The point about the bounce is that the energy is got rid of with the bullet - no dissipation across or permanent damage to the armour, no blunt impact to the wearer, at least one of which you'd need if the bullet just stopped.

you wouldn't have reported it at all if it hadn't been dressed up with the "bullet-proof" bollocks; and the IOP knows this, because the IOP knows well enough how the media functions.

And the way the media functions pretty much guarantees that no science will ever survive media processing.

About the only science that does survive is pictures; but then even that's not so much science as data.

The IOP (etal), of course, are virtually on a hiding to nothing -- their two options are basically:

(a) no science gets reported

(b) spin some science into entertaining-sciency-bollocks, and hope.

So, they got you to notice and report; but all you did was shoot it down in flames. Is that a draw, or did you win?

I was reading the whole article thinking "for every action there is an equal and opposite reaction".

Maybe they are planning on taking potshots at the assailant with the bouncing :)

"The point about the bounce is that the energy is got rid of with the bullet - no dissipation across or permanent damage to the armour, no blunt impact to the wearer, at least one of which you'd need if the bullet just stopped."

Kinetic energy is (from rusty memory) (Mv^2)/2 and, if you remember, the difference between velocity and speed is that velocity takes direction into account. Now the word "bounce" suggests (to me, at least) a change in direction and, hence, a change in velocity. Which means a change in kinetic energy, which has to come from or go somewhere. In other words, the energy is not "got rid of" with the bullet; in fact, the energy required to "bounce" a bullet back at the same speed would be twice that required to bring it to a stop. (the change in velocity from x to -x is twice the change from x to 0.)

of course, it's possible that there was some special stuff that was glossed over for the A-level syllabus, and there have been some interesting developments in the umpty years since I sat the exam, but I'm pretty sure I'd have noticed if the fundamentals had been re-written to the extent that everything I've just coughed out of long-term storage was utter tosh...

Could someone just shoot a gecko and see if the bullets bounce off?

Then we could resolve this debate once and for all.

So that the enemy won't be able to see the soldiers inside their invisible tanks.

Er, does this make sense?

Wasn't this in the news only last week only with a Cambridge lab having got the technology to weave the stuff out of some 'carbon smoke'?

http://news.bbc.co.uk/1/hi/sci/tech/7038686.stm

So, if you made a bullet out of this bouncy nano-whatsit, when would it stop bouncing?

Geckos: is there anything they can't do?

Boffins are quoted as inventing all sorts of weird stuff with unbelievable claims. It's clear that 1. we haven't got the whole story here and 2. it's going to be a while before they've made anything and tested it so we can see some results instead of speech and 3. when they have made something, it'll probably include some extra details that aren't in the article we just read.

There are a number of materials that are flexible under normal circumstances but can become rigid on their own under certain stresses or can be made rigid electrically. That wasn't mentioned here, but if that concept can be combined with an extremely springy material, they could solve some of the things we're worried about here, by making the entire garment very temporarily conduct the distribution of force against the wearer.

But I suggest at this phase of any story the read just say "standard speech-blather to make the boffins feel like they're accomplishing something more quickly than one invention in ten years" and wait until they actually film a test.

But thanks for the gecko angle. ;)

never seen anyone get hit with a bullet either, however, if every action has an equal and opposite reaction, then the person firing the bullet would surely be knocked over as well?

A thick covering of red mercury.

from my understanding they are trying to turn the kinetic energy of the bullet into potential energy-- without imparting much kinetic energy to the wearer. So basically (heavily simplified) they are turning the bullets energy into potential energy via the tensile strength and elasticity of the nanotubules which function similar to a spring. Of course some kinetic energy will still transfer to the wearer, yet this will be a lot less than if it was just a bullet being blocked by a kevlar vest (transferring all of the kinetic energy to the vest). Which is still better than the relatively small change in kinetic energy for the bullet as it pierces an unarmored target (the bullet transfers very little kinetic energy to the target and 'pierces their body, causing ripping because of the fluid dynamics).

