Moving gun firing sideways to the motion of the transporter.

[Robittybob1]

They talk about a bullet getting extra velocity when fired along the direction of travel, and likewise slowed when fired to the rear.

But what happens to a bullet fired from a gun either moving sideways or rotated (swung sideways) rapidly.

 

I saw a YT where the Mythbusters proved a bullet does not curve in flight after being fired from a gun being swung around rapidly.

I had a feeling if a gun is being swung at the same time as being fired, the gun momentarily stops, or slows at least, for there is no way to add the angular momentum to the bullet in the short time it is within the barrel.

Any ideas?

[StringJunky]

From the shooter's perspective, I think the path of the bullet appears to curve, but in reality, it follows a normal straight path. Angular momentum is not induced in that scenario... how can it? The bullet is not going to rotate about a point once it has left the barrel.

[swansont]

For simple translation, analyze the behavior in the shooter's frame. It goes straight, while the target is moving.

 

Curving in flight requires a force, since a curve represents a change in velocity and thus an acceleration.

[imatfaal]

 

Look at around 30sec -1:25 for a slow/freeze frame from the car

[Robittybob1]

From the shooter's perspective, I think the path of the bullet appears to curve, but in reality, it follows a normal straight path. Angular momentum is not induced in that scenario... how can it? The bullet is not going to rotate about a point once it has left the barrel.

There is no way the shooter can affect the trajectory once it has left the gun. I was talking about the angular momentum while it is still inside the barrel. If the bullet goes at 1200 m/sec it might be in the barrel for 1/1200th of a second if the barrel was a meter long. OK that is not long to affect the path of the bullet but motion is motion.

[swansont]

There is no way the shooter can affect the trajectory once it has left the gun. I was talking about the angular momentum while it is still inside the barrel. If the bullet goes at 1200 m/sec it might be in the barrel for 1/1200th of a second if the barrel was a meter long. OK that is not long to affect the path of the bullet but motion is motion.

 

What about the angular momentum? There is rifling in modern firearms which causes the bullets to rotate along the longitudinal axis, and the effect is to stabilize the bullet and keep it from tumbling. That has no direct effect on the ballistic path, though.

 

What is it that you think is happening here?

[Robittybob1]

For simple translation, analyze the behavior in the shooter's frame. It goes straight, while the target is moving.

 

Curving in flight requires a force, since a curve represents a change in velocity and thus an acceleration.

If the target and the shooter were going the same speed parallel to each other, ignoring wind resistance the bullet would go straight across.

If the target was stationary the gun would need to track the target. If the bullet starts off at say 0.5 meter from the point of rotation and the barrel is say 1 meter long is the swinging action of the gun transmitted to the bullet?

I'll need an estimate of the mass of a bullet and we should give an estimate of the angular speed and see how much angular momentum is required.

[StringJunky]

There is no way the shooter can affect the trajectory once it has left the gun. I was talking about the angular momentum while it is still inside the barrel. If the bullet goes at 1200 m/sec it might be in the barrel for 1/1200th of a second if the barrel was a meter long. OK that is not long to affect the path of the bullet but motion is motion.

If the barrel was 100 metres long and did a 1000 revs a second it will still come out straight. Think of a cricket bowler fast over-arming, there's no angular momentum there, is there? There's no fixed connection between the gun and bullet, once it's left the barrel for angular momentum to be there.

[Robittybob1]

 

What about the angular momentum? There is rifling in modern firearms which causes them to rotate along the longitudinal axis, and the effect is to stabilize the bullet and keep it from tumbling. That has no direct effect on the ballistic path, though.

 

What is it that you think is happening here?

OK I wasn't thinking of the rotation of the bullet along its path. Ignore that effect.

Swansont -- What I find is I don't understand your way of asking questions sorry.

"What about the angular momentum?" To me that is an ambiguous question.

"What is it that you think is happening here?" I'm not sure what aspect you are referring to by "here".

 

Off to work. Later!

If the barrel was 100 metres long and did a 1000 revs a second it will still come out straight. Think of a cricket bowler fast over-arming, there's no angular momentum there, is there? There's no fixed connection between the gun and bullet, once it's left the barrel for angular momentum to be there.

But I'm wanting to know what is happening while it is still in the barrel. At the moment don't mention what happens once it has left the barrel. What happens while it is still in the barrel.

"barrel was 100 metres long and did a 1000 revs a second it will still come out straight" agreed but can you turn a gun at 1000 Hz while a bullet is going along the barrel? And if you can what effect would it have on the torques required.

