# Search results

1. ### Appropriate distribution for minimum distance

Ah, okay - that makes sense. I was kind of hoping someone might say, "Oh, this is what so-and-so developed the so-and-so distribution to describe." There are so many distribution functions out there, and I know very little about most of them. I wish I could post some data or plots or...
2. ### Appropriate distribution for minimum distance

I apologize again for over-simplifying the problem. These trajectories are actually the output of fairly complex 6-degree-of-freedom simulations, so I can't really even attempt to give the all the details. (Truthfully, I don't know all of them - there are all kinds of mechanical systems, the...
3. ### Appropriate distribution for minimum distance

Sorry - didn't mean to imply that they're 2-D. In fact, they're 3-D trajectories, and they're not truly ballistic (there's powered flight involved). For the sake of this question, I think we can make the approximation that they are projectiles that originate from a fixed point in space, and the...
4. ### Appropriate distribution for minimum distance

I have what I think is probably a basic question from probability and statistics (about which I'm pretty ignorant). If I have a set of projectile trajectories that were generated by a Monte Carlo process, and I'd like to know the probability the projectile will come within distance d of some...
5. ### How does GR describe acceleration from non-gravitational forces?

Yes - I was speaking sloppily again. What I really meant was that rocket guy is not near a gravitating body, so his spacetime is flat (that's all I meant by "local"), and mine is very much near a massive body, so mine is curved. Thanks again.
6. ### How does GR describe acceleration from non-gravitational forces?

I think I would have said that even before today, although I suspect I wouldn't have understood what I was saying - or slightly less than I do now, anyway. So, let's see - when I stand on the surface of the Earth, I am prevented from following a geodesic on my locally curved bit of spacetime...
7. ### How does GR describe acceleration from non-gravitational forces?

Thanks to all - I knew I had a pretty fundamental misunderstanding, and these responses have all helped a lot. I think I was suffering from the cartoon understanding of the observer in a uniformly accelerating elevator who cannot distinguish between that and a stationary elevator in a uniform...
8. ### How does GR describe acceleration from non-gravitational forces?

I have a pretty basic question - one that should have occurred to me long ago, but I never really thought about before. We all know how the effects of gravity are described by the curvature of spacetime - rubber sheets and all that - as well as the equivalence of inertial and gravitational...
9. ### Who first assigned negative to electrons and positive to protons?

I find this to be an odd discussion. The assignment of a sign to the current was made long before there were any direct observations of the actual charge carriers (as was noted in an earlier post). As far as the scientists at the time were concerned, electricity was the flow of some kind of...
10. ### Fortran How to move on from Compaq Visual Fortran

Thanks, all. I chose Simply Fortran because it is based on the GNU compiler but also provides a nice IDE. It's inexpensive - compared to Intel Fortran, at least - so I'm trying to see if I can get everything working with it before I bite the bullet (or have my employer do it) to get Intel...
11. ### Fortran How to move on from Compaq Visual Fortran

My group at work uses several pieces of legacy Fortran code which we've always compiled using Compaq Visual Fortran. Since CVF has not been supported for while now and is not (easily) compatible with Windows 7, I'd like to move to a more up-to-date compiler and IDE. I'm trying Simply Fortran...
12. ### Web link for crackpot test?

Update: After announcing the he has been told "never to try to do physics again", he has now compared himself to Gallileo and Copernicus. 40 more points!! :biggrin:
13. ### Pair-production - how come it's always e+e-, never muons or tauons?

When I was in grad school there was an older professor who consistently called it a "tauon". He also consistently referred to the muon as the "mu meson" - as it had originally been called, due to the similarity in pi and mu masses - although by then that was considered to be flat-out wrong...
14. ### Web link for crackpot test?

Actually, he didn't score too badly. He's not as far over the top as others I've seen, but he does suffer from the syndrome of saying things along the lines of, "I have a novel approach to the fundamental underpinnings of all of Physics, which explains a host of things the 'Standard Model' [his...
15. ### Web link for crackpot test?

