Bullet dropped lands the same time as a bullet fired..

[Mn Fats]

I just seen a mythbusters clip where a .45 acp bullet was dropped from roughly 3 feet and one was fired horizontally from a 1911 from the same height at the same exact time. The fired bullet landed at about 400 ft away. And, like the title says, it hit the ground the same time as the dropped bullet.

Im sure this has been discussed here somewhere. Im pretty simple. Im trying to wrap my simple head around the idea that 'does this only happen with a .45 acp traveling at about 825 fps?'

What if one were to drop say, a .22 bullet and fire a .22-250 traveling at 4000 fps at the same time, wouldn't it "hang" in the air longer and the bullet dropped would land before the bullet fired?

 

[edwardware]

Assuming flat ground, and horizontal firing position, yes. . . the fired bullet falls to earth at exactly the same acceleration as the dropped bullet. It's just moved over a couple hundred feet on its way to the ground.

It's possible the the bullet creates a vanishingly small net lift due to flying nose-up, but I don't think you could measure it.

 

[MidRoad]

I would think it applies to any bullet. As long two of the exact same bulletss are shot on a level and the bullet is dropped from the same height as the fired bullet gravity should apply the same downward force whether the bullet is moving forward or not. So they should reach the ground at the same time. Unless one or there other is affected by an outside force. But maybe I'm wrong?

 

[denton]

It applies to any projectile fired horizontally.

You can split the forces on the bullet into vector components. The vertical acceleration of gravity is the same for the dropped projectile and the fired projectile.

 

[Mn Fats]

But... If a .45 acp at 825 fps lands at 400 ft fired from 3 ft off the ground, wouldn't a bullet traveling much faster at 4000 fps land further out if it was also fired 3 ft off the ground? Say 1200 ft? Giving it milliseconds to stay in the air longer than the .45?

 

[danez71]

But... If a .45 acp at 825 fps lands at 400 ft fired from 3 ft off the ground, wouldn't a bullet traveling much faster at 4000 fps land further out if it was also fired 3 ft off the ground? Say 1200 ft? Giving it milliseconds to stay in the air longer than the .45?

No.

It doesn't stay in the air any longer... it just travels further before it hits the ground because it's doing 4000 ft/sec instead of 825 (in your example)

 

[DavidABQ]

The force of gravity is constant, they are the same distance from the surface of the Earth. As was pointed out if you break up their trajectories into vectors you can see that the vertical vector is the same.

 

[WestKentucky]

Disregard distance travelled as that is irrelevant. The simple way to look at it is that both bullets are being released from 3 ft height at the same time and gravity is allowed to pull them down to the earth without any interference. Again distance traveled is irrelevant because you are only concerned with the vertical drop.

 

[reddog81]

Gravity doesn't care how fast the bullet is traveling horizontally. Both bullets drop at the same rate (assuming the fired bullet is going perfectly horizontal).

If you fired both a 45 ACP round and a .30-06 at a 30 degree angle the .30-06 would remain in air much longer.

 

[Theohazard]

What if one were to drop say, a .22 bullet and fire a .22-250 traveling at 4000 fps at the same time, wouldn't it "hang" in the air longer and the bullet dropped would land before the bullet fired?

No, it’s the same for any bullet at any speed.

Removing variables like air resistance, uneven terrain, and the curvature of the Earth, a bullet dropped from the hand will always hit the ground at exactly the same time as a bullet fired horizontally. It doesn’t matter what bullet and what velocity.

Think of it this way: What force would cause the bullet to “hang” in the air? Gravity works the same way on all objects, so the way to make your bullet work against the force of gravity and stay aloft longer is by having the bore of the firearm angled upwards. But if the bore is exactly perpendicular to the force of gravity (horizontal), it’s not working against or with gravity.

 

[primalmu]

Theoretically, if the earth was a perfect sphere a bullet that was fired from a gun would land slightly after the one dropped from the hand. This would ONLY be due to the curvature of the earth and have nothing to do with the speed of the bullet.