[swansont]

Swansont -- What I find is I don't understand your way of asking questions sorry.

"What about the angular momentum?" To me that is an ambiguous question.

"What is it that you think is happening here?" I'm not sure what aspect you are referring to by "here".

 

Your claim about angular momentum is rather nebulous. You said you were talking about the angular momentum while in the barrel — well, what about the angular momentum while it's in the barrel? There is no connection to the subsequent trajectory that I can see, and I am not the only one who has said so — you got that in the very first response. But in your subsequent phrasing you are not asking about angular momentum, you are insisting it is involved in the problem.

 

What is happening here - by "here" I mean "in the scenario you have described"

 

But I'm wanting to know what is happening while it is still in the barrel. At the moment don't mention what happens once it has left the barrel. What happens while it is still in the barrel.

"barrel was 100 metres long and did a 1000 revs a second it will still come out straight" agreed but can you turn a gun at 1000 Hz while a bullet is going along the barrel? And if you can what effect would it have on the torques required.

 

The numbers don't really matter. That affects the size, but not the existence, of angular momentum. The bullet will have an angular momentum about the rotation axis. L = r X p

[Robittybob1]

 

Your claim about angular momentum is rather nebulous. You said you were talking about the angular momentum while in the barrel — well, what about the angular momentum while it's in the barrel? There is no connection to the subsequent trajectory that I can see, and I am not the only one who has said so — you got that in the very first response. But in your subsequent phrasing you are not asking about angular momentum, you are insisting it is involved in the problem.

 

What is happening here - by "here" I mean "in the scenario you have described"

 

The numbers don't really matter. That affects the size, but not the existence, of angular momentum. The bullet will have an angular momentum about the rotation axis. L = r X p

Yes a "cross product"! I do wish I was better at math. OK r is the radius but as I look at it the radius is going to increase as the bullet travels the length of the barrel. Now is a man's arm powerful enough to add the required angular momentum to the bullet in 1/1200th of a second? A man's reflexes would not be able to anticipate the force required so does the gun momentarily halt in its rotation? For that is what I was seeing in the Mythbuster video on whether a bullet curved in flight.

P is the symbol for momentum but we are only interested in the momentum in the tangential direction and that will be increasing as the radius increases too won't it?

[swansont]

Yes a "cross product"! I do wish I was better at math. OK r is the radius but as I look at it the radius is going to increase as the bullet travels the length of the barrel. Now is a man's arm powerful enough to add the required angular momentum to the bullet in 1/1200th of a second? A man's reflexes would not be able to anticipate the force required so does the gun momentarily halt in its rotation? For that is what I was seeing in the Mythbuster video on whether a bullet curved in flight.

P is the symbol for momentum but we are only interested in the momentum in the tangential direction and that will be increasing as the radius increases too won't it?

 

You say the gun's rotating, so it's rotating. You're overthinking that part of the problem — deal with the physics, not the complications.

 

We care about the transverse momentum at the point it leaves the barrel, not as it travels down the barrel. r is then the location of the end of the barrel.

 

But what you should find is that when the bullet is further away, r will change but so will the angle from the axis of rotation, and the cross product (which gives you a sine(theta)) will remain constant. As it should. There's no torque on the bullet after it leaves the barrel, so its angular momentum can't change.

[Robittybob1]

 

You say the gun's rotating, so it's rotating. You're overthinking that part of the problem — deal with the physics, not the complications.

 

We care about the transverse momentum at the point it leaves the barrel, not as it travels down the barrel. r is then the location of the end of the barrel.

 

But what you should find is that when the bullet is further away, r will change but so will the angle from the axis of rotation, and the cross product (which gives you a sine(theta)) will remain constant. As it should. There's no torque on the bullet after it leaves the barrel, so its angular momentum can't change.

That is right its angular momentum won't change after the bullet has left the barrel. But as I have been asking is, what happens while it is in the barrel?

"Mythbusters: Is it possible to Curve a Bullet part 2" 1:38 in the video we get a slow motion shot of the gun and as the puff of smoke comes out it looks like it is arrested in rotation. Do you agree?

[swansont]

1:38 in the video we get a slow motion shot of the gun and as the puff of smoke comes out it looks like it is arrested in rotation. Do you agree?

No. All they've done is transition from normal speed to slo-motion at that point. The barrel is still moving.

[Robittybob1]

No. All they've done is transition from normal speed to slo-motion at that point. The barrel is still moving.

I'll have to get the guns out and try it.