at the end of an article in guns and ammo the author says try and figure out what would happen if you fired a bullet straight out the back door of a jet which is going exactly the same speed as your bullet travels?

I'm at a loss. I want to say it would leave the door of the jet at normal speed, since I can't comprehend it just dropping as soon as it leaves the barrel and hitting the floor, but thats kind of what my public school education in general physics makes me want to beleive.

then I think the direction of it's momentum is changed upon ignition, making it fly normal speed and moving away from the plane 2x speed
but if you think about it even as it's moving down the barrel it still has the same velocity of the jet going the other way. Help! this is bugging me.

The bullet would be moving at normal velocity in relation to the weapon, whether the bullet is moving east while the weapon is moving west.  Ther have been cases of jet jocks in supersonic fighters overtaking their fired projectiles and shooting themselves down......
The bullet speed in relation to the ground would be zero.

The bullet would APPEAR to move twice as fast from your perspective (if you were able to actually view it) than if you were standing still and shooting it.  The actual velocity of the bullet (leaving the barrel) would be the same as if you were shooting from a non-moving position.

As for the bullet drop...hmmmmm.  The air is thinner at higher altitudes so there would be less air resistance for the bullet to overcome.  Theoretically the bullet would have a flatter trajectory but as Striker noted, I have no desire to be in a jet with a door open.

but would it fall to the ground at the exact spot it was fired?

forgetting air resistance for a second, if I threw a rock out the back of the plane it would go forward the same speed as the jet minus the speed of my throw, and gradually lose speed and hit the ground miles or so ahead depending on altitude right.

Pretend you could instantly stop the plane midair right as the bullet left the barrel, could you see it fall to the ground right in front of you?

Dude read this response from AssaultWeb

[url]http://assaultweb.net/ubb/Forum1/HTML/204953.html[/url]

Paspercops is right.
The bullet would leave the gun at a velocity relative to the forward motion of the jet and appear to drop because the forward motion of the jet would cancel the velocity of the bullet in relation to the ground. Think about it as the jet driving out from under the bullet so it wouldn't have enough time to hit the floor before the jet moved on.

I've heard of this, is it because of the bullet losing energy after a while? would the bullets from a forward firing gun on a plane have the velocity of their normal stationary ground fired speed + the planes speed initially then as soon as the bullet loses the amount of energy equal to it's energy fired stationary on the ground the plane hits it? I'm still confused.

Alright everyone...here is the official answer:
[url]http://www.physlink.com/ae128.cfm[/url]

Think of it this way. You are in a jet traveling at 800 feet per second, you stand in front of the back door with a .45 pistol and fire a 230 grn bullet out the back door. the velocity of the bullet is also 800 fps. so while the bullet is still in the chamber unfired think of it as haveing a velocity of    -800 fps. now you pull the trigger and the bullet instantly reaches a speed of 0 fps (bullet velocity 800 fps + velocity of aircraft -800 fps = 0 fps. no the bullet will not drop to the floor because it (the bullet) will instantly become motionless in air and you and your plane will be traveling out from under the bullet at 800 fps. now if on a range you fire a round at empty space it will take your bullet several seconds to fall to the ground. likewise in the aircraft by the time your bullet has fallen several feet to what would be the floor you will have traveled the same distance away from it that if on a range it would have traveled away from you. hope this helps.

but thats firing straight down, I was talking about firing straight out the back, opposite the planes direction. But it is interesting, wonder if the two bullets here would hit the same place on the ground or if the droppped one would go forward the direction of the plane more. since it doesn't have the initial downward velocity that the gun-fired one has, that might help it (gunfired one) overcome it's jet-direction momentum by it being shot perpendicular to the plane of horizontal momentum it already had. i'm getting more confused.

Wrong question, sorry dude.

The orginal question is a thought exercise on relativity. You had it right on your first answer:



My college physics professor was an avid shooter, we got a ton of these type questions through the year.

A jet is constantly accelerating, A bullet is only accelerated at the initial propellant explosion and looses velocity in relation to its sectional density (shape, cross section size and its relation to air friction). Although you add the initial velocity from the forward momentem of the jet the velocity quickly decreases so if the jet is also loosing altitude or if initally the bullet is fired at a upward angle it can (run) into the bullet.
the last part is added because no matter how fast the bullet is fired it has no lift and falls at 32 ft per sec, per sec.

The question wanted to know what an observer on the ground would see, if I remember correctly.  In that case, the person on the ground would see the bullet droping straight down with an acceleration of 9.8m/s^2.  Basically the bullet would just be free falling straight down.  Now I'll go see what the "official" answer is on the website mentioned previously and see if I'm right or just made an ass of myself.

edited because I found the article and still think I'm right.

brian73

dam! this is almost as bad as time travel

If the jet is moving at a fixed speed, the bullet will leave the muzzle at the normal
muzzle velocity.

ummmmmmm, ahhhhhhh, errrr, ahhhh, no, i don't have a clue. i guess the WW11 boys just watched where the tracers went? [:P]

I think I get it now, let's forget the back door thing, if the jet had a glass bottom and the pilot shot a bg at the back of the plane  the bullet has zero velocity and is motionless to a ground observer, but the bg's velocity is 900fps and he runs into the bullet. that right?

For starters, what reference frame are you in? Are you the guy shooting the gun or are you the guy standing on the earth with some optics? If you are the guy one the plane then you would feel like you were not moving(given you were not looking at any reference points), you would just simple think well I am standing on something solid and the plane isn't going anywhere relative to me. This would lead you to believe that you and the plane are one reference frame and therefor it would seem like any other shot you have made in your lifetime. The guy one the ground would not see things like you see them because to him you are moving. He is standing on the earth and watching you fly by. So if you are flying by at the same speed he would expect to see the bullet go when you fired the gun it would look as if someone was driving down the road and quickly painted a spot on the ground by reaching out the window. The thing would just sit there. In this case however the bullet would fall at 9.8 meters per second per second towards the ground. you who brought up that the bullet would only accelerate till the end of the barrel nedd not bring this up. when the bullet leaves the barrel it will be going the same speed as the plane in the opposite direction ahich means since it is sitting there in air it wil not deaccelerate and will keep a constant velocity of 0 relative to the guy on the ground.

If you're in a spaceship travelling at the speed of light and you turn the headlights on,  what happens?   [:D]

Ask any member of the PLO. One of them's bound to have tried it during one of their many "flights."