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Intro To Paintball Physics


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#1 cockerpunk

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Posted 10 October 2008 - 08:36 AM

alright, we'll see how this works, ill edit it along the way to add whatever i forget (i will forget ALOT)

Physics

Newtons Laws -

1. objects tend to keep whatever state of motion they have, unless an unbalanced (ie net) force (like friction) acts on them

2. objects tend to keep the motion they have as a factor of there Mass.

Force = Mass x Acceleration or simply F=mA

both force and Acceleration are vectors (have magnitude and direction), and thus the acceleration (change in motion) will only be in the direction of the net force.

3. every force has an equal and opposite force. i push on the wall, the wall pushes back on me.

Work, Energy and Power

Energy and work have a really circular definition. energy is the ability to do work (called joules), and work is defined as the changing of an objects energy(also measured in joules). Power is defined as the amount of work in a time interval(joules per second). the English units typically used are ft*lbsf (called foot pounds) and ft*lbsf/sec. 550 ft*lbsf/sec = 1 hp.

Work = Force x Distance (the force must actually move the object to do work)
kinetic Energy = (1/2)mv^2
Gravitational Potential Energy = ghm

so, an example of energy and work is simply lifting a stone.

a stone on the floor unmoving has no mechanical energy. when i lift the stone, i am doing work on it (applying a force and moving it a distance). so now the stone has more energy. the time it took me to raise the stone is how much power i have (the faster the more powerful).

Conservation of Energy

this is a fundamental tenant of the universe. under some circmstances it can be broken (near the speed of light...), but then we are talking nuclear mechanics. NOTHING like has ever or will ever take place in a paintball gun. for all realistic modeling of paintball and anything related to it, this is a law harder than a 17 year old who took 5 Viagra.

ENERGY MAY NOT BE CREATED OR DESTROYED, IT CAN ONLY CHANGE FORMS.

the perfect example of this is the coaster example. in a roller coaster, you build up a tremendous amounts of gravitational potential energy, and though the course of the ride you trade your potential energy to kinetic energy and back again maybe a dozen times.

of course eventually the coaster will come to a stop right? doesn't that seem like energy is being destroyed?

gravity is the force converting all potential energy to kinetic, and then it is doing negative work to the coaster when it takes the potential back from the kinetic. gravity is doing alot of work. however, another force is doing alot of work too - friction. friction is doing work (transferring energy) from the coaster the whole time. friction is converting kinetic energy to heat energy. another force is also doing work - air resistance. air resistance is transferring kinetic energy into kinetic energy of air. in fact, one might even say that the coaster is doing work on the air around it, to move it.

Friction

all this talk of friction means i should probably define it for you. friction is caused by the molecules of one object being attracted to the molecules of another object. think of it like velcro, hooks on one side, fuzzy stuff on the other. now, what causes friction is somewhat minor when we talk about mechanics because we simply dont care. friction is real, and thats really all that matters.

there are two types of friction, kinetic, and static friction. kinetic friction is between two bodies that are moving relative to each other, and static is between two bodies that are stationary to each other.

to calculate friction, we need to use something called the co-efficient of friction. this a a property value that we look up in a book. obviously since different types of objects have different amounts of friction its easy to see that the surfaces pushed together and what they are made of is important.


Materials in Contact........Coefficient of Static Friction* S........Coefficient of Kinetic Friction * K
Wood on wood................................0.5.....................................0.3
Waxed ski on snow..........................0.1 .......................................0.05
Ice on ice .......................................0.1................................0.03
Rubber on concrete (dry)..................1.0.......................................0.8
Rubber on concrete (wet).................0.7........................................0.5
Teflon on Teflon .............................0.04......................................0.04
Synovial joints (in humans) ..............0.01.....................................0.01
* These values are approximate and intended only for comparison.

now, another thing you can see is that static friction is ALWAYS greater than or equal to kinetic friction. that makes sense, it explains why you can do break stands in a cars and such. it explains why drifting and burning out are bad, and why indy cars and drag racers never do that.

so the equation for calculating friction is - Force of friction = Coefficient of friction x the force pushing the two surfaces together

usually shortened to F = (mu)N

notice, surface area is nowhere in that equation. the size of the surfaces being pushed together does not matter when calculating friction.

