This is a pretty cynical place, mostly - although admittedly with good reason most of the time - but I'd like to take a moment to reco'nise some serious fucking ballsy bravery:

Marine jumps on grenade to save squadmates.

Doubtless you've heard this on the news already but I just think it's worth taking a moment to think about what sort of a phenomenal person you have to be to take such a massive risk with your life for the sake of your friends. I'm not 100% sure I'd be able to do it, but I'd still like to think I could if it came to it. Still - well fucking done, sir, and a long and peaceful life to you - you absolute hero.

And - no injuries! Fuck me, but I was surprised. When the BBC Breakfast thing said they were having a spot about this guy who jumped on a grenade, I was expecting a quadraplegic guy in a hospital bed, not a big, fit, fully-limbed bloke.

It is highly impressive that a man's instinctive reaction in those circumstances would be self-sacrifice.

Yes, he thoroughly deserves his highest-medal-possible award. I loved the SMS he sent to his mum; "Being put forward for a citation, might meet the Queen." and that was all he said. Top bloke.

Also: how did his backpack absorb the blast? Did he jump on it face-up? I didn't really understand that bit.

What a guy.

I started reading the article expecting him to be dead. I had to backtrack when I noticed they had a quote from him after the event.

Nobody can jump on a grenade and not die. And especially not come away with only a nosebleed.

Yeah, he's pretty much a badass.

Though couldn't he have just booted it? That's what computer games have taught me.

I would be more likely to grab him and throw him on the grenade in the same situation.
Didn't this happen (with worse consequences) on Soldier Soldier?

To be fair, surely the shrapnel accelerates away from the blast point - so if you're right on top, you'll avoid most of the lethal force of the shrapnel, and instead only be hit by the percussive force of the blast. That's what threw him up in the air, and apparently he landed without doing himself any damage.

CoD4 has taught me that I should pick grenades up and attempt to throw them back, blowing my own arm off in the process.

CoD4 has taught me to press X to respawn. I am unsure how this plays out on the modern battlefield.

I *think* he picked it up or took it off and then held it in front of him.

Lave - I wondered that, too - although I guess that's a good way to lose a leg.

A bloke I know in the TA Paras regiment was in Iraq and one of the guys on base was kicking rocks around. They'd been warned not to do this as some "rocks" are unexploded cluster munitions. There was a predictable result.

Also - I am now put in mind of Woody Harrelson in Thin Red Line losing his ass when he sits on a grenade that he dropped... Heh.

Don't you just press RB when the message flashes up on your HUD? Fair play to him, though.

Eh? The shrapnel will be pushed out equally in all directions from the grenade, including into whatever's lying on it. That's why his bag was shredded.

Oh God, here they go again.

Yes indeed. However force imparts acceleration, not velocity, so if you're right on top of it, the shrapnel won't be going as fast. Or have I completely made that up?

*ascii finger* [@ myp]

It's not about force, it's about kinetic energy.

Haha, it seems like aeroplanes on conveyer belts, it's been done by Mythbusters
http://mythbusters-wiki.discovery.com/p ... uts?t=anon

F=ma, yes. But this does not mean that the closer you are to a grenade the less it hurts. Which is the upshot of your little "thesis".

But surely at point of explosion, acceleration is huge but distance is almost zero, therefore velocity is almost zero, that's the basis on which I'm working. I'm fairly sure I'm talking cobblers, but I'm not sure why.

As HappyCopse has said, it's about kinetic energy, not velocity.



Why? Because you are opening and closing your mouth. Metaphorically, of course.

But kinetic energy is 1/2mv^2, so it is about velocity.

As I said up there ^^ it's about kinetic energy, not force. If you lie on the grenade you get all the KE and not just a bit of it. Expect to be fucked. However I believe (and the Mythbusters link confirms) that given the explosive force in a typical frag grenade, you will save your comrades, although in the process you'll be reduced to something that resembles an explosion in the Campbell's soup factory, extra chunky tomato flavour room.

Craster, you couldn't be talking any more cobblers if you were discussing in intricate detail how to resole a shoe. The fragments inside a grenade are effectively travelling at some significant speed instantly after the grenade goes off, the don't need distance to accelerate though and don't have any less energy because your body is there.

Whoah, whoah, whoah. Surely you can only translate acceleration into velocity via the medium of distance?

EDIT - time, in fact, but that's effectively the same thing in this case.

Look - go back up a few posts and you'll see the logical reason why you're talking arse - you don't even need to go into the science of it. If you're right, then the closer you are to a grenade the safer you are. Which is patently utter, utter bollocks.

We can of course send you over there to try out your theory?

Wouldn't the confinement of the grenade by said Marine cause the blast to exit at the sides faster, in the same way as a bullet comes out of a gun?

Erm I would presume that a greanade explosion only loses energy as it expands. I wouldn't have thought shrapnel speeds up. Surely it's like flicking a matchbox car across a table, it doesn't pick up speed... But there would be a region over which the bits of grenade went from still to fast*, though I suspect that distance was about the size of the grenade, meaning covering it wouldn't reduce the speed of the shrapnel.

* In the same way that a flicked toy car goes from 0 to fast in less than a wheel length.

