physics question -- jet firing bullets

[Bob]

You're in a jet.  You fire bullets.  

Do the bullets go faster than the jet, slowed down by air at some speed?


You're in the jet and fire bullets.  The bullets are now travelling faster than the jet (assuming that for now).  You speed the jet up and match the speed of the bullets, travelling along side them.  Assuming it's possible, at this point, you could just reach over and pick up the bullets, right?  You're travelling the same speed, but that's zero mph compared to the bullets if you and the bullets are going the same speed.  So you could pick up a bullet with no harm, right?  And if you set it back where it was, it would continue on its way and hit what all the bullets where originally going to hit in the first place.  Right?

Or another variation.  You're in the jet, you fire the bullets.  After that it's a little bit fuzzy for me on how things work in real life.  But say the bullets are fired and travel slower than the jet, or if you speed the jet up and catch up to the bullets but aren't going much faster than they are.  At that point, either running into the bullets by speeding up into them or by slowing down a bit and having them run into you -- at that point, the difference in speed wouldn't be so great, so the bullets would just tap your jet and fall to the ground.  Right?  So you could fire the bullets and be travelling nearly the same speed as those bullets and have them hit your jet, but have no real effect -- ie You could shot your own jet with bullets and have no effect, have the bullets bounced off your jet.  Right?

[gyzzzmo]

Right.
Sounds like a variation on somebody trying to explain what the complications are of lightspeed.

[timothy42b]

This is one of those thought problems that seems confusing in a  vacuum.

Except you don't fly jets in a vacuum, you need air for the wing surfaces to work.  LOL.

But anyway.  Put some real data on this and the solution becomes obvious.  In particular, what is the normal speed of a jet, and what is the speed of a bullet?  You'll have to look it up, and may need to do some unit conversion.  If you're in the US, you'll probably get the speed of the jet in mph, and the bullet in ft/sec.  the rest of the world uses metric and no conversion is necessary. 

[communist]

It also depends on which way you are firing and how strong the wind current is.

[lostinidlewonder]

You're in a jet.  You fire bullets.  

Do the bullets go faster than the jet, slowed down by air at some speed?

The bullet goes faster than the jet if you fire the bullets while inside the jet.

You're in the jet and fire bullets.  The bullets are now travelling faster than the jet (assuming that for now).  You speed the jet up and match the speed of the bullets, travelling along side them.  Assuming it's possible, at this point, you could just reach over and pick up the bullets, right? .... ....So you could pick up a bullet with no harm, right?

If there is no friction (i.e: the bullet was fired in space), then you could accelerate the jet, catch up with the bullet and pick it out of the sky (although the rotation of the bullet would probably cut up your fingers). The Jet would have to travel at the speed in which it was travelling when the bullet was fired + the velocity of the bullet produced from firing it from the gun. You would need an extremely long jet to allow for safe acceleration, or you would be ripped to shreds.

Or another variation.  You're in the jet, you fire the bullets.....  But say the bullets are fired and travel slower than the jet .......... So you could fire the bullets and be travelling nearly the same speed as those bullets and have them hit your jet, but have no real effect -- ie You could shot your own jet with bullets and have no effect, have the bullets bounced off your jet.  Right?

The thing is, when you are on the Jet, the bullet in the unfired gun is travelling at the speed in which the Jet is travelling. So when you shoot the bullet it will go the velocity the jet is going + the velocity produced from firing the bullet. So for the bullet to harmlessly bounce off the wall, you would have to suddenly accelerate to match the speed of the bullet which would pretty much tear up the jet and yourself.

This only applies for when we are not travelling at the speed of light. When traveling at the speed of light strange things start to happen (time dialates and mass increases in objects). When you travel the speed of light experienced time actually goes faster (The faster your velocity the faster you move through time relative to others.)

So if you traveled for say 1 year at the speed of light, 30 years might pass for other people but you only experienced 1... strange stuff yeah. But if you fired a gun while traveling at the speed of light, the energy produced in the gun would not be sufficient to increase the speed of the bullet so it would stay in the gun and nothing would happen. No amount of energy will accelerate a mass past the speed of light.

[communist]

The bullet goes faster than the jet if you fire the bullets while inside the jet.

