Breaking the sound Barrier Pics.

[Walshy]

Originally Posted by nuxie1

It means it is travelling faster than its sound wave, so you will only hear it after it passes you.
Wikipedia and Google can no doubt explain it better.

Yeah, have a read. Wikipedia has a reasonable article, including a nice little figure near the top explaining how it's the build-up of sound (i.e. pressure) waves in front of a moving jet/bullet/whatever that causes the shock wave & thus the sonic boom.

The reason there appears to be a delay between flypast & boom is that it still takes time for the shockwave to reach you. If the jet's slow & close, the sound will 'appear' to come from pretty close behind it (but still noticeably behind it, if you pay attention to the direction). If the jet is distant or fast, the offset becomes much more noticeable. If you're talking about Mach 1, the jet will travel about the same speed forward as the shock travels sideways, so if the jet does a flyby at, say 1000m up, it'll be a kilometre past you before you hear it.

Of course, a subsonic jet also announces it's approach (because the sound it generates travels faster than it does) whereas for a supersonic one, it's outrunning its own soundwaves, thus you hear nothing before the boom.

Just to complicate things, shockwaves generally actually travel slightly *faster* than the speed of sound, depending on the strength of the shock (which depends, of course, on the speed & shape of the object making the shock). It starts to get complicated, there, IIRC. Been too many years since I did compressible flow at uni...

There are some good pics available on the net showing shockwaves from different things. Some examples are on this page , this page (a nice pic of a gunshot), and this page, including some nice pics of the exhaust shocks from the space shuttle

There's a video out there showing off the LOSAT anti-tank rocket - the high res version (not the dozens on youtube) actually let you spot the shockwaves coming off the missile during the supersonic portion of its flight. Neat stuff!

[R3Dl2ULZ]

ok on the subject of reaching max velocity 10 or so metres from the end of the gun. thinking back to calculus (i think it was calculus) you have the rate of change of displacement (velocity). you also have the rate of change of velocity (acceleration). BUT there is also the rate of change of acceleration (forgot the name).
so for an example:
a bullet leaves a barrel travelling at 900m/s, acceleration at time of exit= 100m/s^2 and the rate of change of acceleration is -10m/s^3. (EDIT: we are in a low friction environment. i didnt realise when i made up the numbers. but i dont want to have to change them and they demonstrate well enough)
at t=0:
v=900m/s
a=100m/s/s

at t=1:
v=1000m/s
a=90m/s^2

t=2
v=1090m/s
a=80m/s^2

t=5
v=1300m/s
a=50m/s^2

t=10
v=1450m/s
a=0m/s^2
in this example the bullet would reach max velocity 10 seconds after firing.
so my accelration retardation value was way off reality, but it still shows that the bullet continues to accelerate after leaving the barrel, it just accelerates more slowly over time
until in this example
t=12
v=1440m/s
a=-20m/s^s
the bullet starts to slow down at an increasing rate.

so its not accurate to the actual application but it shows the basic math behind Xxain's claims. i understand its more complicated than that but it still proves that the bullet continues to accelerate after leaving the barrel.

[Plastik8]

Originally Posted by R3Dl2ULZ

so its not accurate to the actual application but it shows the basic math behind Xxain's claims. i understand its more complicated than that but it still proves that the bullet continues to accelerate after leaving the barrel.

What force is providing the acceleration after it leaves the muzzle?

No force => No acceleration.

I really have no idea either, just wanna argue, but keeping it simple

[R3Dl2ULZ]

there isnt a force providing the acceleration. but as the force stops and the air resistance starts there is a rapid retardation of the existing acceleration. im not sure if im right or not. but in maths at high school and calculus at TAFE one of the common differentiation and integration examples involved displacement, velocity, acceleration and rate of change of acceleration. i dont know if ive misunderstood the concept or not. but it does explain the results that Xxian claims to have.

[dohzer]

Surely the only way it could accelerate after the end of the barrel is if the expanding gasses behind it continue to propel it forward.
As Plastik8 said, there is no force after that, so the bullet will decelerate due to resistance of the air.

[Rt!]

Wouldn't there be air pressure inside the barrel on the front of the bullet?

I can see how maximum acceleration may be reached right as the bullet leaves the barrel..

[wahoo84]

what ever happened to this post being "post pics of breaking the sound barrier" ?!

[dohzer]

Originally Posted by wahoo84

what ever happened to this post being "post pics of breaking the sound barrier" ?!

It got split in two with the "pics" part being moved to the photography forum.

[Walshy]

Originally Posted by R3Dl2ULZ

there isnt a force providing the acceleration. but as the force stops and the air resistance starts there is a rapid retardation of the existing acceleration. im not sure if im right or not.

I'm afraid not, but it was a good effort.

Newton's second law: F = m * a

If there's no force on the bullet, there's no acceleration. In the case of a gun, the force that provides the acceleration forwards is the gas pressure behind it, generated by the burning propellant.

The moment that force from the propellant is removed, you've got a fast-moving bullet in air, acted on by gravity which pulls it downwards, and air resistance, which slows it's motion. There's nothing which pushes it forwards, thus the net acceleration will be backwards and downwards, and it's velocity will decrease and change direction. There's no slow or steady decrease in acceleration. Remove the force instantly, and you also remove the acceleration instantly.