So, there's the combatant disruption associated with momentum transfer involved in absolutely revectoring a high speed mass, and then there is the question of where it will get revectored!

Upon impact, the bullet will bounce in a different direction just as quickly as it came in.

Imagine the possibilities!

In a MOUT situation, one could then take a shot at any portion of the vest visible to the shooter and then 'bank on' (poor pun) the slug(s) going off into some other portion of the room.

Pump a few rounds into that vest and shit will be flying in all directions in the room until it finally comes to rest inside something soft and squishy.

Awesome! What an amazing improvement in combatant survivability.

Provided that you convince the enemy to wear them...

If the bullets were made of nanotubes too would they bounce or stick?

And the relevant part is

F.t = impulse = d(mv) / dt

So if you've got twice the d(mv) (which is probably going to be the same as m.dv, unless bits fly off the bullet when it hits) then you're either going to get twice the force exerted against the target from a bullet reversing direction as compared to one that just stops and falls to the ground - or if you want the wearer to absorb the same amount of force, you've got to make the slowing-down-and-reversing of a bullet hitting the geckojacket take twice as long as the slowing-down-and-stopping when the same bullet hits ordinary kevlar.

This is the same principle how crumple zones work to protect you in car crashes: they reduce the force by making the impact last longer. So, it's possible this new jacket could reduce the force - for instance, if the elasticity of the nanotubes is so much greater than kevlar that it takes the bullet four times as long to slow down and reverse, it'll actually only exert half the force on the wearer.

Of course, it will travel further in that longer time too, and given that this stuff is supposedly so thin, that suggests that it would likely penetrate some distance into your body, stretching the jacket material ahead of it, before the elasticity of the jacket gets enough to ping it back out of you in the direction it came from. And I haven't actually done the math, so maybe the distances we're talking about here are only a matter of millimetres to start with and the further travel will still stop it short before it hits your body, but it should all be work-out-able in theory.

To answer your questions, if the bullet was made of carbon nanotubes then due to quantum entanglement and the requirement for an equal and opposite reaction, the moment the person hit with the bullet falls over then the person who shot the bullet will fall over as well.

Yours Sincerely

Chief scientific advisor to the House of Lords

The carbon nanotube has the highest tensile strength of any material yet fabricated by mankind. Unlike Kevlar, the strategy discussed in this article (did anyone read the source article) is to elastically store the kinetic energy of the incoming round, and then to return the energy to the bullet. A kevlar vest absorbs energy in the breaking fibers and requires multiple layers to prevent penetration. The layers that do not break transmit the remaining energy of the round to the body, painfully, but over a large enough surface area that ideally penetration does not occur.

A couple of interesting things. The energy not returned to the bullet is dissipated along the length of the nanotube fiber, not at right angles to it. This has to be a good thing. Also, has anyone considered the inherent karmic justice of a bulletproof vest that return the projectile to the shooter at significant ballistic velocities?

As far as conservation of momentum goes, guys, little common sense here, if a guy can fire the gun and not be thrown backwards, the smaller rebound velocity will probably be manageable for the victims body.

Best comment in weeks. Well done, that man!

...film or still photos of what happens when a real bullet is fired at a real layer of this nanotube fabric that has been placed against a block of clay or plasticine, I sha'n't be turning handsprings or dancing in the streets.

I've seen photos of the above done with kevlar armour, both with and without a Sorbothane (tm) blunt trauma pad, and the results are hard to refute - you look at the cross-section of the plasticine and work out if you want your ribs deforming that much. (Sorbothane does wonders for decreasing the deformation, btw.)

If the nanotube stuff perfoms as expected and only leaves minimal or negligible deformation of the plasticine when struck by a projectile likely to be encountered on the streets, well and good, time to celebrate.

****

According to my very rusty physics, since E=0.5.m.v^2, then for a 80kg squaddie stood on a frictionless surface (any ice rinks in Iraq?), a perfectly bounced 320J bullet will near-instantly impart a speed of 2.82 m/s, or about 6.3mph.