If there is no friction (i.e: the bullet was fired in space), then you could accelerate the jet, catch up with the bullet and pick it out of the sky (although the rotation of the bullet would probably cut up your fingers). The Jet would have to travel at the speed in which it was travelling when the bullet was fired + the velocity of the bullet produced from firing it from the gun. You would need an extremely long jet to allow for safe acceleration, or you would be ripped to shreds.
The thing is, when you are on the Jet, the bullet in the unfired gun is travelling at the speed in which the Jet is travelling. So when you shoot the bullet it will go the velocity the jet is going + the velocity produced from firing the bullet. So for the bullet to harmlessly bounce off the wall, you would have to suddenly accelerate to match the speed of the bullet which would pretty much tear up the jet and yourself.

This only applies for when we are not travelling at the speed of light. When traveling at the speed of light strange things start to happen (time dialates and mass increases in objects). When you travel the speed of light experienced time actually goes faster (The faster your velocity the faster you move through time relative to others.)

So if you traveled for say 1 year at the speed of light, 30 years might pass for other people but you only experienced 1... strange stuff yeah. But if you fired a gun while traveling at the speed of light, the energy produced in the gun would not be sufficient to increase the speed of the bullet so it would stay in the gun and nothing would happen. No amount of energy will accelerate a mass past the speed of light.

You are smart 

[iroveashe]

Right.
Sounds like a variation on somebody trying to explain what the complications are of lightspeed.

True, like if you could make a satellite with a really powerful camera that can take very detailed photos, and if you could make it travel faster than the speed of light very far away, and you make it take pics of our planet, could we actually see the past?

[scottmcc]

a high velocity bullet, such as one fired by a rifle, has a typical muzzle velocity of about 2000 feet per second, or 1300 miles per hour.  this speed diminishes over time due to friction from the air.

a typical jet will fly in a range of speeds from 150-1000 mph, although typically about 400-500 mph for subsonic cruising.  thrust from the engine offsets drag.

the bullet will fly significantly faster than the plane that shot it, as evidenced by the fact that planes are shot down by bullets, as seen frequently on the history channel.

if you could conceivably very rapidly accelerate to the speed of the bullet, then match speed with it, you could indeed "catch" the bullet, but in all practical terms it's not going to happen.

alternatively, you could jump out of one plane and into another, if it went into a carefully orchestrated dive.  it's been done several times:  www.skydivingstunts.com

[Bob]

I saw something on tv last night about a jet firing bullets and how they have to be aware of their mutions.

And I remember a teacher in the past saying how pilots ran into their own bullets when they first started using jets.

So then I'm wondering... If they're just slowly catching up to their bullets, they're not really going to be injured by them, right?  It would be like getting hit by a bullet going 1 mph.  


I've heard these two people --- previous teacher and guy on tv -- say that a jet can catch up or pass the bullets it fires.  But then... So what?  Besides just getting in the way of their ammution and preventing it from hitting the target, the jet isn't going to be damaged by bullets because the difference in speed isn't that great.

[scottmcc]

if running into one's bullets was a problem, they wouldn't put a gun on an airplane. 

the very first wwi planes had a problem with the bullets not being timed to the propeller, such that pilots would inadvertently amputate prop blades.  but this was quickly remedied with a timing belt.

modern jet fighters have no problem firing bullets at targets both stationary and airborne, but most air-to-air combat nowadays is beyond visual range, and thus conducted with various guided missiles.

of course, the AC-130 carries a really big gun on its side, which doesn't risk the plane running into it, but does deflect the plane several degrees each time it is fired.  and the A-10 has a pretty big gun around which the entire airframe was built, which can exert thrust significant enough to slow the plane down noticeably.  but that's not what you were asking.

your documentaries seem to have mislead you.

furthermore:  helicopters don't fly.  they are so ugly the ground repels them. 

[richard black]

No amount of energy will accelerate a mass past the speed of light.

No - no amount of energy will accelerate a mass _up to_ the speed to light. You can in theory get arbitrarily close to the speed of light, but not reach it.

If you fire a bullet forwards from a plane, the bullet is going a lot faster than the plane, though it slows down due to air friction. At the same time, however, the bullet accelerates downwards due to gravity, so unless the plane is in a dive it will never bump into the bullet, though it may eventually pass directly above it.