 

[mcb]

It would also have to do with the speed of the bullet. Your right it is not quite true that the fired bullet and dropped bullet will hit the ground at the same time. Within the distances and velocities encompassed by conventional firearms and the simplifying assumption made here in the thread and by Mythbusters it's accurate enough (we probably cannot measure the difference in this setting) and basically true but not strictly true all the time.

If we get nit picky its not true as its based on some simplifying assumptions. The big assumption we are making is the ground is flat and the gravity vector is perpendicular to it and parallel to itself. This is not true, we live on a ellipsoid and the gravity vector is not parallel to itself since it always points to the center of mass of the earth. Thus if you fire a bullet horizontally fast enough the fact that we are on ellipsoid would come to play. Orbital velocity at sea level is 25,900 fps, assuming we had no air resistance or elevated terrain to run into a bullet fired horizontally at 25,950 fps would never touch the ground it would be in orbit. If you fired it horizontally at say 90% of the orbital velocity it would still hit the ground but it would take measurably longer to reach the ground than the bullet that is simply dropped. If we fire that bullet horizontally at 1000 fps or even 3000 fps with air drag playing a role the difference is not long large enough to be measures even in the best lab setups. Though if you got really neurotic we could probably calculated if for an ellipsoid the approximate shape of the earth.

The assumptions made in the Mythbuster story are only correct for relatively low velocities (as far as orbital mechanics go not firearms they are too slow)

Yes I play to much Kerbal Space Program

 

[Fine Figure of a Man]

It would also have to do with the speed of the bullet. Your right it is not quite true that the fired bullet and dropped bullet will hit the ground at the same time. With in the distances and velocities encompassed by conventional firearms and the simplifying assumption made here in the thread and by Mythbusters it's accurate enough (we probably cannot measure the difference in this setting) and basically true but not strictly true all the time.

If we get nit picky its not always true as its based on some simplifying assumptions. The big assumption we are making is the ground is flat and the gravity vector is perpendicular to it and parallel to itself. This is not true, we live on a ellipsoid and the gravity vector is not parallel to itself since it always point to the center of mass of the earth. Thus if you fire a bullet horizontally fast enough the fact that we are on ellipsoid would come to play. Orbital velocity at sea level is 25,900 fps, assuming we had no air resistance or elevated terrain to run into a bullet fired horizontally at 25,950fps would never touch the ground it would be in orbit. If you fired it horizontally at say 90% of the orbital velocity it would still hit the ground but it would take measurable longer to reach the ground than the bullet that is simply dropped. If we fire that bullet horizontally at 1000 fps or even 3000 fps with air drag playing a role the difference is not long large enough to be measures even in the best lab setups. Though if you got really neurotic we could probably calculated if for an ellipsoid the approximate shape of the earth.

The assumptions made in the myth-buster story are only correct for relatively low velocities (as far as orbital mechanics go not firearms they are too slow)

Yes I play to much Kerbal Space Program

The point is, a bullet fired out of a gun falls at the same speed as a bullet dropped.

 

[primalmu]

Yes I play to much Kerbal Space Program

It's amazing how much you learn about orbital mechanics playing KSP.

 

[Captcurt]

It is a theory that they will hit at the same time, but only in a vacuum. The projectiles are both affected equally by gravity, but friction or resistance with the air will impede projectile speed though only minutely. You need a vacuum, a perfectly flat surface, perfectly horizontal barrel and perfectly timed execution.

 

[Arkansas Paul]

Gravity is the same 9.8 m/s squared, no matter how fast an object is moving horizontally.
Gravity effects it the same.

 

[mcb]

It is a theory that they will hit at the same time, but only in a vacuum. The projectiles are both affected equally by gravity, but friction or resistance with the air will impede projectile speed though only minutely. You need a vacuum, a perfectly flat surface, perfectly horizontal barrel and perfectly timed execution.

