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Ballistics Pendulum

A place to ask general spud cannon related questions.
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Ballistics Pendulum

Unread postAuthor: Lockednloaded » Sat Dec 19, 2009 12:13 am

In Physics the other day we did ballistic pendulum calculations, and it seemed like a great way to measure velocity for all of us who don't have chronys or anything else for measuring this(besides the fact that the calculations can be a bi@tch). Basically the setup is a block is suspended on two wire and massed. The bullet is then massed and shot into the block, and if the bullet does not bounce off (inelastic collision) you see how hight the block traveled and then you can calculate the velocity of the bullet with a basic collision grid (i'm not goin to explain the calculations if some other member wants to fine, but i don't really want to :lol: ) This system works for real guns, so it should work for our creations.
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Unread postAuthor: rp181 » Sat Dec 19, 2009 12:20 am

yep, people have been using those for quite a while :)
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Re: Ballistics Pendulum

Unread postAuthor: Ragnarok » Sat Dec 19, 2009 12:24 am

Lockednloaded wrote:...besides the fact that the calculations can be a bi@tch...

You'll have to explain why pushing around nothing more complex than square roots is a "bitch".

I would derive the equations for you, but I'm up at stupid o'clock, and thus, going to bed is more important.

Either way, they're perfectly feasible - Larda used a ballistic pendulum for the 28.8kJ ETG. I'm a little surprised they don't see more use, but some things are just odd.
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Unread postAuthor: Lockednloaded » Sat Dec 19, 2009 12:32 am

well maybe not for you but in my physics class we have to use wacky numbers, and I'm only in eighth grade. I might set one up and compare it to a GGDT reading, and see how close they are if its not a huge margin i might start using one. I just made this topic so that everyone knows that there are many ways to calculate the velocity of your cannon, and even one as simple as a pendulum and some calculations
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Unread postAuthor: SpudBlaster15 » Sat Dec 19, 2009 2:19 am

For those who are wondering, this equation will give you the velocity of the round.

v<sub>r</sub> = sqrt(2*h*g)*(m<sub>p</sub> + m<sub>r</sub>)/m<sub>r</sub>

v<sub>r</sub> = velocity of the round, m<sub>p</sub> = pendulum mass, m<sub>r</sub> = mass of the round, h = maximum height of the pendulum, and g = gravitational acceleration.
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Re: Ballistics Pendulum

Unread postAuthor: jackssmirkingrevenge » Sat Dec 19, 2009 2:27 am

Lockednloaded wrote:it seemed like a great way to measure velocity for all of us who don't have chronys or anything else for measuring this


You canmake your own chrony in a much simpler manner that will be infinitely cheaper, compact and easy to use.
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Re: Ballistics Pendulum

Unread postAuthor: Ragnarok » Sat Dec 19, 2009 12:43 pm

jackssmirkingrevenge wrote:You can make your own chrony in a much simpler manner that will be infinitely cheaper, compact and easy to use.

<Pedantism>Infinitely? So it costs no money, takes up no space whatsoever and requires absolutely no effort at all on the part of the user?</pedantism>

Some people like to ignore that the two methods of velocity measurement have their own advantages and disadvantages.
Limiting yourself to just one of them for all purposes puts you at a disadvantage when you come to something where the other method would be better put to use.
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Re: Ballistics Pendulum

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Re: Ballistics Pendulum

Unread postAuthor: jackssmirkingrevenge » Sat Dec 19, 2009 1:55 pm

Ragnarok wrote:Some people like to ignore that the two methods of velocity measurement have their own advantages and disadvantages.


Other than not needing batteries, mention one advantage of the ballistic pendulum, especially for larger launchers :P you'd also never have an accurate result because of energy lost through projectile/block deformation.
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Re: Ballistics Pendulum

Unread postAuthor: Ragnarok » Sat Dec 19, 2009 2:42 pm

jackssmirkingrevenge wrote:...you'd also never have an accurate result because of energy lost through projectile/block deformation.

Um...

... what you just said is entirely wrong.

The ballistic pendulum relies on momentum conservation, not energy conservation.
Momentum is always conserved in a collision, be it elastic or inelastic. Kinetic energy isn't (as you say, energy is lost to deformation, heat, etc), but that's irrelevant to the pendulum.

Unless you've been a total idiot*, a ballistic pendulum is always accurate.
It may not be hugely precise, but accurate and precise aren't the same thing.