Air Resistance and other damping forces

air resistance is a pickle. see air resistance is a differential relationship. meaning, you need to know calculus to solve most fluid resistance problems. i cannot possibly teach you calculus but i will say a bit about this.

the reason it requires calculus is because air resistance is a function of velocity. velocity is the derivative of the position function, so theirs where you get your calculus.

but one thing i can tell on relatively simple terms is that the faster an object is moving (velocity) the larger the force. often times its modeled as a power function, meaning the resistance is equal to the velocity squared. other inputs into the force of air resistance are shape and surface texture.

fortunately in paintball, most of the time shape, mass and all that jazz is a constant when comparing things because most paintball are pretty similar.

The Calculus of Motion or kinematics (sort of)

ok, i figured since i mentioned velocity and that whole bag i would explain a bit about that. Sir Issac Newton in england and Libneitz (sp?) in germany both discovered calculus almost simultaneously because both were studying motion and realized how important rate of change is to the study of motion. they are both given credit for the discovery.

anyway, the important thing is the rate of change of an object.

the position function describes the path an object takes, and you can use it to find where the object is at any given time.

if we take the derivative (otherwise known as finding the rate of change) of the position, we get velocity. velocity is the rate of change of the position. velocity is alot like the speed an object has, however speed is simply a number, and velocity has a direction and a magnitude. it is enough to say that the velocity is how the position of the object is changing, how fast, and in which direction is the position changing.

if we take the derivative of the velocity, the rate of change of the velocity, we will have the acceleration. acceleration describes how the velocity is changing, and in which direction. the acceleration is very important because of the previously mentioned F = mA. Acceleration, like velocity is a vector, and has both a value and a direction.

Pressure

something we deal with ALL the time in paintball. pressure is simply a Force over an Area. whenever we have a force that is pushing against an area, we have pressure. no, pressure does no only apply to fluids but to anything that has a force and an area.

ideal gas

the ideal gas law is the start of ideal fluids. there are about a million and half differnet versions of the ideal gas law putting it in different terms, like molar mass, mass, and whatnot. the basic chemestry form is

PV = nRT

where P is pressure, V is volume, and T is absolute temperature. R is a constant, and n is the molar mass (if i remember right)

basically, as long as your working with a fixed amount of mass (like we typically are in paintball), pressure and volume will go up if temperature rises, and if temp remains the same, then pressure and volume must trade off.

ideal fluid flow

the only real way to understand fluid mechanics is to use energy. as stated before, energy is neither created, nor destroyed. so the energy at the start of your dynamic system must be equal to the energy at the end. so fluids have many different types of energy -

Pressure is a measure of potential energy (P)
Velocity is a measure of kinetic energy (V)
and gravitational potential energy (z)

later, i will discuss thermodynamic properties of fluids. in most cases in paintball, heat and that energy is just not a very large factor, so we can neglect it. in most cases we neglect the gravitational effect aswell because air is not very heavy, and its not moving in that direction very much.

what those three terms build for us is the Bernoulli equation. again, it has a few different forms but most commonly stated -

(P/density*g)+(V^2/2*g)+z = constant throughout the system.

in this equation, the first term is pressure potential energy, the second is kinetic energy, and the third is the gravitational potential energy. every term in this equation translates directly into feet, or meters. in fluids we call feet of elevation (what it represents), head. yeah, i know. funny stuff right?

anyway, we use this equation to solve for all sorts of different stuff. if we know the pressure in a dump chamber (velocity of the fluid is zero, and z term is neglected) and we vent it, we can predict how fast the fluid will be moving (pressure drops to zero, z term still neglected) because we know that energy is constant, it just changes form.

another useful equation in basic fluid mechanics is for solving flow. the letter Q is often used.

flow is defined as the amount of stuff going though a specific area. the equation is simply Q = Area(V)

this again, must be constant (assuming no leaks). so basically it means that if the cross sectional area shrinks, the velocity must be faster.


