How far do you have to be from a nuke for it to just knock you off your feet?

Ignoring radiation…As for a nuclear bomb lets say an “average” size nuclear bomb is dropped at ground zero and goes off. Now if you are standing at ground zero the bomb falling on you would probably kill you before that reaction :wink: . 100 meters away the reaction would kill you, 1000 meters away the same story. Just how far do you have to be from an average size bomb before the force just knocks you over?

What’s an average size bomb? 20 kt, like the Trinity Test? 100 kt? 1Mt?

No. Nuclear weapons, in order to maximize effectiveness, are airburst weapons. Otherwise they’ll waste half of their energy digging a crater and generating massive amounts of fallout.

The McDonald Ranch House sustained minor structural damage and shattered windows two miles away during the Trinity test. If it’s enough pressure to shatter a window it could probably knock you over if you’re not prepared for it.

To me a typical bomb would be a warhead from a strategic missile. They’re usually listed as a few hundred kilotons, at least in the USA’s missiles.

I have no idea about it knocking people down. It might not be like in the movies where people get thrown around by explosions and then they get up. It might be that a blast strong enough to knock someone down would rupture internal organs at the same time. Certainly I would think it would rupture your eardrums. That’s a pet peeve of mine, people in movies never lose their hearing around gunfights and explosions.

There are some DVDs you can rent of old nuclear test films, and in those they talk a lot about overpressure in PSI, usually on the order of a few PSI or maybe tens. Houses will be knocked down at a certain number, and so forth. If you knew how many PSI it would take to knock a person down, you could probably figure out the range.

Great! I was just thinking of renting a DVD for this Saturday night, for me and my wife.

If you are 3 feet away from the Bomb when it goes off, I guarantee you will not remain on your feet.

For conventional explosives, blast weight 250,000 lb or greater, the distance calculation is as follows for a surface explosion:


Let’s say 20kilotons, 40,000,000 lb; cube root is 341.995 time 30 = 10,260 ft.

You are feeling around 1.7psi and are expected to not suffer serious injury from the blast. Note that effects are not solely based on blast overpressure (PSI) but need to figure the impulse duration for a completely accurate figure.

That’s surface again. As noted above, most nuclear blasts are atmospheric and more energy is available for overpressure effects (none expended in cratering). Also a near surface phenomenon known as the Mach Front will enhance blast effects from an above ground explosion. Basically the blast wave partially reflects off the surface and reinforces the wave traveling radially from the point of explosion.

Fragments, radiation, and thermal effects from a nuclear blast are a whole different story. The thermal and radiation effects arrive at the speed of light. Not an instant kill on the radiation but your clothes, hair, and/or skin could be alight real quick. I guess STOP,DROP, AND ROLL counts as getting knocked off your feet.

Some google fu on Hiroshima blast effects could give you a better understanding.

One thing that’s always been bugging me, does the shockwave travel at the speed of sound, and does the speed change during propagation? Sometimes when I’m on an outcrop overlooking a distant city, I wonder if I’d be able to watch the approaching shockwave and have enough time to take cover before it arrives. Normally I use the speed of sound in my mental calculations, but I wonder if that isn’t a valid assumption.

I was watching The Day After recently and I’m afraid I had to giggle. Jason Robards is trapped in his car on a road when lo and behold, just over the hills, there’s a huge mushroom cloud.

I was always under the impression that if you could see it, you were dead. Maybe not right away, but if you were close enough to see it, you were going to get a lethal dose of radiation.

Am I wrong?

On flat ground IIRC you can see roughly six miles before curvature of the earth blocks your view. Taller things could obviously bee seen further away. something several hundred feet tall like a mushroom cloud can probably seen for at least 20-30 miles on a nice clear day.

More like several tens of thousands of feet.

What does the wave actually consist of anyway? It’s not like a lightwave in a vacuum is it? More like a soundwave in the air, right? So is it a fast moving wave of superheated air?

Yes. Prompt radiation is unlikely to be a problem unless you are so close that the other effects (blast, thermal) will kill you first. The immediate effects of nuclear weapons scale differently with yield.

For a typical modern weapon, say 300 kt, the result is:

thermal 7 km
blast 5 km
radiation 2.5 km

Yes, you are wrong.
Assuming that you are shielded from the radiation (photons and neutrons) generated in the initial explosion, the residual radiation from fallout is going to take quite a while to affect you. During A-bomb testing, the Army had soldiers shield themselves from the initial blast, and then walk towards “ground zero.” Note that modern H-bombs are so big that fatal flash burns might result as far as 30 miles from the explosion.

Absolutely. Check out this video of the explosion of a rocket fuel factory shot from a nearby mountaintop. You can see the shockwave propagating across the desert floor. The guys filming it even have time to say “Wow, that’s going to be loud!”

Of course, you’d still get a dose of radiation from the blast and you might be blinded if you’re looking straight at the bomb when it detonates, but you would definitely have time to take cover.

It’s a pressure wave, just like sound, only with a much larger pressure differential. The air itself, however, doesn’t move anywhere; the molecules just sort of swing back and forth as the wave passes by. Here is an animation of a longitudinal pressure wave (sound, in this case, but it applies equally to a shockwave). If you watch closely, you can see the particles move back and forth as the waves pass; their net movement is zero.

Moving over to stand on the large X labeled corrected…

Little known fact: eating a peanut butter sandwich can help you survive an atomic blast.

I remember reading in “Surely You’re Joking, Mr. Feynman” that Richard Feynman, while witnessing the first A-bomb blast, daringly wore only a pair of UV protected, coated sunglasses. I hope I’m remembering that correctly, and I don’t think he was joking.

Actually, he used the windshield of a truck as the only protection…

Of course, it would have been doubly hilarious if afterward someone said “Wow, did you hear that?”