17 replies to this topic

[madmax]

Link:

Well, so I have successfully completed the physics project I was previously talking about (if anyone is interested in the spring constant of a crossbow, its about 412 n/m) The video isn't all that entertaining, but it does have a potato cannon at the end, and a small guide on how to record the fps or m/s of a nerf gun.

spoilers:
Crossbow with no extra bungees and a 12'' brass barrel shooting converts gets 200 fps
Potato cannons go BOOOOM!

[Shrub]

Nice putting nerf into your physics very cliché . Was it a school project or just a "I was bored" project.

Edited by Shrub, 06 June 2009 - 10:04 PM.

[madmax]

It was a school physics project, I figured I've always wanted to know how fast my crossbow shoots in relation to my airsoft gun, and now I know that it shoots about 100 fps less. And I like potato cannons, they make booms.

also: Cliché

[Blue]

That was actually a pretty cool project.

Edit: What grade did you get on it?

Edited by Blue, 06 June 2009 - 10:05 PM.

[madmax]

I'm turning it in on Monday, so I don't know yet. Seeing as our final was watching star wars (Yay after AP test AP physics) and picking out the physically impossible things (like deflecting lasers with light(sabers)) I think I should do pretty well. Though its kind of tame compared to my friends coil gun, but thats not going up on youtube.

[Doom]

Good to see someone learning a thing or two about science.

You probably won't be surprised to learn that there is NO simple way to accurately estimate the performance of a spring or pneumatic gun. No analytical (i.e. plug the numbers in) solution to the problem exists. I've written a computer simulation for pneumatic guns (see BAGS in my signature) and messed around a bit with very basic spring gun simulation. Pneumatic and spring guns in the simplest accurate representation are modeled by a system of non-linear ordinary differential equations. Anyone with experience with ODEs knows that analytical solutions exist only in a limited number of cases, and this case isn't one of them. Hence computer approximations are the best we can do.

If I wanted a semi-accurate guess I'd use an energy based approach like you did at first, however, I'd assume a low efficiency. Not all of the energy will end up in the dart. Barrel friction, the energy of the gas, plunger kinetic energy, etc., are other places the energy will go. 5% efficiency is a good minimum for pneumatic guns, however, theoretically 60% or higher is possible. As for the efficiency of spring guns, I honestly couldn't make an accurate guess. Your Crossbow is 39% efficient given the information you've provided, which is very respectable.

Maybe you could incorporate a thing or two I've said into your report. Also, if you have any questions I probably could help.

[cheesypiza001]

I'm assuming this is the Crossbow you used........(?)

My Crossbow:




Name: Zeus
Mods Done: Level 3.5, no reinforcements on the inside besides plunger shaft, but I do have two bungees not in the picture that I use. Barrel is brass inside PEX tubing. The barrel will have a bolt action breech when the brass finally comes.
Integrations: None, but at some point maybe a AT2K
Comments: I got this for my birthday and proceeded to hack it up and make it look like crap. This was when I didn't care how my guns looked. Now that I do I got a new case off of a friend and made this. I think it looks nice with the flame things on the side. Name comes from the Greek god of the heavens. He throws thunderbolts at things. Not in the picture is another stencil on the other side of a thunderbolt.

[madmax]

Cheesypiza: Yeah, that is the crossbow I used, I also finally integrated a 2k, so I have to edit the post on the crossbow page.

Doom: That is really cool, I cant believe you were able to make a computer simulation for the air guns. I moved away from the potential energy all going into the dart, because I realized that the potential energy of the spring really on acts on the plunger rod, and then fluid dynamics because of different sized tubes. Though next time I try this (I want to test max shot for fps, maybe some other stuff) Ill try to do it using like a 40% efficiency or something like that. I think most plunger guns are more efficient than the crossbow because the plunger tube gets wider towards the back, allowing air to escape around the plunger.

[Shrub]

I'm turning it in on Monday, so I don't know yet. Seeing as our final was watching star wars (Yay after AP test AP physics) and picking out the physically impossible things (like deflecting lasers with light(sabers)) I think I should do pretty well. Though its kind of tame compared to my friends coil gun, but thats not going up on youtube.

Impossible? I think not. The very in depth and non-explained things are what we don't know.

And if you knew anything about star wars you know that light sabers are really "laser-swords" so laser deflecting laser. Also blasters must have some type of mass in their ammo allowing it to shoot so it may just be for a visibilty factor (lasers shooting out of guns). Also "THE FORCE" IS real. If you imagine. LOL

[madmax]

Well, lasers are concentrated monochromatic light, just so you know. Also, the laser blasters have no mass, they are lasers. It is impossible to deflect a laser with a laser. What happens is that depending on their phase they either constructively or destructively interfier for a split second, before passing through each other unefected on the same path. You would need mirrors to deflect the laser. Even if the ammo might have mass, the actual laser would not, the ammo may just store an amount of energy that the gun initiates to fire a burst of electromagnetic energy, a laser.

Edit: to be honest I don't argue the force, it doesn't go against any laws of physics that I can think of.

Edit: I've managed to derail my own topic, score!

Edited by madmax, 08 June 2009 - 07:37 AM.

[zaphodB]

Actually, a blaster bolt is a combination of plasma and laser. Depending on the blaster it has higher concentrations of plasma/light. The "ammo" that gets used up is the plasma cell.

A lightsaber is also a plasma weapon.

[Shrub]

Ah ha I knew there was some physical way of doing so.

[madmax]

Is it actually possible to create a plasma-sword? The plasma would need to be kept in sword form by a crazy strong magnetic field. I don't think its possible to have something that can create the field necessary mounted in the hilt of the sword. Also, I think if there was a field it would create a circle around the handle, not a sword shape. Though if it was possible to create a plasma-sword, besides the fact that you would have to react at the speed of light (the force takes care of that) it might be possible to deflect the lasers, i'm not sure if lasers are effected by magnetic fields, cause they would actually pass through the plasma (I think).

[Shrub]

It's called the force remember that's why only Jedi can use lightsabers.

[Darth Freyr]

Star Wars blasters are not lasers. From Wookiepedia:

Blasters were a considerable improvement over the archaic laser design. Instead of a coherent beam of light and heat like a simple laser, the blaster fired a highly compressed, focused high-energy particle beam that was very deadly to most creatures. The energy that made up the blaster bolt depended on the type of blaster being used.

Whether or not lightsabers deflect lasers is a moot point. in response to your "circular blade" statement, how about a very long, thin ellipse, with the majority of the length outside the handle and a small section inside. Reacting at the speed of light is unneeded as the projectiles are flying at a fairly low speed, which you would have noticed if you actually watched the movie.

[Talio]

You really should have just used a chronograph.

[madmax]

Talio, thank you for re-railing this thread. I didn't use a chronograph mainly because I don't own one. I think that chronographs also function very similarly, except instead of using sound the chronographs use lasers to determine where the dart is. I wish I had been able to use a chronograph, but I think the way I did it is decently accurate.

Shrub: Very true. The force is kinda like a get out of jail free card, it explains almost anything that Jedi's can do. For example: Plasm-sword. The handle could not possible house the source of a magnetic field strong enough to contain plasma, but if your a Jedi, just use the force!

[Talio]

Since it was for school I'm not gonna bust on you too much. Chrono's use different means for testing and there is some room for error, but you could have literally gone to any paintball field and asked them to use their chronograph and I can't believe any of them would say no. But then again, I understand doing things for the sake of doing them too, so good for you.