If we are making the flat surface and parallel gravity vector assumption (ie ignoring orbital mechanics) than air resistance should make no difference. The horizontal velocity and associate drag are and orthogonal vector to the falling acceleration and velocity and its associate drag and thus separable. As long as both bullets are in the same orientation they should fall the same air resistance or not.

 

[aarondhgraham]

This is a classic example of a lesson in Physics 101,,,
Every textbook written uses it.

Gravity always works,,,
And it's always constant.

Aarond

.

 

[ATLDave]

It is a theory that they will hit at the same time, but only in a vacuum. The projectiles are both affected equally by gravity, but friction or resistance with the air will impede projectile speed though only minutely. You need a vacuum, a perfectly flat surface, perfectly horizontal barrel and perfectly timed execution.

You don't need the vacuum.

 

[mcb]

This is a classic example of a lesson in Physics 101,,,
Every textbook written uses it.

Gravity always works,,,
And it's always constant.

Aarond

.

Technically gravity various with the distance (inverse of the distance squared) between the two bodies centers of mass. Average gravity at sea level is 9.80665m/s^2 at 1600m altitude (Denver CO) its only 9.80173 m/s^2. It also varies with location on earth since the earth is lumpy and not uniform in density but that is getting really pedantic.

 

[bhk]

When I was around 10 or 11, I (with major help from my father) used this theory to calculate the average velocity of a BB fired from my Red Ryder. At some distance above the ground (3 to 5 feet, can't remember exactly) a BB would take exactly 1/2 second to fall to the ground (32 ft per second/second) in a vacuum. At that age, I couldn't control for the vacuum part, of course. We then repeatedly shot the BB gun from a level position at the correct 1/2 second height and calculated the average distance of the BBs flight. Took that flight distance time two, resulting in the average velocity in ft/sec over the entire flight of the BBs flight. Didn't worry too much about the curvature of the earth!

This was over 55 years ago and the details may be somewhat fuzzy and the results were certainly rough, but is was an interesting educational project at the time.

 

[ATLDave]

One way to think about this - a bullet is not an airplane. Airplane wings provide lift because they asymmetrically create lower pressure above the wing than below it and/or by deflecting air downwards but not upwards. It's the asymmetry that provides lift.

Bullets are symmetrical. And they spin, so any deviation from that symmetry spends equal time up and down. There's no asymmetry over the course of flight to generate lift. Just because something is going fast doesn't mean it is flying. Flight requires lift, which requires asymmetrical forces pushing up relative to gravity.

No asymmetry for a bullet means no lift means it's not flying. It's falling. It is also moving horizontally very fast. But it's not actually flying.

 

[Mn Fats]

Once again THR makes me feel...that word for not smart. Thanks guys, very educational.

 

[Chaparral]

When I was around 10 or 11, I (with major help from my father) used this theory to calculate the average velocity of a BB fired from my Red Ryder. At some distance above the ground (3 to 5 feet, can't remember exactly) a BB would take exactly 1/2 second to fall to the ground (32 ft per second/second) in a vacuum. At that age, I couldn't control for the vacuum part, of course. We then repeatedly shot the BB gun from a level position at the correct 1/2 second height and calculated the average distance of the BBs flight. Took that flight distance time two, resulting in the average velocity in ft/sec over the entire flight of the BBs flight. Didn't worry too much about the curvature of the earth!

This was over 55 years ago and the details may be somewhat fuzzy and the results were certainly rough, but is was an interesting educational project at the time.

O

Exactly - this is a key principle in ballistics, and explains why a higher velocity round will be flatter-shooting (out to a given distance) than a lower velocity round.

 

[cp1969]

Other symmetric bodies DO develop lift. Baseball, ping-pong ball are examples of this. A slider is moving through the air exactly like a football or bullet. People say 'it's the seams' but the ping-pong ball does the same thing without seams.