[lostinidlewonder]

True, like if you could make a satellite with a really powerful camera that can take very detailed photos, and if you could make it travel faster than the speed of light very far away, and you make it take pics of our planet, could we actually see the past?

When you look up at the stars you could be looking at stars that no longer actually exist, their light source is still travelling to our planet but they have long gone. So if you where say 1 light year away from Earth and had an unbelievably powerful telescope that could look at the surface of the Earth, you could see the past, and see things that happened 1 year ago. Stars that we see can be millions of light years away (so we are observering them one million years earlier than their current state), galaxies that astronomers observe with telescopes can be a billion years behind the state that they are actual in because the light has taken so long to get to us.

[iroveashe]

So if the universe had a beginning in time and space, and we were far enough from that place, and if we had a powerful enough telescope we could actually see the beginning of the universe. That's a lot of if's but I find it more fascinating than jets and bullets.

[gyzzzmo]

True, like if you could make a satellite with a really powerful camera that can take very detailed photos, and if you could make it travel faster than the speed of light very far away, and you make it take pics of our planet, could we actually see the past?

Occording to current theories its alot easier to bend time towards yourself (so you can see history), than influencing lightspeed itself (wich wouldnt be possible). This all due to that fact that lightspeed seems the same for everybody, no matter what speed the spectators have relative from each other.


 

[gep]

So if the universe had a beginning in time and space, and we were far enough from that place, and if we had a powerful enough telescope we could actually see the beginning of the universe. That's a lot of if's but I find it more fascinating than jets and bullets.

The universe did have a beginning, and one can "see" its beginning no matter haw far away from it you are (in other words, how many years have passed since its beginning). The first thing we can really SEE is called "the cosmic background radiation", currently a "black body" radiation of some 3K (so you cannot see it with your eyes, you have to detect it using a radio dish) which looks something like this:

It represents the "light" of the (whole!) universe at the moment of decoupling, i.e. the moment the universe had cooled enough for photons to travel freely, which is roughly 300,000 years after the Big Bang. It isn't smooth, and the details of that smoothness gives indications of what the conditions were before that time. Currently, science can tell with quite some accuracy what these conditions were back to an all but infinitesimal small interval after the Big Bang (or "Fiat Lux", if you like), something like 10-35 seconds after the very beginning.
Which is pretty astounding, I think….

[pianogeek_cz]

When you look up at the stars you could be looking at stars that no longer actually exist, their light source is still travelling to our planet but they have long gone.

But how are we to say what "actually" exists? That would mean there still is some giant "space clock" ticking by which we could set the universal "now" applying everywhere in space - but the point is, there -isn't- one!

Events (such as a star ceasing to exist) are not instantaneous, they occupy certain surfaces in space-time: in some regions of space-time, the star exists, in others, which are included in the event, it is gone. The speed of light is something like (God, my head's spinning like hell now) a delimiter of events - it determines the "shapes" of event-surfaces.

Now, space-time is non-uniform: gravity "curves" it. Therefore, the shape of an event-surface is not the same for all events (they would be in a perfectly uniform universe, then again, in such a universe the notion of an event is highly problematic). It is never guaranteed that the order of events is the same for two observers or that according to an observer's perception of time, the same event won't happen twice! Black holes would be a prime example of the curvature: they are regions of space-time so curved that from a point on the event-surface inside the black hole, it is impossible to "travel" to a point outside.

It's not a very intuitive concept and I find it highly daunting - and it has great implications for philosophy of matter and, well, of pretty much everything else as well. But it is, in a weird and disturbing way, fascinating. :-)

[communist]

Am I the only one on the forum who does not know physics 

[iroveashe]

But how are we to say what "actually" exists? That would mean there still is some giant "space clock" ticking by which we could set the universal "now" applying everywhere in space - but the point is, there -isn't- one!

Events (such as a star ceasing to exist) are not instantaneous, they occupy certain surfaces in space-time: in some regions of space-time, the star exists, in others, which are included in the event, it is gone. The speed of light is something like (God, my head's spinning like hell now) a delimiter of events - it determines the "shapes" of event-surfaces.