No - the Baez one was it - but this one is more serious and has some value for that reason. Thanks.
16. ### Web link for crackpot test?

Sheesh - how embarassing. I should have been able to find that - sorry! Yes, it was indeed John Baez's page that I had remembered. Thanks!
17. ### Web link for crackpot test?

I'm sorry - I know this is archived somewhere around here but I couldn't find it ... Can anyone give me a link for the set of tests used to identify a crackpot (either the theory or its author). I've forgotten who had it on his website - someone at Princeton, maybe?? I've got a live one on...
18. ### What is a String Mass-Shell? and what does it do?

I don't have a clue! :approve: But since no one else has answered your question, I'll toss out a tidbit, in case it is actually the same thing. In "normal" Quantum Field Theory, when people speak of a particle as being on its mass shell, what they mean is that its mass is equal to its physical...
19. ### Merry go round

If you're in the rotating frame and want to explain the weird curved path taken by the ball as you see it, you have to invoke the inertial forces, i.e Coriolis and centrifugal forces. But if you do that and calculate where the ball should go while under the influence of those forces, you should...
20. ### What do they mean by force particles exactly?

You won't hear people use the expression "wave/particle duality" much in the context of modern quantum theory. It's more a term that was used in the early days (first half of the 20th century) when people were still struggling with the question of whether the fundamental constituents of nature...
21. ### Why can't we detect off-shell particles?

I think of "off-shell" as being kind of synonymous with "non-physical", as in a photon with non-zero mass. You probably know this, but such non-physical states are necessary in order to conserve energy-momentum, but they are also allowed by the energy/time version of the Heisenberg Uncertainty...
22. ### Neutrino Oscillation ?

To understand how neutrinos oscillate, you have to understand the concept of mixed states, which is why "neutrino mixing" is sometimes suggested as a better term than "neutrino oscillations". As you might know from Quantum Mechanics, particles are described as existing in quantum states...
23. ### Calculations for flying ball

Are you familiar with the ballistic coefficient of an object? It's just mass divided by drag coefficient times cross-sectional area, and it's a good measure of how the object will respond to drag forces. In your example, you've kept everything fixed except that you've increased the ball's...
24. ### Kernel density estimation for non-I.I.D. data

Unfortunately, the problem is a little more complicated than that. I need to consider all orbiting objects currently being tracked and to select a time for which a specific event that produces debris will not put any of those objects at risk of damage. Thus, the time at which my debris is...
25. ### Kernel density estimation for non-I.I.D. data

I'm not sure what kind of breakup you have in mind, but I am talking about debris from collisions between two objects. In such cases, then yes, up to variations in momentum transfer, the center of mass of the cloud of debris from each object tends to follow the original trajectory of the...
26. ### Kernel density estimation for non-I.I.D. data

I'm coming to the same conclusion. I've seen papers suggesting that KDE approaches become more difficult in more that one dimension because of the need to choose some kind of "directionality" for both the kernel function and the associated bandwidth selection. In my case that's very likely to be...
27. ### Kernel density estimation for non-I.I.D. data

Okay, I need to provide a little more detail. I'm analyzing the results of collisions in space that produce a large number of debris fragments, and the risk imposed on orbiting satellites. I have a model for the debris production, which is what determines the distributions of mass, velocity...
28. ### Kernel density estimation for non-I.I.D. data

The only variable I'm concerned with is particle number density as a function of position and time. This is for safety analysis, and the safety requirements are specified in terms only of the probability of collision (between a debris fragment and something of value) with debris of any size...
29. ### Kernel density estimation for non-I.I.D. data

I am wondering if Kernel Density Estimation (KDE) is appropriate for some data analysis I'm working on. I have a simulated process that produces a large number N of pieces of debris, and I want to know how these objects are distributed spatially. In other words, I'd like to estimate a density...
30. ### Ball in a complex 2D environment

Okay, I think that defines the problem well enough to get started - missing details will reveal themselves in due course. I would attack it this way, off the top of my head: 1. Get the center of mass (COM) motion right first. Use the rule that the angle of incidence = the angle of...