Stats and Experimental Design

i really hate stats. unfortunately, knowing the basics of stats is HUGELY important in scientific testing.

the reason we use confidence intervals and other stats methods to look at our data (deals with gun consistency) -

the issue is finding the "true mean" vs the "sample mean" if i want to find the average age of all poeple in the entire world, what do i do?i decide on what would be a representative sample of the world, poll those poeple, and calculate the mean right?

when we do that we are finding a sample mean.

the question is - how close to the true mean (if you actually polled everyone on the entire planet) age?

that's where the CI comes in.

so yes, we can do a comparative study by agreeing on a common sample size and say "well that guns three shots are more consistent than that guns three shots"

however, what we really want to know is how consistent EVERY shot from the gun is.

to do this, we are comparing a sample mean (the shots fired over the chrono) to the true mean (the velocity of ALL shots the gun ever fires).

a CI simple compares the variation in the data (standard deviation), with the number of samples taken, and then shows you how accurate to the true mean our sample mean is.


so you see, we use confidence intervals to look at how different our sampled mean (the data points we take in a test) to the true mean (if we had data on every single event under those conditions).

the equation for a two sided confidence interval is (mean) +/- (tvalue x standard deviation)/ Sqroot(number of events)

the t value is a textbook value based on a normal distribution, and we can decide how inclusive our results are by judging if we want to 90%, 95%, 99%, or even 99.9%. what that means that if we take a sample size of say muzzle velocities, then calculate the CI around it, we choose how inclusive our range is. if we calculate the CI based off a 90% t value, then we can be 90% sure that our next group of samples of velocity will fall in that range. with a CI we can look at exactly how good our sample is compared to the "true" values of ALL shot velocities.

MORE to come. comments please?
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#2 howudoin

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Posted 10 October 2008 - 09:02 AM

this kinda helps but not really it dosnt have much to do with paintball at the moment but im sure u will make more use full aka telling why and how much a ball will drop when fired at a certain Fps :rolleyes:
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#3 cockerpunk

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Posted 10 October 2008 - 09:05 AM

this kinda helps but not really it dosnt have much to do with paintball at the moment but im sure u will make more use full aka telling why and how much a ball will drop when fired at a certain Fps :rolleyes:


this has EVERYTHING to do paintball physics. pretty much all motion in the universe can be described with newtons laws.

the tricky part is how it all fits together to describe paintball physics.
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#4 masspb

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Posted 10 October 2008 - 09:26 AM

you might want to give them gravity

g = 9.806 m/s

#5 masspb

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Posted 10 October 2008 - 09:38 AM

this kinda helps but not really it dosnt have much to do with paintball at the moment but im sure u will make more use full aka telling why and how much a ball will drop when fired at a certain Fps :rolleyes:


I hope you are kidding right?

#6 Lord Odin

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Posted 10 October 2008 - 11:29 AM

I know someone is gonna ask why surface area doesn't matter with friction and you should probably explain why. Otherwise that episode of the phone books that couldn't be pulled apart easily will come up all over the place.

#7 cockerpunk

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Posted 11 October 2008 - 10:16 AM

I know someone is gonna ask why surface area doesn't matter with friction and you should probably explain why. Otherwise that episode of the phone books that couldn't be pulled apart easily will come up all over the place.


no, surface area does not matter. again, this is the issue of applying the rules correctly.

the reason why the phone books do not come apart is not because there is a tremendous amount of surface area. if you look in the the formula for friction you can see why they are very hard to tear apart.
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#8 Lord Odin

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Posted 11 October 2008 - 10:48 AM

Looking at the formula, I understand that. If there isn't a variable for it, then it doesn't apply.

Just trying to give a heads up for people not so inclined to look into things on their own.