Now, space-time is non-uniform: gravity "curves" it. Therefore, the shape of an event-surface is not the same for all events (they would be in a perfectly uniform universe, then again, in such a universe the notion of an event is highly problematic). It is never guaranteed that the order of events is the same for two observers or that according to an observer's perception of time, the same event won't happen twice! Black holes would be a prime example of the curvature: they are regions of space-time so curved that from a point on the event-surface inside the black hole, it is impossible to "travel" to a point outside.

It's not a very intuitive concept and I find it highly daunting - and it has great implications for philosophy of matter and, well, of pretty much everything else as well. But it is, in a weird and disturbing way, fascinating. :-)

That's basically what Einstein called relativity =P

[go12_3]

Am I the only one on the forum who does not know physics 

No, you aren't.   

But physics is just a topic about matter and how things are built with matter, by chemicals and compounds and atoms and such....no matter what we touch or feel or see, it's all matter, solid or liquid.  Matter can change it's form and it usually does and it's an ongoing process for all time and eternity.  We are only living in a  millimeter of  space and time.  Man is the only creature on earth that has invented time.  Time really isn't time.....if you think about it.
We only measure time for our convenience in going places and keeping our appointments and of course keeping track of our limited or unlimited practice time.  Nature has its way of changing the seasons and also time.  Physics is everywhere and I might just study about it afterall.   

best wishes,

go12_3

[iroveashe]

No, you aren't.   

But physics is just a topic about matter and how things are built with matter, by chemicals and compounds and atoms and such....no matter what we touch or feel or see, it's all matter, solid or liquid. 

Not only matter, there's also antimatter

[go12_3]

Not only matter, there's also antimatter

Yes, coming from you, it would be anitmatter!  lol     

[Bob]

No more lightspeed stuff.  Start another thread.  I'm still confused about the jet and bullets -- Can a jet shoot really itself or would the bullets not be travelling fast enough?

[gyzzzmo]

No more lightspeed stuff.  Start another thread.  I'm still confused about the jet and bullets -- Can a jet shoot really itself or would the bullets not be travelling fast enough?

Its not all that hard. The terminal velocity of a bullet can be much slower than a jet, so in theory a jet should be able to shoot itself.

[timothy42b]

There is supposedly an actual case of this happening.

As the story goes (I have never been able to verify, but it is widely claimed to have happened) a Grumman F-111 (sometimes the model is different) fighter plane was doing gunnery practice.

In gunnery practice one shoots at a target towed by another plane, presumably with a sufficiently long tow line.

At any rate, the pilot fired 20 mm cannon at the target sleeve while in a slow climb.  After shooting his burst, he dived and accelerated.  Supposedly the projectiles followed their normal parabolic arc, slowing against both gravity and air resistance, and the plane intersected the arc and was hit.  While the bullets were going slow, they got sucked into the air intakes and destroyed the plane, while the pilot ejected safely. 

Did it really happen?  I dunno.  It's a pretty well known story though, and reasonably plausible though requiring incredibly bad luck. 

[Bob]

So it's more of an issue of flying into a swarm of metal bits rather than getting pierced by flying bullets.

[prometheus]

It would be intuitive to add velocities. But this is fundamentally wrong. Why? Because from every frame of reference the speed of light must be the same.

But for your case this can be ignored and you can classically add the velocities using trig. Since you have an airplane you use 3 components for the 3 dimensions. You define the 3 axis and from that you can define the velocities.

For air drag you have the formula F=.5ρv^2CA

Where ρ is rho, the density of the air.
v is the velocity
C is the drag coefficient
A is the area.

The velocity is with respect to the air. So if your airplane's velocity was with respect to the ground you have to consider the winds.
The drag coefficient is basically the aerodynamics coefficient. It is dimensionless. And for bullets they call it the ballistic coefficient.
A is the area of the bullet perpendicular to the direction of movement. So like a silhouette.

The heavier the bullet the slower it will decelerate because f=ma.

The bullet will also fall because of gravity. So it will always accelerate with the value g towards the earth. But if the plane is really really high and above an ocean near the equator this can really change it from the usual 9.81 m/s^2. Maybe you want to use F=m1m2G/r^2 for accuracy.

That's it.

