Gun Myth

[CypherNinja]

JesseL and sacp beat me to it, but yeah, at extreme velocities the curvature of the Earth would be a factor.

I.E. If you had a rail-gun in Kansas that could fire a bullet at 2-3 miles/sec or so, the fired round would hit a fraction of a second later because the surface was "falling away" from the bullet.


Also, doesn't CheyTac's ballistic computer account for curvature of the Earth as well?

 

[donttellthewife]

I believe if you were on the moon doing this experiment the two bullets would hit the ground exactly at the same time. On this planet we have an atmosphere which will have an affect on the fired bullet, granted for all practical applications they will hit at the same time, but in science even 000001 of a second counts

 

[Eightball]

Gravity is one law you can't break.

Watch me. I'm a criminal that way .

 

[JesseL]

I believe if you were on the moon doing this experiment the two bullets would hit the ground exactly at the same time. On this planet we have an atmosphere which will have an affect on the fired bullet, granted for all practical applications they will hit at the same time, but in science even 000001 of a second counts

Nope. Atmospheric forces don't figure into this scenario.
This is because (as has already been noted) the fired bullet and the dropped one have identical vertical velocities.

You would actualy see a greater difference between the impact times on the moon because of the moon's smaller radius and lower gravity. Those factors would make the experiment that much less like the classic flat-earth physics scenario and more of an orbital dynamics problem.

 

[Colt]

I asked a similar question to a college physics professor, and I believe he gave me a BS answer.

Q: If a jet is flying at the speed of a bullet, and a bullet is fired from a rearward-facing gun mounted on the jet, would the bullet fall straight to earth?

He said no. He said the bullet would travel "backward," in the direction it was fired, but at a slower rate than if it had been fired from at rest. He said it was due to the "instant explosion of the propellent."

I didn't refute his answer, but instead asked at what speed the jet would have to be traveling before the bullet would fall straight to earth. He said it would never happen.

So I asked "You're telling me that if the plane was flying at 5-gajillion miles per hour, the rearward-fired bullet would still travel backward of the point from which it was fired?"

"Yes."

I want my tuition money back.

 

[donttellthewife]

It is true that the horizontally fired bullet and the dropped bullet would hit the surface at the same moment, if the experiment happens in vacuum. In vacuum there is only the force of gravity which affects both objects in the same way.
However if the shooting occurs in air there is the additional force of drag.
Both objects - let us assume spheres - experience drag. The difference however is, that the horizontally fired bullet has a much higher velocity. Only the "downward" velocity components vy at t=0 are the same (vy=0) for both bullets.
The force of drag is (roughly) proportional to the square of the velocity v (v = sqrt(vx2 + vy2)) and not only to the vy component! Thus, the drag experienced by the fired bullet is much higher than the drag experienced by the dropped bullet. As a consequence the fired bullet will reach the surface later.
Example: Sphere of 10 mm diameter, 10 g mass, fired at 500 m/s from 10 m height
1. Horizontally fired: flight time 1.649 s; terminal velocity 160.2 m/s; point of impact at approx. X = 400 m (range)
2. Dropped: fall time 1.432 s, terminal velocity 13.9 m/s

I hope this explaines it better

 

[mpmarty]

Believe it or not, that is a genuine problem with ballistic projectiles in hypersonic aircraft (read bullets as opposed to self propelled projectiles such as rockets). It is possible to "fly into" your own bullets in certain circumstances of dogfighting. Although the velocity of the aircraft is added to forward fired weapons normal velocity this added velocity actually increases the rate of decelleration in the bullets i.e. sub sonic exterior ballistics vs. super sonic ballistics have different ingalls values for drag and decelleration.

 

[avpro]

I think that the bullet would travel backwards relative to an observer on the aircraft, but drop straight to the ground relative to an observer on the ground.

 

[javacodeman]

My 2 cents:

True but for two things:

1) The Earth is curved. If your projectile has enough velocity, it could enter into orbit where it fell at the same rate that the Earth curves away from it. This, of course, doesn't happen with bullets. But the general effect would happen to a lesser degree.