#9 scoreshot

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Posted 11 October 2008 - 07:18 PM

[quote name='cockerpunk' date='Oct 10 2008, 07:05 AM' post='35723']
this has EVERYTHING to do paintball physics. pretty much all motion in the universe can be described with newtons laws.

the tricky part is how it all fits together to describe paintball physics.
quote]

How about this:
At what degree of underboreing does the kinetic coefficient of friction begin to effect air efficiency?
We know eliminating "blow by" increases air efficiency, but at what point does the friction caused by underboreing begin to reduce that air efficiency by requiring more air to push the ball through a tight insert at the same velocity?





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#10 toxic city pb

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Posted 11 October 2008 - 07:32 PM

You a physics major?
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#11 howudoin

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Posted 11 October 2008 - 08:49 PM

my bad didnt read the full post :( but nice job its really good it helps me but i don think im gonna calculate on the field in the middle of a gun fight lol
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#12 pb101

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Posted 11 October 2008 - 09:44 PM

How about this:
At what degree of underboreing does the kinetic coefficient of friction begin to effect air efficiency?
We know eliminating "blow by" increases air efficiency, but at what point does the friction caused by underboreing begin to reduce that air efficiency by requiring more air to push the ball through a tight insert at the same velocity?


I think it would have to be an extremely tight fit where you'd likely end up breaking balls.

#13 scoreshot

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Posted 12 October 2008 - 12:48 AM

I think it would have to be an extremely tight fit where you'd likely end up breaking balls.


Maybe so. My question addresses the "point of diminishing returns" with regard to underboreing as it may apply to air "efficiency." However, you bring in an interesting suggestion regarding ball breakage and underboreing. I addressed my view on that subject with the following quote from the thread on "Underboreing in real paintball".

"To determine "how much" to underbore, you need to first take into consideration the "size range" of the balls you are shooting. As you know, all paintballs have a variance in size and shape within the same case. (Presumably, to a lessor degree with the higher quality tournament grade paint). You should gage your underbore relative to the smallest balls in the lot you are currently shooting. The underbore needs to be just small enough to provide a paint/bore match that completely eliminates "blow by" with the smallest balls, but no smaller. By doing so you provide a 100% ball/bore seal for 100% of the balls being shot. Underboreing more than is necessary to eliminate "blow by" for the smallest balls you are shooting, would probably unnecessarily increase the risk of barrel breaks as the largest balls in the lot are forced to contend with the same underbore as that of the smallest balls in the lot."

As far as the "efficiency" question is concerned, I would tend to agree that the amount of underbore induced friction which would require additional air pressure to maintain velocity,(reduced air efficiency), may be at or even below the underbore size where ball breakage is negatively impacted. In other words, we may encounter increased barrel breaks before underbore friction becomes an "air efficiency" issue.





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#14 brycelarson

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Posted 12 October 2008 - 08:12 AM

[/i]As far as the "efficiency" question is concerned, I would tend to agree that the amount of underbore induced friction which would require additional air pressure to maintain velocity,(reduced air efficiency), may be at or even below the underbore size where ball breakage is negatively impacted. In other words, we may encounter increased barrel breaks before underbore friction becomes an "air efficiency" issue.


We would love to see other people also get some real data on this - but CP and I did a break test - and found no statistically significant difference in barrel breaks for any size between .679 and .695.

barrel break test

based on that test I don't believe that with any commercially available barrel at this point you can underbore too much.

#15 Lord Odin

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Posted 12 October 2008 - 08:24 AM

We would love to see other people also get some real data on this


I agree. I would like to see other variables affecting the paint to be tested as well. Not everybody gets to play with fresh paint and perhaps that's what may give people more barrel breaks.

For example, someone should do a similar test with aged paint. Take a case of fresh paint, do 20 shots for each insert (assuming 8 inserts), which equals 160 rounds per test. Conduct the same experiment over 6 months and you have 960 rounds (just under half a case) of paint.

Another would be paint exposed to heat and humidity. Each would have to be tested seperately, then jointly, along with a control.

