Infosponge,

You seem to be very well versed in these areas of physics...but you also use some very contradictory statements at times.

Throughout your posts you have, at different times, stated that:

1. Gravity has absolutely no effect on a spinning bullet
2. Gravity has a negligible effect on a spinning bullet

Which is it? Absolutely no effect...or a small insignificant effect? It can't be both, can it?????

Of course, this is a bit of nit-picking...but you seem to enjoy that so much I figured you would mind.

Rick

Tin Can,

Your job is much harder and less profitable...when I fall down I get paid extra for it.

The worst part though is doing it on pupose!!!

I once had a producer at lunch ask me why I was sitting there so glum and quiet. I reminded him that after lunch the only thing I had to look forward to was an automobile accident! He said: Oh, yeah...that must really weigh heavily on your mind. I replied: Yeah...but not as heavy as it's going to weigh on your wallet!

Rick

No, I don't mind, and you are exactly right to pick up on the difference. That difference is simply a matter of what model you are using. There's no such thing as an exact physical model, since we don't know exactly how the real universe works in all of its details from top to bottom. (In fact, we don't know for certain whether there is just one universe or several, one set of physical laws or several, and other basic stuff -- but that's waaaay off the topic.) Now, that doesn't mean that discussing physics is meaningless or of no worth in dealing with the real world. For an extraordinarily broad range of real world problems, the unknown bits of physics play such a small role that it simply doesn't matter in any practical sense that we don't know how to factor in their contributions (if any.)

A similar sort of thing is also true for the parts of physics that we do understand well: sometimes the contributions of even those well-known factors are so small that we are simply better off ignoring them when considering a practical, real world problem. In those cases, a simplified and idealized model is much easier to deal with and provides the same answer as a more complete and accurate model -- at least within the limits of practicality (and often within the limits of precision measurement.)

In the particular case of a rotating bullet in a gravitational field, the bullet is so close to an ideal, perfectly rigid mass -- given the nature and magnitudes of the forces acting on it -- that it makes good sense to talk about gravity having no effect on the rotation and angular momentum of the bullet. If you want to step beyond merely considering practicality or the limits of measurement, and instead want to engage in a more complete theoretical discussion, then gravity does have an effect on rotating masses. That is because the rotating mass is not perfectly rigid, the acceleration due to gravity is not the same at every point throughout the mass, and as a result, the mass is deformed by gravity. That is exactly what causes the deformation of the earth's oceans known as tides, so the forces due to variation in the gravity field across a particular region of space are known as tidal forces.

In the case of a bullet, those forces are incredibly tiny. It takes very precise measurements to record variations in the earth's gravity field taken over relatively large distances -- such as at sealevel and in an airplane at 30,000'. The diameter of a bullet is much, much smaller than 30,000', and the tidal forces are correspondingly much, much smaller. The overly belabored upshot of all of this is that tidal forces are simply not worth considering in any practical consideration of the effects of gravity on a bullet, even if in a more precise, theoretical sense they are not exactly zero. My seemingly contradictory statements are simply a reflection of which model I was thinking of (even if I wasn't perhaps being careful enough about making my usage of that particular model explicit and clear.)

Rick, I actually have a little experience at that but it weren't at high speed. Used to strap on a minigun before strolling around in Nam. Thought I'd shoot down a hawk one day when we were bored...far as I know he's still flappin'. An illuminating experience indeed. Air to ground is much easier. Choppers are a lot of fun!



From memory only but as I recall from McCoy's work, the difference in POI from a .50 cal moving at 250 knots is about 16' from 9 o'clock to 3 o'clock, range being a few hundred yards. This is from the influence of aerodynamic jump. Air to air is tough!

A bullets rotation would be perpetual only if it (the bullet) did not encounter resistance, which it most certianly does. The lionshare of said resistance is not on the side of the bullet but in the frontal area due to its forward movement. The rate of decay of the spinning motion is very slow in relation to the forward motion, but not unlike a top spinning on a hard floor, the little resistance there is will eventually have its way with the energy causing it to spin.

Anyone who has experienced heavy for caliber cast bullets tumbling at long ranges can testify to this.

I wrote this in 1996:

Now wait! If the bullet gits a spinin one way and the gun gits a spinnin the other, and the target's on earth, all spinnin will stop sooner with tha mushroomed bullit.
Or are you in outer space?

Quote:

"If you're cross-eyed and have dyslexia, can you read okay?"

I think you can...but only in a vacuum when the tide is out and the Moon is in its seventh house... and even then I believe you have to stand on your head!

On the other hand...I know people who are not cross-eyed, don't have dyslexia...and still can't read.

Rick

PS...If you can "strap on" a mini-gun and "stroll around" you are a far stronger man than I.

I've lost my grip on more than one occasion.

This is quite good�
http://www.nennstiel-ruprecht.de/bullfly/index.htm#Top_of_page

"one word, rick: "rental"

And no crashing cars

Harry Madson yes...the others I don't recall ever meeting.

Rental, bought...what the hell do I care, they ain't my cars I'm crashing!

On Howard the Duck they made us use rented cars for the whole chase with the Ultra-Light plane and we were informed that anyone who even scratched one would be flying home the same day.

Two days later I completely destroyed a rented cop car during the chase and pretty much totalled the rented stake-bed truck I hit with it...and the director loved it so much I became his favorite stunt guy on the show. The shot is in the film and it was about the hardest hit I ever took in a car wreck.

Rick

"Rental, bought...what the hell do I care, they ain't my cars I'm crashing!"

I think we are talking Equipment Rental, the kind you don't smash up.
Verry Profitable.

yes specialist, it's the equipment kind, altho my kit is tools and material handling equipment, not "shooting" equipment.

I rely on the slow nickel rather than the fast dime

Well, I'm really pleased we got all that ballistic stuff sorted out! Whew, it was getting serious for awhile! I thought I was gonna have to speak some more to the subject.

You stunt guys and grips and all, I admire your courage. Or at least your lack of fear. Thanks for the side bar on that.

Rick, you know I was talkin' casual and all...not even Arnie could do that...when he was young! In all reality I strapped on an OH-6 and hoovered around. Did you know that some Cobras had twin minis in the turret, and the recoil thrust was powerful enough to keep them from rolling into a rocket run until the burst limiter shut it down? ratta-tat-tat. And you're right. Upside down in a vacuum is the trick.

Wow! So that was you in Beetle Juice?

got back my replies...

i was wrong.. in time of flight, gravity has no measurable effect

jeffe

4 Stars for you Jeffe!

Dan,
thanks partner... when I am wrong, I fess up quick...eating crow sucks, but cold and moldy crow is even worse

jeffe

Well...you grips can say equipment rental all you want to but stunt guys get to say stunt adjustments and residuals!!! Of course, we also get to say knee, shoulder, back surgery... and early arthritis.

The best thing about stunt work is where else can you have a third grade education and make $400 grand a year by playing cowboys and Indians and cops and robbers?

Rick