that was required to put the bow into this cocked condition.

I would imagine the bolt is far more dangerous at peak speed than it is at first acceleration, even with the string pushing it.

[Kawasicki]

youngsyr said:

Caddyshack said:

youngsyr said:

There's a fixed amount of energy stored in the bow part of the cross bow. Once the string is released it will transfer into the bolt and ghrough the bolt into the zombie's head.

If there's enough force to oenetrate at range, there's enough force to penetrate point blank.

If you fired it point blank into an immovable object like a solid brick wall, the force would be pushed into the wall, which wouldn't move and would push back against whatever was holding the cross bow, I.e. your hand/shoulder.

Thanks, so do you think the bolt is doing the same feet per second in the first 1cm of travel on the rail of the bow as it is when the flights pass the end of the rail (fully left the bow) as I wonder if the bolt accelerates as the energy might not be fully released until the curve of the bow has fully gone back to un cocked….so then energy is released in proportion to the bow returning to un cocked state?

The bolt's speed is irrelevant/ a red herring.

The bolt needs a certain amount of energy to penetrate the zombie's head. The bow either has enough energy stored to penetrate or it doesn't. The bow will transfer all its energy to the bolt regardless of how far the bolt travels.

It’s not the energy that gets you, it’s how quickly the energy transfer happens.

[Caddyshack]

Ah, I get you. The speed and the energy contained are not the same thing, the energy is accelerating the bolt but the energy is already there to penetrate.


So, another question: if you follow my thoughts of the curve of the bow going from cocked (fully bent) to straight, fired and unloaded and you could stop the curve on stoppers then the bow would push back in to the shoulder therefore the energy that released the bolt is now transferring to the shoulder therefore the bolt has been released but the full energy has not transferred to the bolt as it is now pushing the bow back.

Or instead of point blank against the wall you fire it 1 cm away from the wall which doesn’t allow the bow to fully straighten. It would surely push the bow back still which means not all the energy has transferred to the bolt until that curve is fully straight. On that basis the bolt would have less penetrating power against the zombie head if fired very close or point blank than if fired 1 foot away?

[Caddyshack]

It seems from this (below) that the draw length is important to pass that energy so it needs some distance to receive all of the energy.

An earlier answer is that you can easily hold a the tip of a drawn bow against a tree.

There is a gradual acceleration as the bow returns to its un-drawn position. The distance over which this force is applied is just under the length of the arrow continually giving energy to the arrow, in math: [;E=\int_0{r} F_{bow}(s) ds;].
When the bow hits the target the stopping distance is smaller but the Energy is the same so we know the force must be greater.
The reason why it doesn't penetrate when pressed against the tree is because the launch force is smaller than the required force to penetrate and so the tree just exerts the same force on the arrow as the bow does and the arrow doesn't move. When launched from a distance, there is no force acting on the arrow when it reaches the tree but it does have a lot of energy, when it hits the force from the tree slows the arrow across a short distance, meaning the force is very large and therefore the reaction force (of arrow on tree) can penetrate the bark.
If you have a softer substance the force will be less and the arrow will penetrate more deeply.

[Simpo Two]

I think that to impart max energy to the bolt, the string has to become straight.

[Caddyshack]

Simpo Two said:

I think that to impart max energy to the bolt, the string has to become straight.

Yes, I have now come to that conclusion from what I have been reading up.

Have toyed with the idea of a cheap cross bow, some water melons and use my air rifle chrono to do some scientific experiments but last knights reading makes me agree 100% with your post.

[Russ35]

Not a crossbow, but there are plenty of archers who have put an arrow though their hand between the forefinger and thumb due to the arrow coming of the rest as they loose the arrow.

[Caddyshack]

Russ35 said:

Not a crossbow, but there are plenty of archers who have put an arrow though their hand between the forefinger and thumb due to the arrow coming of the rest as they loose the arrow.

I imagine that it depends on the draw poundage of the weapon. Even 30% of a powerful bow is enough to penetrate I would assume...following the logic.

[Simpo Two]

Caddyshack said:

Simpo Two said:

I think that to impart max energy to the bolt, the string has to become straight.

Yes, I have now come to that conclusion from what I have been reading up.

Have toyed with the idea of a cheap cross bow, some water melons and use my air rifle chrono to do some scientific experiments but last knights reading makes me agree 100% with your post.

On reflection I think it would be wiser simply to keep a further distance from said zombie...