#16 scoreshot

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Posted 12 October 2008 - 11:58 AM

We would love to see other people also get some real data on this - but CP and I did a break test - and found no statistically significant difference in barrel breaks for any size between .679 and .695.

barrel break test

based on that test I don't believe that with any commercially available barrel at this point you can underbore too much.


Yes, I have read the barrel break test results, and most of the replys. Nice work to be sure guys.

"We chose a paint on the brittle end of the spectrum to make sure that we had events to record. 6 month old monster ball would prob have given us no breaks - which would have made the test useless."

I have never used monster balls but I have herd that it is very large and of poor quality and size consistency. If that is true, respectfully, I would be very surprised if a .679 insert with fresh monster balls would not produce a "statistically significant difference in barrel breaks" as compared to the smallest, tuff shell, good quality paint available.

As suggested, the paint being used, quality, size and condition needs to be taken into consideration when we determine what underbore (if any) constitutes too much as to induce a "statistically significant difference in barrel breaks".

Indeed more and varied testing is needed.





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#17 Lord Odin

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Posted 12 October 2008 - 01:31 PM

Actually, Monster balls are really small. I had a few to try out and they were even rolling through my .682 insert.

They are also extremely tough. I've seen them on a warm, humid day turn really squishy. They wouldn't break for the life of us.

#18 scoreshot

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Posted 12 October 2008 - 03:16 PM

Actually, Monster balls are really small. I had a few to try out and they were even rolling through my .682 insert.

They are also extremely tough. I've seen them on a warm, humid day turn really squishy. They wouldn't break for the life of us.


Then they are obviously not a good test ball. My point is simple. The balls they shot were a blow through match with a .689 insert. Quote: "We shot balls that matched the blow through test on the .689 insert..." Maybe those balls are the largest and most brittle available, in which case they covered a good assortment of paint. If not, and if that were the only size ball and shell characteristics they shot for the tests in question, then as has already been suggested, more testing needs to be done with other paint sizes and shell properties to substantiate the claim that it is impossible to underbore too much with the commercially available barrels.

Here's a quote from another forum member:
"So far under boring I have shot fluid, bronze, gold, evil, ultra evil etc. On uncapped semi on my, ND Shocker, Vice, Quest, Ego, Ion, legend, CCM ss-25, and autococker SR. Did not affect accuracy at all was able to hit everything I pointed at. I did get alot of barrel breaks when I underboared too much. But when I went 1-2 sizes under barrel breaks where not an issue at all. I under bore all the time for me it seems to be the way to go."

(I did the underlining to make the point)

Seems simple enough to me...squeezing a big ball into smaller and smaller bores will eventually lead to and increase in the frequency of barrel breaks. I suspect that this can be shown to be the case with currently commercially available barrels, inserts and paint.

Tell ya what...steer me to the largest available paint and I'll do a paint /barrel break test myself, using all of my SP inserts to see if I can show a difference in the frequency of barrel breaks between any of the underbore sizes. I will post the results on this forum. After all, more testing is what we need.





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#19 Tagline

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Posted 12 October 2008 - 10:20 PM

What are we looking at for efficiency if we underbore?

I think another test is needed for that. That way we can find a median between increased chance of ball breaks and more shots per tank.



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#20 cockerpunk

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Posted 13 October 2008 - 09:06 AM

How about this:
At what degree of underboreing does the kinetic coefficient of friction begin to effect air efficiency?
We know eliminating "blow by" increases air efficiency, but at what point does the friction caused by underboreing begin to reduce that air efficiency by requiring more air to push the ball through a tight insert at the same velocity?


the short answer is we dont know.

the long answer is that we have not hit that point with .005 or .007 inches underbore. if there was a way to underbore farther, we'd love to see it. you could extrapolate the graphs we did on the original paint to barrel match test and see whats going on. my bet is that you will end up breaking paint before friction really becomes an issue.
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#21 Lucas

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Posted 13 October 2008 - 09:45 AM

Wow your smart! loll, your either a genius or you found this like dozens of other things on the internet! lol!

