# GUNS and a TRAIN

Hi
If you are on a train moving at the speed of 100km east and you fire a bullet west at 100km, do the bullet move away from the spot it was fired from or does it just drop to the ground?

Just a brain teaser that has been annoying me.

Would it just drop to the ground as the speed of the train would cancel out the speed of the bullet?

Well, if this is a puzzle, then you start by realizing that the "km" units represent distance, not speed. Ignoring that, the bullet drops to the ground while the train speeds away.

Depends who's looking at it. To you on the train it will fly away from you like any other bullet. But, to someone on the ground it will indeed drop straight down.

Except hot bullets move MUCH faster than 100 km...like an order of magnitude faster. And they (and everything else) move at km/hour, or km/ sec or any other distance/ time.

Depends who's looking at it. To you on the train it will fly away from you like any other bullet. But, to someone on the ground it will indeed drop straight down.
Won't it allso do something that would look odd to the person on the ground.
The bullet does not drop straight down it will spend a certain amount off time stationary in the air before it falls, equal to the time of flight the bullet would normally take.

Won't it allso do something that would look odd to the person on the ground.
The bullet does not drop straight down it will spend a certain amount off time stationary in the air before it falls, equal to the time of flight the bullet would normally take.

This is wrong. It will fall down normally. The flight time of the bullet is the amount of time it takes to fall. How could it possibly stay suspended in the air?

phinds
Gold Member
This is wrong. It will fall down normally. The flight time of the bullet is the amount of time it takes to fall. How could it possibly stay suspended in the air?

I think what is meant here is that the drop to the ground will start off gradually and speed up BUT that is, as you believe, incorrect. Some folks believe that if you plot the vertical position of a speeding bullet against time that it is different that what you would get if you just held the bullet in your hand and dropped it (which of course, is not true).

Danger
Gold Member
I think what is meant here is that the drop to the ground will start off gradually and speed up BUT that is, as you believe, incorrect.
I feel obliged to disagree, but it's probably just a matter of semantics. The drop of the bullet is an acceleration rather than a speed, so it does indeed start gradually. That, of course, is not the same as the coyote standing on thin air until he realizes that there's nothing under him and then suddenly dropping (with the tips of his ears lagging behind).

Drakkith
Staff Emeritus
I think what is meant here is that the drop to the ground will start off gradually and speed up BUT that is, as you believe, incorrect. Some folks believe that if you plot the vertical position of a speeding bullet against time that it is different that what you would get if you just held the bullet in your hand and dropped it (which of course, is not true).

Are you saying a bullet doesn't fall to the ground as quickly while in flight as it does if it was simply dropped? I could see that being the case since it's travelling through the air.

Danger
Gold Member
Are you saying a bullet doesn't fall to the ground as quickly while in flight as it does if it was simply dropped? I could see that being the case since it's travelling through the air.

If the gun is fired parallel to the ground, the bullet will hit the ground at exactly the same time as one that is dropped from the muzzle simultaneously. There is no noticeable aerodynamic lift on a bullet, and hence no opposition to gravity.

Drakkith
Staff Emeritus
If the gun is fired parallel to the ground, the bullet will hit the ground at exactly the same time as one that is dropped from the muzzle simultaneously. There is no noticeable aerodynamic lift on a bullet, and hence no opposition to gravity.

Ah Ok. I just realized that when firing bullets at a long range target you aim UP a bit...hence they can travel for several seconds longer because of this.

This is wrong. It will fall down normally. The flight time of the bullet is the amount of time it takes to fall. How could it possibly stay suspended in the air?
If the gun is pointed up as danger has pointed out.The bullet will rise slightly then fall.
If pointed down it will fall quicker.
So it has to be parallel for it to fall at the same rate as one that is dropped.

phinds
Gold Member
I feel obliged to disagree, but it's probably just a matter of semantics. The drop of the bullet is an acceleration rather than a speed, so it does indeed start gradually. That, of course, is not the same as the coyote standing on thin air until he realizes that there's nothing under him and then suddenly dropping (with the tips of his ears lagging behind).

Yes, you are right of course. My point, incorrect as this part was, was that the bullet would fall at the same rate as if it were just dropped (although as others have correctly pointed out, that's only true when the gun is fired parallel to the ground, an assumption that I was making and should have said so)

Danger
Gold Member
that's only true when the gun is fired parallel to the ground, an assumption that I was making and should have said so)

No worries, mate. The fact is, I always assume that a horizontal position is implied unless otherwise stated. Where the problem arises is that people will generally reference personal experience (if they have any) in cases such as this. There is one point only at which any gun can be truly fired horizontally, and that is at the muzzle. Instantly after discharge, the slug begins to drop. (And I've deliberately ignored the fact that there's no such thing as "horizontal" because of Earth's curvature...) (Also, if the barrel is not properly crowned, there could possibly be a slight upward, downward, sideways or some combination thereof thrust imparted to the bullet immediately upon exit.)

edit: Oh, yeah... and barrel tuning. For maximum accuracy, the harmonic oscillations of the barrel are matched to the ammunition detonation characteristics.

