As the world turns fact or fiction?

[natman]

The Earths rotating and moving under the bullet is true.

Yes the bullet was moving at the same speed as the Earth, but not after it left the barrel.

Ok, what made the bullet stop moving the same speed as the Earth just because it is no longer in the barrel?

 

[eye5600]

What about relativistic effects?

In college physics, we did a lab with firing a .22 short into a ballistic pendulum in order to measure... I don't remember what. The prof did murmur something about the uncertainty principle for the bullets though it was not relevant to the experiment.

I was just bowled over by the college giving kids guns and bullets in the lab with not particular instruction or supervision. I suppose the experiment had been in use since the time when any and every kid in Maine knew about guns. And I suppose they don't do it anymore.

As for including the Coriolis effect in your ballistic program, I think it could make a difference of a quarter or half a bullet width at the target for a long range shooter, well worth the effort.

 

[Dr. Tad Hussein Winslow]

Ok, what made the bullet stop moving the same speed as the Earth just because it is no longer in the barrel?

Ahhh, good question. Nothing made it stop, but what made it *slowly slow down slightly, relatively speaking*, I believe, is the fact that it's no longer touching the ground, nor touching anythiog which is touching the ground, and therefore no longer being slightly nudged along by contact with the ground. Witness what happens when you shoo a fly our your car window, or jump high while standing on a rotating merry-go-round - you don't fall back to the exact same spot. Now it IS still nudged along somewhat by the air in the atmosphere which is matter that has mass and is also moving along with the earth - which is why a fly flying around inside your car does not fall back to the back window (the air mass inside the car is being pushed along by the interior extremities at the back of the car, pushing the whole air mass forward - but again, shoo it out the window and everything changes). But I think that in the air, the air cannot push the bullet along *quite* as much as being in contact with something which is in contact with the earth would push it along, so you get a small amount of coriolis. 'Course, I could be all wet on this.

You know, I think I AM full of it, because the 'earth spin' vector would not stop, due to the law of inertia, would it? ... not even in a vaccuum would that sideways vector & velocity change, let alone when also surrounded by an air mass which is moving along with the ground! Hmmmm - now I'm at a loss to understand the coriolis....

 

[Driftertank]

Benezra, thanks. I think I knew that, but reading all this stuff confused the simplicity if the concept in my mind....

The easiest way, at least in my mind, to think about the Coriolis effect is to relate it to the Mythbusters episode where they debunked the bit in the movie "Wanted" about curving the bullet around an obstacle: no matter how fast you 'fling' the gun, as the bullet leaves the barrel it assumes a straight velocity vector. While it is effected by air resistance and gravity, movement of the barrel does not induce a progressive vector change after the bullet leaves it: the bullet travels, in a straight line, in the direction it was travelling at the moment it left the barrel. For the same reason, the shooter, moving along an arc as he would be on the surface of the earth, doesn't impart the movement of the earth's arc into the bullet after it leaves the barrel; it continues to travel in a straight line (barring wind changes). The earth rotates under it, inducing a RELATIVE change in the bullet's vector, which is minor, but over the tame lapsed from firing to impact on a very long-ranged shot, can noticeably change the bullet's impact point.

But the biggest influence, aside from the shooter's movement, which changes POI, is aerodynamics...manifesting in both windage and spindrift. Gravity falls right behind it, as the shot's elevation has a marked effect on POI as well. Unless you're making REALLY long shots, the coriolis effect is negligable as an influence on POI.

 

[rcmodel]

What happens when the wind is blowing 20 MPH, 180 degrees opposite the direction of the earths rotation?
Or if the wind changes and blows 180 degrees the other way, it is still a 20 MPH wind.
And you are shooting 90 degrees north or south too it?

You still have a 20 MPH crosswind to contend with when shooting long range.
You have to allow for that wind, but you don't have to allow for the earths rotation.

rc

 

[taliv]

the most important part is if you're shooting straight up with no wind. someone should calculate whether the coriolis effect would be enough to cause it to miss the shooter

 

[Driftertank]

Depends on the type of round. Most rounds have sufficient flight time to make the answer yes, but in the real world, wind makes much more difference.

Mythbusters did another episode where they fired several guns straight up. Their rounds landed all over the place.

 

[benEzra]

What happens when the wind is blowing 20 MPH, 180 degrees opposite the direction of the earths rotation?
Or if the wind changes and blows 180 degrees the other way, it is still a 20 MPH wind.
And you are shooting 90 degrees north or south too it?

You still have a 20 MPH crosswind to contend with when shooting long range.
You have to allow for that wind, but you don't have to allow for the earths rotation.

Wind effects do dominate, yes. Coriolis only comes into play when you're using really heavy projectiles, shooting at really long ranges (many miles), and/or punching rounds into the stratosphere. Think "long range artillery".

Of course, with modern guided munitions, Coriolis inertial effects are almost as irrelevant to projectiles as they are to airplanes and cars; if the projectile is actively steering itself where it is supposed to go, the initial trajectory only has to be approximate.

 

[SpamHandler]

Art, I was just showing that your example was not of the Coriolis effect, but of another effect acting on the projectile. (Very illustrustrative example that you provided, BTW).

The direction of rotation creates a pressure differential between the sides, as viewed from the axis of spin. The behavior, then, is much like an airplane wing, with lower pressure on the high-speed, upper, side and higher pressure on the low-speed side. The differential creates lift forces on the wing--and pushes a bullet sideways.

The lift created by a wing is due to the Bernoulli effect, and relies on air following a longer path along the upper surface to lower the pressure above. Since a bullet doesn't have such an irregular surface, being a circle in cross-section, it is much more dependent upon the wind velocity to effect any change in lift or drop. The pressure differential is responsible for both.....but due to 2 different effects.

 

[Ohio Gun Guy]

I'm jumping in late here and only read the last few posts.... I'm not convinced.

Not for small caliber. MAYBE for a .50 cal fired at a VERY long range. Wouldnt the bullet have to be free from the earth for enough time for the earth to essentially spin under it? I would think that since the atmosphere is rotating also, at nearly the same speed this effect, if any, would be minimal and only have effect on shots that have a much longer flight time than most small arms.

However, I may use this as an excuse next time at the range.....

 

[hogshead]

Didnt you guys watch shooter, Mark Walberg said you had to take into account the rotation of the earth. Came from Hollywood and I know they know. Obtw I'm just a peckerwood with to many guns too.

 

[Quentin]

I wonder just how much Mark Walberg and the Hollywood crowd know about rifles. Less than we do, that's for sure!

 

[scythefwd]

I have only heard of this mentioned once by long range shooters that happen to be friends. One is on the Army shooting team, he doesn't care. One was a scout sniper, and as long as he isn't traveling he doesn't care. The scout said that it only became a consideration when he was drastically changing latitudes and taking long shots. Since he still practices at 600-1k yards (or is it meters) I would be interested in what he considers long shots.

I take it that if you sighted in in Gnome, Alaska, and are sent on a sniping mission in Brazil it would matter. You don't get a ton of shots to sight in and adjust your POI vs POA when you are trying to sneak in without being noticed. If you have all day to sight in I wouldn't bother with considering it for small arms.