#22 cockerpunk

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Posted 13 October 2008 - 07:17 PM

functional question - should i add more to this, or do we have enough to talk about? do you guys want a thread like this? can you handle a thread like this without it going all crazy off topic?
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#23 brycelarson

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Posted 13 October 2008 - 07:32 PM

functional question - should i add more to this, or do we have enough to talk about? do you guys want a thread like this? can you handle a thread like this without it going all crazy off topic?


I'm going to suggest that we lock this thread - anyone who has something physics related - please PM us and we'll add it to the initial post. That will keep relevant info at the top of the thread.

#24 Lord Odin

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Posted 13 October 2008 - 07:35 PM

I would at least recommend how to get Standard Deviation to the list as well. I think that's essential to a lot of our testing. Letting Excel do it is one convenient but it does nothing on implying the meaning or importance behind it.

#25 brycelarson

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Posted 13 October 2008 - 07:43 PM

I was thinking that a second thread about the basics of stats would cover that stuff.

#26 Lord Odin

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Posted 13 October 2008 - 09:27 PM

Ahh, ok. Cool.

Physics related, I would also include formulas related to gases, such as pressure.

#27 Captain Hollywood

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Posted 14 October 2008 - 11:50 AM

functional question - should i add more to this, or do we have enough to talk about? do you guys want a thread like this? can you handle a thread like this without it going all crazy off topic?



Yes! More! I personally am super interested in learning about this stuff, My teacher actually was suprised when she saw what I was reading. I say you should definitely do more on the subject.. As for the locking of the thread part, Not really nessesary unless some silly people come in here and make it super off topic in a bad way...

#28 cockerpunk

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Posted 14 October 2008 - 01:42 PM

ok, got some stats and other stuff in there.
The ultimate truth in paintball is that the interaction between the gun and the player is far and away the largest factor in accuracy, consistency, and reliability.

And yes, Gordon is the sexiest manifestation of "to the front."


#29 moyster14

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Posted 14 October 2008 - 01:54 PM

Ideal gas law maybe appropriate.
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#30 cockerpunk

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Posted 14 October 2008 - 01:55 PM

Ideal gas law maybe appropriate.


yeah fluids stuff will be comming up.
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#31 moyster14

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Posted 14 October 2008 - 01:56 PM

Coolness, that's were my weakness lies.
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#32 chainsaw

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Posted 21 October 2008 - 09:00 PM

you might want to give them gravity

g = 9.806 m/s



Im not trying to be too picky, but this seems like a crucial difference. The equation is actually:

g= 9.806 m/s2

The reason it is squared is because gravity is being measured as an acceleration not just as a speed.
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#33 brycelarson

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Posted 21 October 2008 - 09:12 PM

Im not trying to be too picky, but this seems like a crucial difference. The equation is actually:

g= 9.806 m/s2

The reason it is squared is because gravity is being measured as an acceleration not just as a speed.


isn't it more accuratly meters per second per second? not meters per second squared.

#34 pb101

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Posted 21 October 2008 - 09:43 PM

isn't it more accuratly meters per second per second? not meters per second squared.


Yeah, but they mean the same thing. I would maybe put in power is measured in Watts. 1 hp = 746 watts.

If some of you kids are interesed in this stuff then I would suggest taking Physics.

#35 chainsaw

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Posted 21 October 2008 - 10:00 PM

isn't it more accuratly meters per second per second? not meters per second squared.



Sorry if what I typed was confusing. When I said m/s2 I meant, (m)/(s)2 a.k.a. (m)/(s2) and (m)/[(s)(s)]. I didnt mean (m/s)2. Again sorry for any confusion.
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#36 brycelarson

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Posted 21 October 2008 - 10:12 PM

Sorry if what I typed was confusing. When I said m/s2 I meant, (m)/(s)2 a.k.a. (m)/(s2) and (m)/[(s)(s)]. I didnt mean (m/s)2. Again sorry for any confusion.


there we go - just wanted to clarify that for everyone.

