[Lord Odin]

cockerpunk, on Oct 10 2008, 08:36 AM, said:

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.

[cockerpunk]

Lord Odin, on Oct 26 2008, 10:21 PM, said:

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.

[Snipez4664]

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

[Leftystrikesback]

Snipez4664, on Oct 26 2008, 09:45 PM, said:

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

[cockerpunk]

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.

[Leftystrikesback]

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

[cockerpunk]

Leftystrikesback, on Oct 27 2008, 12:25 PM, said:

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.

[Leftystrikesback]

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

[Snipez4664]

Leftystrikesback, on Nov 14 2008, 06:15 AM, said:

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.

[JeffdeFlatulent]

In the ideal gas law:
PV = nRT

n = number of moles

[gdot5]

masspb, on 10 October 2008 - 09:26 AM, said:

you might want to give them gravity

g = 9.806 m/s

two things
1) gravity would not EVER be 9.806 m/s, it would be 9.806 m/s/s (m/s/s = metres per second squared)


2) gravity is not a constant

depending on where you are in the world gravity will be different, (mass and distance between two objects affect gravitational pull)

Gravitational pull can be determined with the universal gravitational law
F = G * m1 * m2
............r * r

this also means if the moon is above you gravity will be different


HOWEVER since it is such a minute difference, you can just use 9.8 m/s/s

[MW BSPB]

smart people

[Eskimo]

toxic city pb, on 11 October 2008 - 07:32 PM, said:

You a physics major?

most of this besides fluid dynamics and a few other concepts are a really quick summary of grade 11 - 12 physics.
Good job sir, I shall forever remember this as a post of interest.

[Eskimo]

gdot5, on 01 March 2010 - 10:09 AM, said:

masspb, on 10 October 2008 - 09:26 AM, said:

you might want to give them gravity

g = 9.806 m/s

two things
1) gravity would not EVER be 9.806 m/s, it would be 9.806 m/s/s (m/s/s = metres per second squared)


2) gravity is not a constant

depending on where you are in the world gravity will be different, (mass and distance between two objects affect gravitational pull)

Gravitational pull can be determined with the universal gravitational law
F = G * m1 * m2
............r * r

this also means if the moon is above you gravity will be different


HOWEVER since it is such a minute difference, you can just use 9.8 m/s/s

someone took a course of planetary motion lol
Anyways but once again this doesnt effect the paintball or us as its simply so small it wont make any noticeable difference. Kind of like how the faster you move the slower time moves. or the closer you are to an objects center of gravity, and the larger that gravity is. time moves slower.
Or how if you shoot a paintball and you observe the paintball in flight for 3 seconds but the paintball might time the flight as 3.1 seconds.

YAY physics!

and if we REALLY want to be anal crack heads, were only assuming that the physics we apply is for a euclidean flat space!
WERE fucked if spacetime is open or closed.

This post has been edited by Eskimo: 16 April 2011 - 11:42 AM