2) As someone mentioned, the wind resistance would play a role. I'm not speaking of the resistance to the bullets horizontal vector, but to the vertical vector. I can only imagine that a spinning bullet would handle this differently than a non-spinning bullet, but how I do not know. Additionally, any other defects would also play a role in this wind resistance, just as dimples in golf balls help them stay aloft.

 

[TimboKhan]

But a bullet can travel a mile before hitting the ground, its just hard to understand it hitting the ground the sametime.

Most bullets can only travel a mile when fired on a ballistic (parabolic) arc. I might be wrong, as I usually am when it comes to physics questions, but I don't know any bullet that could travel a mile if fired from a level rifle at a reasonable height.

 

[Hazzard]

I think the timing of each bullet's fall (one from the barrel, one dropped from the hand) depends on how they fall.

In my defence, I did say "But 2 objects of equal terminal velocity will fall at the same rate on earth."

If you fire the bullet fast enough (and neglect aerodynamic resistance, and you don't have any backstop) it will go into orbit at the altitude it was fired from.

However, if you drew a tangent line from the curvature of the earth out into space, the dropped bullet would hit the earth at the same time that the fired bullet intersected the tangent line if they have the same terminal velocity and air density hasn't changed..

 

[RecoilRob]

Colt..

Any projectile fired rearward from a vehicle at the same, exact speed as the vehicle is travelling forward....would drop straight to the ground. Easy to picture a slow moving car with you heaving a baseball out the back window. If you did it right, the ball would bounce straight up where it hit....from falling straight down.

I think a refund from the professor in question is warranted.

 

[Wes Janson]

Assuming a perfectly flat plain, several miles long, and perfect timing...in a vacuum I have no doubt that both would hit at the same time. But I'm strongly suspecting that in an atmosphere there's going to be a very minute difference. When you read about all of the factors that affect accuracy for benchrest shooters, I can't help but suspect that something is going to cause a slight difference.

 

[Molon Labe]

There's a lot of guessing going on in this thread.

Time for a real answer: The bullet dropped from your hand will reach the ground first. The bullet fired from the gun will reach the ground last.

Why? Answer: lift. There's lift due to the bullet's positive angle of attack. Why is there a positive angle of attack? Because when the bullet is flying through the air, the longitudinal axis of the bullet is not at exactly the same angle as the flight path. The bullet points "up" a little bit relative to the flight path, thereby giving the bullet a little bit of lift, and thus staying in the air longer than the bullet you dropped from your hand.

 

[Hazzard]

Answer: lift. There's lift due to the bullet's positive angle of attack. Why is there a positive angle of attack? Because when the bullet is flying through the air, the longitudinal axis of the bullet is not at exactly the same angle as the flight path

Regardless of angle of attack, there is no lift. Lift is caused by a lower air pressure on top of the object than below the object (like an airplane wing). The bullet is symetrical and spinning, so the pressure on the upper side will be equal to the lower side. The rise due to angle af attack is due soley to trajectory. If fired parallel to the ground, angle of attack makes no difference.

 

[c_yeager]

This is true. The reason that it is true is because bullets DO NOT generate any lift whatsoever, a bullet is perfectly symetrical (ensured by the spin imparted with the rifling, thats why guns have rifling). A bullet will not generate lift no matter what angle its fired at. The foreward motion of the bullet does nothing to slow its fall to the ground.

In fact, just to make the experiment more confusing you dont even need to drop a bullet of the same weight. The experiement words if you drop a wooden bullet weighing 20 grains and fire a bullet weighing 230 grains, weight has no bearing on the experiment whatsoever, neither does velocity, the only thing that really needs to be constant is the shape of the bullet, and even that isnt a big variable.

OK, there is one variable here that probably only matters for thousands of a second (at best). The curvature of the earth means that a bullet fired level with the horizon does in fact have an infintesimal upward trajectory compared with the surface of the planet, theoretically the further the bullet travels the longer it will take to land, again we are talking about an amount of time that probably cant even be measured with regular equipment. For all intents and purposes the fired bullet lands at the exact same time as the dropped bullet, and no, wind resistance doesnt matter.

You really should have learned this in highschool physics, its an easy experiment to conduct.

Just to fuel the fire, consider this: You fire a 32 grain .22 short from a 3 inch pistol barrel at an angle perfectly even with the horizon. Simultaneously you fire a 55grain .223 from a 20 inch barrel at the same attitude, which one lands first?