#37 oerllikon

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Posted 21 October 2008 - 11:25 PM

Excellent write up. This is the jackpot of knowledge, for those who know how to use it to explain why certain things happen.
Judging by the teflon coefficients, wouldnt a J&J ceramic, or other barrel that has teflon in the bore, and a marballizer, for example(or other paint that has teflon on the shell), have a lot of a lower coefficient than a typical setup?
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#38 pb4bean

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Posted 26 October 2008 - 12:50 AM

cocker punks my hero! go mechanical engineers in training!
What it do


#39 ikaneatmetal

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Posted 26 October 2008 - 04:49 PM

my mind. blown. im gonna have to reread most of that. curse me for not taking physics in highschool.
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#40 Leftystrikesback

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Posted 26 October 2008 - 07:41 PM

no, surface area does not matter. again, this is the issue of applying the rules correctly.

the reason why the phone books do not come apart is not because there is a tremendous amount of surface area. if you look in the the formula for friction you can see why they are very hard to tear apart.


It's actually not that easy to see from the formula and I wasn't able to convince myself until I did a free body diagram. So I'm gonna take a stab at explaining it if that's OK with you. From the formula, the friction of the two phone books should be (u * W) where W is the weight of the phone book, since that's the force pushing down and u is the coefficient of friction for paper on paper. This does not equate to a whole lot of force, only about 10 pounds or so.

The explanation is as follows:
say there are 1000 pages in a phone book
each page lays on top of another, so the force for friction on the first page is the weight of that page times the coefficient of friction, lets say the weight of that page is 1/1000 of the total weight, W => weight = W/1000 and Friction = u*W/1000.
The weight on the second page is its own weight plus the weight of the pages above it, so the friction = u*2*W/1000.
and so on until you get the friction force on each page. So add up all those friction forces and what do you get?
Well you get a sum from 1 to 1000 times u*W/1000. The sum from 1 to 1000 = 1 +2+3+...+999+1000 = 500500.
So the total force from friction is 500*u*W.

lets say the coefficient of friction for paper on paper is .4 (just a guess, could be close though) and that the weight of a phone book is 15 pounds (just a guess) that means that the force it would take to pull the phone books apart is 500*.4*15 = 3000 pounds.

So surface area is not the reason for this large friction, its due to the fact that you add the friction from each page together, and that there are a lot of pages.
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#41 Lord Odin

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Posted 26 October 2008 - 09:21 PM

Stats and Experimental Design

i really hate stats. unfortunately, knowing the basics of stats is HUGELY important in scientific testing.

the reason we use confidence intervals and other stats methods to look at our data (deals with gun consistency) -



so you see, we use confidence intervals to look at how different our sampled mean (the data points we take in a test) to the true mean (if we had data on every single event under those conditions).

the equation for a two sided confidence interval is (mean) +/- (tvalue x standard deviation)/ Sqroot(number of events)

the t value is a textbook value based on a normal distribution, and we can decide how inclusive our results are by judging if we want to 90%, 95%, 99%, or even 99.9%. what that means that if we take a sample size of say muzzle velocities, then calculate the CI around it, we choose how inclusive our range is. if we calculate the CI based off a 90% t value, then we can be 90% sure that our next group of samples of velocity will fall in that range. with a CI we can look at exactly how good our sample is compared to the "true" values of ALL shot velocities.

You should probably mention that samples of under 30 (general rule of thumb) will require the use of t-values instead of z-values because the smaller the sample size, the poorer the estimate of standard deviation becomes. T-values are used to widen the confidence intervals to reflect the poorer estimate. It would be advised to do samples of at least 30 to give a better estimate of the population parameters. The larger the sample, the smaller the confidence internal and the more exact the approximation of the population standard deviation. In other words, use z-values for large samples (30+) and t-values for small samples (-30).

I would also add that the higher the confidence level (99% instead of 90%, etc), the wider the confidence interval becomes. And the larger the sample size, the smaller the interval comes.