Which is (I think) to say it will be like being run into by someone your own mass at 12.6mph, i.e. a fast run.

Still, being knocked flat might make you a smaller target.

*****

My physics are even rustier, but consider the logic of what you're saying -- any energy imparted by the bullet to the shootee when it bounces off, is identical (or a little less, due to losses) to the energy imparted on the shooter when it leaves the gun. In other words, if the ricochet would knock the victim on his ass, the recoil would also knock the perp on his ass. Since it doesn't, it won't.

Ron

Sorry, mate, but there is an error in your comment, based on an urban myth (which I only learned about recently myself.)

Teflon does not help a bullet in the slightest when it impacts a BPV. The "teflon lets it slide between the fibres" is a mis-apprehension based on the common use of Teflon-coated AP rounds.

The truth is, the Teflon is used to improve things at the gun end, not the target end. Using Teflon reduces friction when firing the gun, leaving slightly more energy for the bullet (instead of wasting it on frictional heat.)

By the time the bullet hits anything, the Teflon has long since done its job and contributes nothing.

Buggered if I can remember where I saw this, though. Try Wikipedia. (I wouldn't trust my life to info gleaned from WP, but since neither of us are (hopefully!) ever going to be struck by an AP, tt doesn't really matter. :-)

As for DU, as I understand it, is useful for two reasons: It's very dense, i.e. weighs more for the same size bullet, so it carries more more energy. Secondly, when it strikes armour, it doesn't "mushroom" like lead, or even steel. It stays sharp ("self-sharpening"), thereby penetrating better.

Modern tank AP rounds don't even bother with explosive - they consist purely of a very thin, arrow-like tungsten or DU penetrator. During Gulf War 1 such a penetrator went straight through the thick sand berm protecting an Iraqi tank, sliced straight through the 50's vintage tank, and ended up deep in the berm on the opposite side. (This paragraph has nothing to do with the topic, it's just a cool story I had to share. :-)

The difference between the effect of shooting on the shooter and being shot by the shootee, in terms of physical knockdown is explained by the timeframe.

Impulse = force x time

A Kevlar jacket (to stick with actual working tech) has to stop the bullet from "zipping along" to zero in the space of a few mm. Maybe 20mm.

The force from the bullet being accelerated from zero to "zipping along" is applied over a much longer time frame. first the barrel of even a short pistol is twice the thickness of a vest, and then the shooter's arms flex, spreading the transfer of the force to the shooter's centre of gravity over even longer (or, in a longarm the barrel is 15 x the thickness of a vest.

Since the Impulse at each end is the same and

Impulse/time=force, the longer timeframe for the application of a force to the CoG of the shooter means that they will experience a fraction of the physical knockdown of the target if the target is wearing a vest.

If the bullet goes right through without stopping, the impact will be lessened, depending on how much of the projectile's energy gets dumped into the target.

It's a pretty simple idea:

if a 5g bullet moving at 330 m/s undergoes an elastic collision (i.e. no loss of kinetic energy) with an 80kg person initially at rest, then after the collision the person will be travelling at about 0.04 m/s (i.e. 4 cm/s) and the bullet will riccochet off at about 329.96 m/s. The target absorbs a mere 0.064 Joules of energy, the same amount as dropping a marble onto your chest from a height of a metre. As Anonymous coward above said (although he/she got the physics wrong too), simple physics - conservation of momentum and conservation of kinetic energy.

As is clear from the article, this won't work unless you fix the nanotube fibres at the ends, so your armour will have to be very stiff - think a mesh of nanotubes like flyscreen, attached at the points that they cross over, so that any nanotubes being hit by the bullet will pull sideways on the perpendicular nanotubes. The armour would be a shell, not a flexible fabric.

But these are trifling details of implementation, once the theory proves it can be done....

It's cause the bullet bounced off my jacket and passed thru his head ...