 

[Hazzard]

You really should have learned this in highschool physics, its an easy experiment to conduct.

With a spring loaded "gun" with a switch on the end, a couple of ball bearings, and an electromagnet. The bearings will hit in mid flight/drop every time. Great experiment that I demonstrated many times.

Just to fuel the fire, consider this: You fire a 32 grain .22 short from a 3 inch pistol barrel at an angle perfectly even with the horizon. Simultaneously you fire a 55grain .223 from a 20 inch barrel at the same attitude, which one lands first?

Same time given the same terminal velocity. The .223 will just travel farther before it hits the ground.

 

[Drewtam]

If it fell fast enough, it would curve slightly left or right from the spin, but not up or down. But wind would have a much more significant effect on direction.

A bullet can only have a significant angle of attack if the center of pressure is different from the center of mass. For a bullet shape, this does not happen. (yes this explanation is oversimplified, but it suffices)

Colt,
The answer depends on perspective.
If you are on ground and plane goes to the left at the speed of bullet and shoots backwards, then to you, the bullet isn't moving and just falls.

If you are on the plane, it appears that the bullet is moving away at the speed of a bullet .

Drew

 

[Molon Labe]

Regardless of angle of attack, there is no lift.

You are wrong.

A bullet will not generate lift no matter what angle its fired at.

You are wrong.

For a bullet shape, this does not happen.

You are wrong.

Lift is caused by a lower air pressure on top of the object than below the object (like an airplane wing).

True, but that's not what keeps an airplane in the air. Contrary to what you learned in high school physics, the differential pressure between the top and bottom surfaces of an airplane wing does not produce enough lift to keep an airplane in the air. In other words, your high school physics teacher was wrong.

Remember when your teacher drew the cross-section of the airfoil and the two air molecules? And then showed how the molecules got separated by the wing's leading edge, and how they were "reunited" at the wing's trailing edge? (There's no such thing as an "air molecule", but we'll pretend there is.) I bet your teacher said something to the effect, "The molecule that travels along the top surface of the wing has a longer path than the molecule that travels along the bottom surface of the wing. Due to Bernoulli's Principle, there must be a lower pressure along the top surface of the wing relative to the bottom surface. This provides lift. Blaa blaa blaa."

Well guess what? Your teacher was wrong on a couple crucial points:

1. Yea, the leading edge splits the molecules. But there's no law that says they must be "reunited" at the wing's training edge. In fact, they're not.

2. Yes, the air pressure along the top surface of the wing is lower than the air pressure along the bottom surface of the wing. But the amount of lifting force produced by this differential pressure is not enough to keep a plane aloft.

If you don't believe me, let me ask you a question: How do stunt planes fly upside down? When a stunt plane is upside down, the wings are inverted! Yet they are still able to fly.

What keeps a plane in the air is not due to "lift" created by differential pressure between the top and bottom surfaces of the wings. An airplane stays in the air because the engine pulls the airplane forward and the wing is at a positive angle of attack. When the air hits the wing, the wing directs the air down. That's important, because air has mass. When you throw mass in one direction, there is an equal and opposite force in the opposite direction. The wings throws air down, which means there is an upward force on the wing. A helicopter operates on the same principle... it also throws air down, which results in an upward force on the helicopter.

Ever stick your hand out the window while driving? If you flatten your hand, and rotate your hand to make a positive angle of attack, your arm will go up. Why? Because your hand is directing air down. Air has mass, which means there's an equal and opposite force pushing your hand up. An airplane wing works the same way.

So can an airplane fly with wings that look like barn doors? Yes. So why are wings shaped like airfoils? A wing that looks like a barn door is not efficient... it creates turbulence, drag, a vacuum behind the wing, is somewhat unstable, etc. An airfoil is more efficient. The reasons are somewhat complicated, so I won't get into it.

For the uber-geeks, more info on this stuff can be found here and here.

Oh, and one more thing... not only does the bullet's positive angle of attack (a.k.a. positive yaw angle) create lift, but by golly the air pressure above the bullet is slightly less than the air pressue under the bullet due to spinning. This also produces a little bit of lifting force via Bernoulli. See pages 15 and 19 in this document for more info.