#42 cockerpunk

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Posted 26 October 2008 - 09:34 PM

You should probably mention that samples of under 30 (general rule of thumb) will require the use of t-values instead of z-values because the smaller the sample size, the poorer the estimate of standard deviation becomes. T-values are used to widen the confidence intervals to reflect the poorer estimate. It would be advised to do samples of at least 30 to give a better estimate of the population parameters. The larger the sample, the smaller the confidence internal and the more exact the approximation of the population standard deviation. In other words, use z-values for large samples (30+) and t-values for small samples (-30).

I would also add that the higher the confidence level (99% instead of 90%, etc), the wider the confidence interval becomes. And the larger the sample size, the smaller the interval comes.


t and z values only really become equal when the sample size is over 128 actually.

as a rule of thumb, i just use t table values for everything.
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And yes, Gordon is the sexiest manifestation of "to the front."


#43 Snipez4664

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Posted 26 October 2008 - 11:45 PM

So, since when are we using friction to rip phonebooks in half instead of tensile strength, where surface area clearly does apply?
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#44 Leftystrikesback

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Posted 27 October 2008 - 12:44 AM

So, since when are we using friction to rip phonebooks in half instead of tensile strength, where surface area clearly does apply?


maybe you don't understand the question being answered... the question is why is it so hard to separate two intertwined phone books by pulling them away from one another... there is a ton of friction, but why so much? I used the phrase rip apart but I realize that's misleading. I used it because I once saw the myth busters try to get two phone books apart and they litterally had to be ripped by APCs. here are the results from that episode
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#45 cockerpunk

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Posted 27 October 2008 - 01:23 AM

lefty, i think you are very close.

but i would use the term "normal force" instead of "weight"

when you have a book spread out to twice (or more) its normal girth (?) and you pull on its ends, there builds a tremendous normal force in the center. the forces pulling tensily are resolved as a compressive force on the pages themselves due to the construction and arrangement of the books. this makes for tons (literally) of friction.

you can image quite a different result if only the center 200 pages or so were weaved together because the forces are not resolved into a compressive load without those wider pages.
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And yes, Gordon is the sexiest manifestation of "to the front."


#46 Leftystrikesback

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Posted 27 October 2008 - 12:25 PM

Yup I agree with that, I was just simplifying it and showing the principle. Something along the lines of how you can simplify a cow to be a sphere ;)
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#47 cockerpunk

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Posted 27 October 2008 - 02:07 PM

Yup I agree with that, I was just simplifying it and showing the principle. Something along the lines of how you can simplify a cow to be a sphere ;)


time for a great engineering joke.

a farmer wants to make a machine to pluck his chickens so that he doesn't have to pay immigrants do it. he asks three people, a mechanical engineer, a software engineer, and a physicist.

the mechanical engineer says "oh damn, thats gonna be hard. i need to make a robotic arm with 6 degrees of freedom and the proper end effector and its gonna be a total pain int he butt!"

the software engineer says "man, thats a tough one. im ognna have to write all these loops of code, sensor control systems, thats alot of math!"

the physicist says "oh thats easy ... assuming a spherical chicken ..."

EDIT - put some basic fluids up there.
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And yes, Gordon is the sexiest manifestation of "to the front."


#48 Leftystrikesback

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Posted 14 November 2008 - 05:15 AM

For those of us looking for more in-depth analysis of the physics of paintball, I found this site. It is VERY interesting and I suggest you take a look if you have some math/engineering background. It describes the important flight characteristics of a paintball very well from what I have read so far. look beyond the first page for the cool stuff.

http://home.comcast....pballphys1.html
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#49 Snipez4664

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Posted 14 November 2008 - 09:26 AM

For those of us looking for more in-depth analysis of the physics of paintball, I found this site. It is VERY interesting and I suggest you take a look if you have some math/engineering background. It describes the important flight characteristics of a paintball very well from what I have read so far. look beyond the first page for the cool stuff.

http://home.comcast....pballphys1.html



his calculator is very valuable.
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#50 JeffdeFlatulent

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Posted 09 April 2009 - 05:27 PM

In the ideal gas law:
PV = nRT

n = number of moles




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