The bullet is also spinning, if you've got a rifled barrel. The spinning is there to stabilise the bullet - gyroscopic forces keep it pointed in the same direction, so it doesn't veer off-course due to wobbling or tilting or slightly uneven air resistance on the nose. I believe that a rifle bullet fired upwards will still be pointing upwards when it hits the ground some time later, but I'm not 100% sure of that (read it somewhere I now can't remember, but it sounds plausible, given the relatively short time of flight).

As for maximum acceleration as it leaves the barrel - well, I think you're right in that air resistance would be lower - partly due to the air being able to move sideways, partly because there's already a column of air in front of the bullet moving forwards... but you've got to remember the gas pressure behind the bullet has also dropped, as the bullet moves and the volume behind it expands dramatically, and the gas is also cooled by the barrel walls themselves. The time the propellant takes to completely burn would have an effect on this, I imagine. It's a complex question, certainly!

[noboundaries-au]

Originally Posted by Walshy

Remove the force instantly, and you also remove the acceleration instantly.

Correct me if im wrong but it wont disappear instantaneously as the gas wont instantaneously stop pushing, ill illustrate

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Red = Gas


As its not possible for all the gas to escape at once after the bullet leaves the barrel. There will still be a SLIGHT forward almost instaneously dissapating force.

[Iceickle]

Originally Posted by noboundaries-au

snip

The second half of this thread is incredibly frustrating, noboundaries you are talking about a different situation...

As the bullet leaves the barrel, the gas behind it is no longer directed to push the bullet forward- as the bullet leaves the barrel, the acceleration decreases RAPIDLY, so much so that you won't really notice the bullet go any faster. It certainly won't make the bullet go twice as fast, thats just insane.

To sum up, this shouldn't really be an argument, the bullet will be going a tiny tiny tiny bit faster 1cm out of the barrel, compared to its velocity when it leaves the barrel. From here we can consider the acceleration to be negative due to wind resistance, and the bullet will slow down.

[Sureshot]

Thinking about the forces being applied to the bullet in this scenario, you have the force of the initial explosion and gas expansion behind it, and the friction of the rifling groves of the barrel applying an equal force behind it, and the initial friction of the cartridge holding the bullet in place.

The trigger is pulled, and a spark starts to make it's way through the gunpowder, turning it into rapidly expanding gas, popping the bullet out of the cartridge and moving it into the barrel.

At about halfway, the gas is still expanding, as small particles of gunpowder, blown away from the spark by the initial exposion are still burning and producing gas. The bullet is still being accelerated at this point.

Now comes the interesting part. If the bullet is heavy, and the wrong type, or incorrect ammount of gunpowder is used, then the bullet will be travelling faster than the gas produced by the explosion, and start to slow down while still in the barrel due to friction with the barrel, and air friction of the bullet displacing the air in front of it.

In an extreme example, where only a part of the powder has ignited, you could get the bullet actually slowing down to the point of stopping while still inside the barrel, or only just making the barrel's end and then falling to the ground.

This is why the composition of gunpowder is specifically designed to burn at a constant rate, to give the least variation in force as the bullet is fired, and the highest velocity at the end.

So in an ideal situation, where all of the powder has burned at a constant rate in the barrel, the barrel itself is of a uniform design and the friction on the bullet is constant and all other things being equal, then the maximum velocity of the bullet would be achieved a minute distance ahead of the end of the barrel where the propellent gas is still accelerating it more than friction with the surrounding air is slowing it.

Iceickle is right on this one.

What's more amazing is at which exact point does the bullet break the sound barrier? When the bullet initially leaves the barrel, the gas behind it will rapidly overtake the bullet so for a little while the bullet is surrounded by gas moving in the same direction and at greater speed. So the sound barrier would only be broken by the bullet when it escapes this cloud of expanding gas and meets the outside air.

[Sureshot]

Originally Posted by UB3R

/cough/bullshit/cough

I have also never heard of a silencer dramatically increasing the speed of a bullet as most are simply a series of baffles inside a cylinder which if anything would decrease the bullets velocity. If a silencer made the bullet faster (and hence fire further, more accurate, and drop less over distance) don’t you think that every single gun since this discovery would have a silencer, especially hunting rifles where the quieter operation would be an advantage or military weapons. They haven’t been illegal for as long as they have been invented and its very rare to see any hunting, sporting, and even military rifles with silencers.

The silencer does slow the bullet, which is part of it's job. It works to slow the expanding gas cloud left over from the gunpowder, so there is less noise produced.

In some cases, it also works to reduce the bullet's speed to below the sound barrier, to quieten the gunshot produced.

[UB3R]

Originally Posted by Sureshot

The silencer does slow the bullet, which is part of it's job. It works to slow the expanding gas cloud left over from the gunpowder, so there is less noise produced.

In some cases, it also works to reduce the bullet's speed to below the sound barrier, to quieten the gunshot produced.

that was my point, some n00b earlier actually tried to make the point that a sliencer makes the bullet go faster.

[wahoo84]

Here guys, look at this:

http://www.youtube.com/watch?v=iTc9Oky4mzA