*Admittedly, as with chronographs, get too close to the muzzle blast, and that can cause issues.

Other than not needing batteries, mention one advantage of the ballistic pendulum...

Depends on what I'm comparing it to.

A sound rec chrony is obviously marred by drag, as well as (on occasion) unclear firing and impact sounds.
A lightgate chrony (at least, a homemade one) can't be used particularly far downrange - try to get one to work 100 feet from the muzzle. Can you really get your projectile to cross that tiny beam?

Also, I've got a number of things going on for a project where they have to be based around ballistic pendulum technology. They quite literally cannot be done with sound-rec or lightgate chronys.
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Re: Ballistics Pendulum

Unread postAuthor: D_Hall » Sat Dec 19, 2009 3:38 pm

Ragnarok wrote:A lightgate chrony (at least, a homemade one) can't be used particularly far downrange - try to get one to work 100 feet from the muzzle. Can you really get your projectile to cross that tiny beam?

Of course, the same thing could be said of pendulums... Can you really get your projectile to hit COM of your pendulum?

Also, I've got a number of things going on for a project where they have to be based around ballistic pendulum technology. They quite literally cannot be done with sound-rec or lightgate chronys.

I don't doubt the sound gate tech. Those are VERY limited (and of questionable accuracy but I'll grant that's another thread).

I'm curious though... What's so unique that you can't use non-pendulum based tech? Honestly, I've never heard of anybody using a pendulum for anything other than cost issues.
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Re: Ballistics Pendulum

Unread postAuthor: Ragnarok » Sat Dec 19, 2009 4:31 pm

D_Hall wrote:Of course, the same thing could be said of pendulums... Can you really get your projectile to hit COM of your pendulum?

With a relatively well done and hinged pendulum, then not being too far out from COM shouldn't too dramatically interfere with the results.

I'm curious though... What's so unique that you can't use non-pendulum based tech?

Well, it's not so much that it's completely impossible to do it any other way, but it's actually the most viable solution.

It's to do with energy used during penetration of given media. Given the potential for spalling or simply the projectile itself disintegrating, most lightgate based methods would be thoroughly foxed by the potential debris flying about.

Maybe if I had a high speed camera I could work from that, but I don't and it wouldn't actually make for a better solution.
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Re: Ballistics Pendulum

Unread postAuthor: D_Hall » Sat Dec 19, 2009 5:09 pm

Ragnarok wrote:With a relatively well done and hinged pendulum, then not being too far out from COM shouldn't too dramatically interfere with the results.

Similar arguments could be made regarding a well done shadow box, of course.

It's to do with energy used during penetration of given media. Given the potential for spalling or simply the projectile itself disintegrating, most lightgate based methods would be thoroughly foxed by the potential debris flying about.

So how does a pendulum solve this problem?
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Re: Ballistics Pendulum

Unread postAuthor: Ragnarok » Sat Dec 19, 2009 10:25 pm

D_Hall wrote:So how does a pendulum solve this problem?

For one of the tests at least, I need to record the energy left over in the flying fragments of target and projectile - lightgate based stuff would probably give me a confused velocity, and I'd still not know how much exactly had survived.

As a pendulum both gives the total momentum of the flying fragments and catches them (to later mass), I can then actually work out the energy left over properly. Less fuss, and a proper answer.

Obviously, the ideal option would still be a high speed camera, but it wouldn't necessarily give a better answer than a well made pendulum.
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Unread postAuthor: D_Hall » Sat Dec 19, 2009 11:42 pm

OK, fair enough. You're interested in the fragments and such too. As originally described, I thought you were saying that the pendulum would somehow descriminate between projectile and fragment.

As you say, there are other ways, but yeah, a pendulum is probably the easiest in that case.
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Unread postAuthor: jackssmirkingrevenge » Sun Dec 20, 2009 12:14 am

Whoops, tripped in my physics there. I still can't see any advantage that would make a pendulum preferable to a lightgate chrony though.

For one of the tests at least, I need to record the energy left over in the flying fragments of target and projectile - lightgate based stuff would probably give me a confused velocity, and I'd still not know how much exactly had survived.


The only decent way to do this is using a high speed camera against a metered screen. Without such equipment, I would use a series of even spaced thick card/thin ply screens, that would not only give me a measure of fragment velocity but also distribution.
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