ANYWAY - CoD4 Bezzie's should honour this man, by whenever they see a grenade icon flash up, they should run over to it, and lie prone on it. DO IT FOR THE BEZZIES!

Of course it is, I know that.

What I'm saying is this. The explosive force imparts acceleration to the shrapnel (f=ma). If we are saying that kinetic energy is the killer, kinetic energy varies on velocity, not on acceleration (.5mv^2). So, how does a flying object gain massive kinetic energy without travelling enough distance to turn that acceleration into velocity?

I'm not arguing, it's just that my GCSE level physics is telling me something that can't be true.

I think so yeah. In principle at least. It would be a bit like putting your thumb on the end of a hose pipe. But where your thumb could turn out to be made of kitchen towel, and so just get shredded instead...

This is true about lots of things. I will write a long post about this later if you want

How's about we just mention the "the smallest particle known to science is [X]" issue with senior school physics and leave it at that.

It's an issue of scale. That acceleration happens over a very, very small distance. A few mm. When you flick a penny across a table at Shunt it accelerates to it's top speed over a distance smaller than it's diameter. Same thing could happen with a grenade.

Even A-level physics is ludicrously, hopelessly simplified. GCSE is just a joke.

I'd rather you wrote a short one highlighting which bit of my logic is wrong

No, because there is signifcant 'give' or dampening force in the human body.

Craster - the shrapnel is given velocity very quickly, but not for very long, as it is stopped by flesh, but not instantly, as it soon becomes fleshy pulp. If you were made of Adamantium you'd be fine, though.

OK, that I can believe. However, if it can get to lethal velocity over such a short distance, I'd expect that the air resistance effect would be very low (sharp pointy bits of metal), so it would in fact continue to accelerate for a much larger distance, which would still mean that you would be safer lying on it that you would if you were 3 feet away.

Damn, have I missed the physics row?

"Fair play" to the grunt fellow in this instance.

That is wrong.

Of course it is.

WHY?

I don't really care about grenade physics.

There's only one reason I'm posting here.















The plane does take off.

No.

I'm not entirely sure what the effects of the explosive charge are w/r/t pressure in air adn expanding concussive waves, but once the force has applied, it's decelerating, just like a bullet is after it leaves a gun.

Being on top of a grenade is theoretically safer purely because the explosion and carnage is localised.

The shrapnel accelerates to maximum velocity in say) 1cm. By then it's being decelerated by the fact that it's in your body... but if you're on top of it then you just get a big hole in one part of you. If you're three feet away you get lots of smaller holes all over you... such as through your head/heart/etc.

F=ma.

No force acting other than air resistance.

Kalmar's here - totally tagging out, and going back to work.

Craster, it can't continue to accelerate unless it's being acted on by an external force. And after the initial explosion there is nothing acting on the shrapnel. In the same way that I only get to flick a coin once across the table.

Huzzah! And now it makes sense - of course that acceleration only applies at the instant of the explosion.

Makes sense, ta.

I assume he'd have had a flack jacket on, though.

You are confusing physics experiment land, where cows are rigid, hollow and spherical, with the real world. You are correct that the shrapnel accelerates from rest (let's not confuse this with frames of reference) to x m/s (where x is big), but confused about how quickly this happens; the reality is that, as Lave has said, it does it in more much more space than the radius of the grenade itself. Plus, even if the shrapnel is still accelerating away as it enters the body, the squishy body doesn't slow it down as much as it is still accelerating, at least for the first inch or so of penetration.

Simply look at energy. A large amount of potential energy (in the explosive) becomes kinetic energy (in the explosion), which is in the form of shrapnel and air movement. All that KE ends up in the body.

No no no. Once the explosion has happened there is no more force accelerating the fragments, only the modest retarding effects of air resistance.

You are trying to apply a basic knowledge of (I'm guessing) Newton's three laws and the SUVAT equations to circumstances (short timescales, non-rigid bodies) where they can't work. That's what you're doing wrong. Lave, start a General relativity thread so we can blow Craster's mind.

No, I really wasn't. I just forgot about my f disappearing

ZOMG.

With explosions of a significant size, you do get a pressure wave which could conceivably cause shrapnel to continue accelerating . Depending on the speed of the pressure wave compared to the initial speed of the shrapnel.

However, a) I have no idea about pressure waves b) it wouldn't make much difference to the lethality of the shrapnel.

I will say this, in this particular case: It doesn't list what sort of shrapnel the grenade produced. Which might have a significant bearing on the lethality of it when the 'jump on it with backpack' method is used.

Surely the other obvious reason why it is less safe to lie on a grenade is that at any distance away you are only going to get hit by a tiny proportion of the amount of shrapnel that you would if you dived on it. Diving away onto the floor with only your feet facing the grenade would seem like the best bet, obviously you aren't going to get any medals or save anyone though.

Yo F=ma F=ma

I don't think you're completely wrong, at least to question it.

The reaction causes a huge pressure increase inside the grenade case, causing it to explode. So yes, most of the energy is imparted to the casing fragments in that instant, but the gases from the explosive are still expanding rapidly. Will they accelerate the fragments further, over a very short distance?

Not that it makes any difference, but academically speaking, as we so enjoy doing.