Don't know if you want to calculate change in momentum and kinetic energy converted in the case the plane gets hit but a bullet. The thing is, bullets usually go a lot faster than planes. And they always fall down. They don't fly. There is no aerodynamic lift as with a wing.

I think it's more convenient to think of the space shuttle docking to the ISS. Both move at something like 7.4 km/s Then an astronaut goes out for a space walk. He has tools hanging from tethers.
They hand over a big box of equipment. All objects move with 7.4km/s and that's a lot of kinetic energy. Then if such an object drifts off into space and gets into the path of a satellite with an entirely different object then depending on the angles between the orbits they will have a big difference is speed. And when they hit you can approx. the forces and the kinetic energy released.


Things do get a bit complicated calculus-wise. You can get some differential equations you really have to twist around and take some integrals to get the data you want. Especially if you want to account for all the subtly changing constants.
And if you have movement in 3 dimensions you have to do 3 calculations for each dimensions.


But really you can ignore the Lorentz-factor.
I think you can see that if you had a space ship moving at 90% the speed of light and you shot both a light beam, from your flashlight, and some SF railgun projectile that moves with 98% of the speed of light towards me, then if I add the speeds I get silly answers.

Light speed is the same from every frame of reference. So I will see the photons arrive with 1 c regardless if if you are moving yourself or not. Even if you move very very fast.
And the same is true for the railgun projectile. It will of course be slower than the light beam and arrive later. But if you add speeds like you can classically you get .9 + 1 is 1.9 times the speed of light.

To account for this time either slows down or speeds up to fix the difference. Or space gets contracted or stretched. Same mathematical effect.

[prometheus]

So if the universe had a beginning in time and space, and we were far enough from that place, and if we had a powerful enough telescope we could actually see the beginning of the universe. That's a lot of if's but I find it more fascinating than jets and bullets.

It's not an if that the universe had a beginning. The universe expanded out of one point. We can call that a beginning.

We can not see back to the beginning of the universe because telescopes use photons. And in the early universe electrons were not bound because energies were too high. This means the universe was opaque. Photons would collide with with electrons.
But at some point the universe expanded enough so it was cool enough for atoms to form. At that point photons were able to move through the universe freely. The universe became transparent.
This is the cosmic microwave background.

So if we look away as far as we can what we see what is called the surface of last scattering. It is the surface of the opaque universe. If we look further away we see no photons because they could not move freely.

Because this happened 400,000 years after the big bang, that's as far as we can see.

In theory you could see farther if you use gravitational waves. But none have been observed. Or maybe neutrino telescopes. But neutrino's were extremely hard to even detect. Neutrino telescopes have been build. But it's extremely impractical. Neutrino telescopes are being used. But observing objects so very far away, donno about that.

We can calculate which light has had time to reach us and if we do we get a diameter of 93 billion light years of the observable universe. But the surface of light scattering reduces it to 78 billion light years.
The most distant observed galaxy is IOK-1 and the distance is 12.8 billion light years. So that's a diameter of 25.6 billion light years for the area in which we can see galaxies.

It's a coincidence that the distance we can see is almost equal to the age of light years of the universe. This IOK-1 galaxy appears as it was 750 million years after the big bang. Not as it was just after the big bang. It took at least 150 million years for the first galaxies to form. We should be able to see the pre-galaxy universe someday between 400,000 and 150 million years old. But not quite yet. And that gets really close to 45 billion light years away.

[gyzzzmo]

The fact that the universe is expanding and therefore should have a point of origin, doesnt mean it is the 'beginning of all we know'. Maybe THIS universe we know appeared because of the collapse of a giant black hole, and is just part of another bigger universe we dont know of yet. Nothing is sure in science, not even the speed of photons

[prometheus]

That doesn't matter because it expanded from a singularity.

[gyzzzmo]

That doesn't matter because it expanded from a singularity.

Does matter, because singularity doesnt mean that its the beginning.

[prometheus]

I didn't say it can't be true. I said it doesn't matter. There would be no way to tell the difference.

[Bob]

Hey, hey, just jets and bullets here.  No more speed of light.

[gyzzzmo]

Hey, hey, just jets and bullets here.  No more speed of light.

Dont you already have your answer to that question then? :p