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There is one point only at which any gun can be truly fired horizontally, and that is at the muzzle. Instantly after discharge, the slug begins to drop.
Only if it's a fluke.
So other assumptions are more accurate and valid.

Danger
Gold Member
Only if it's a fluke.
So other assumptions are more accurate and valid.

I didn't say that it's likely; merely that it's the only point at which it is theoretically possible.

i understand that with the same gravitational constants, a bullet would fall to the ground at the same velocity whether dropped or fired from a gun; that makes perfect sense to me.

what i'm not so clear on, however, is whether or not drag would play a role in which one would hit the ground first. for example, the falling bullet would come into contact with considerably less air than the one from the gun, so would that factor into the descent rate?

i realize the question is sort of a sub-topic of the OP's question, but i was curious, thus i ask.

i'm not a physics major, just a guy seeking knowledge, so please correct my logic or misused terms

phinds
Gold Member
i understand that with the same gravitational constants, a bullet would fall to the ground at the same velocity whether dropped or fired from a gun; that makes perfect sense to me.

what i'm not so clear on, however, is whether or not drag would play a role in which one would hit the ground first. for example, the falling bullet would come into contact with considerably less air than the one from the gun, so would that factor into the descent rate?

i realize the question is sort of a sub-topic of the OP's question, but i was curious, thus i ask.

i'm not a physics major, just a guy seeking knowledge, so please correct my logic or misused terms

Seems to me that the drag you speak of would, assuming the bullet is rotating along the axis of motion and not tumbling, just retard the horizontal velocity and have no effect on the vertical component of the velocity.

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Danger
Gold Member
for example, the falling bullet would come into contact with considerably less air than the one from the gun, so would that factor into the descent rate?

i'm not a physics major, just a guy seeking knowledge, so please correct my logic or misused terms

I'm no scientist either; don't worry about it. PF is here to teach.
While I'm in no way qualified to teach anything, I do believe that I see a flaw in your logic. You are correct that a fired bullet encounters far more air than a dropped one, but that is on the horizontal (or near-horizontal) axis. Vertically, it is impeded by the same 'depth' of atmosphere as the falling one.

edit: Ooops! Hi, Phinds. I see that you beat me to it. That's what I get for trying to post 17 hours into a "Big Bang Theory" marathon.

phinds
Gold Member
edit: Ooops! Hi, Phinds. I see that you beat me to it. That's what I get for trying to post 17 hours into a "Big Bang Theory" marathon.

Hey, happens a lot around here with so many of us answering questions, we step on each others toes, but that's LOTS better for the OP than no response at all.

thank you both for the prompt replies.

that makes perfect sense, i think i was assuming that the velocity would somehow affect the rate at which it falls, but if that were the case, it would be altered far more by the forward velocity, than by the negligible amount of drag.

enjoy the big bang theory!

I didn't say that it's likely; merely that it's the only point at which it is theoretically possible.
What you actualy said was
The fact is, I always assume that a horizontal position is implied unless otherwise stated.
Which is if you consider the odds of actualy happening highly unlikely if at all.
A bullet in flight will be deviated by air fluctuations up or down.
It aint possible to fire a bullet horizontally.
So why you pick the totally improbable rather than the theoretical inevitable is beyond me.

This experiment can be done.
Have a professional golfer stand on a flatbed of a speeding train moving at 160 miles/ hour.
This happens to be the typical horizontal velocity of a golf ball driven by a pro.The balls usually stay airborne for about six seconds.
An observer not on the moving train I think would see the driven golf ball as a stationary golf ball suspended over the railroad track for about 6 seconds and then fall to earth.

cjl
This experiment can be done.
Have a professional golfer stand on a flatbed of a speeding train moving at 160 miles/ hour.
This happens to be the typical horizontal velocity of a golf ball driven by a pro.The balls usually stay airborne for about six seconds.
An observer not on the moving train I think would see the driven golf ball as a stationary golf ball suspended over the railroad track for about 6 seconds and then fall to earth.

Suspended? I suspect they would see the golf ball in a very fast, approximately vertical climb, followed by an approximately ballistic descent. I also wouldn't be surprised if it had a substantially different flight time than normal, since golf balls have substantial spin, and aerodynamic forces would not be identical between a golf ball hit off a moving train and a golf ball hit from the ground.

Depends on the initial velocity of bullet.You haven"t specified speed is in km/s or km/hr of bullet!!!!! If its km/s them bullet will surely drop but it wouldnt be visible to normal human eye(superman is a exception)

Depends on the initial velocity of bullet.You haven"t specified speed is in km/s or km/hr of bullet!!!!! If its km/s them bullet will surely drop but it wouldnt be visible to normal human eye(superman is a exception)

Speculation: At 100 km/s it would probably resemble a "laser bolt" from science fiction due to atmospheric friction.