Physics question - Explosions in space

Post by **[xeno]Julios** » Wed Nov 16, 2005 6:15 am

K just something I was pondering.

From what I understand, explosions generally do their damage through the following ways:

-high velocity shrapnel
-heat
-shockwaves

Now I'm interested in the shockwave aspect - am i correct in understanding a shockwave as essentially a very powerful sound wave?

Now if an explosion occured in a vacuum, and you were protected against heat, and the shrapnel didn't hit you, would you not feel a thing? (by shrapnel i mean any physical part of the device, including particles of the explosive itself)

Post by **Wizard .3** » Wed Nov 16, 2005 6:19 am

<s>What about the material ejected from the explosion itself? In saying that, I refer to the actual explosive material itself, after said explosion. </s>

Also, can you say a shockwave is a powerful soundwave? I'm not so sure... I think it is more just a wave of force travelling through a given medium. It's more just a powerful wave...?

edit: oops, didnt read your whole statement.

Post by **[xeno]Julios** » Wed Nov 16, 2005 6:22 am

Wizard .3 wrote: Also, can you say a shockwave is a powerful soundwave? I'm not so sure... I think it is more just a wave of force travelling through a given medium. It's more just a powerful wave...?

a wave of force - but if it's not electromagnetic, and not a pressure wave, then what is it?

(sound waves are essentially pressure waves which interact with biological systems to produce the perception of "sound", and you can't have pressure waves in a vacuum, since there are no particles to experience compression and rarefaction)

Post by **Wizard .3** » Wed Nov 16, 2005 6:26 am

Yes, I realize what waves are. I was trying to make a point that soundwaves are essentially just waves of force (or pressure if you will), and a shockwave is a powerful one... but it came out differently.

As for the first question, would you feel anything? Well, if there is nothing touching you, no heat, and no pressure or force applied to your body, I don't see how you can 'feel' anything.
And when you say heat, I assume you mean infrared and therefore all EMR.

Post by **Wizard .3** » Wed Nov 16, 2005 6:29 am

Did a little searching, and found this:
http://www.newton.dep.anl.gov/askasci/p ... y00915.htm

Post by **[xeno]Julios** » Wed Nov 16, 2005 6:33 am

tx nice link - so i guess the answer is no.

Post by **mrd** » Wed Nov 16, 2005 6:37 am

From that website:
"(If you divide the area of the
astronaut's silhouette by the area of a sphere whose radius is the
distance from the explosion to the astronaut, you will have the
fraction of the explosion's energy absorbed by the astronaut.)"

That's kinda neat. So it sounds like if you're protected from heat and debris, unless you're really close to the source, nothing "dangerous" happens.

Post by **Pooinyourmouth** » Wed Nov 16, 2005 6:42 am

For the most part I would say no, but really it depends on how far you are from the blast. An explosion is normaly a quick reaction that releases a lot of gas in a small space of time. If you where in a vacuum I imagine that no shockwave would be made at all, but the gas leaving the explosions would have nothing to stop it as well. So I think you would feel it, but not as rough as it would be in an atmosphere.

Post by **r0n1n** » Wed Nov 16, 2005 8:21 am

mrd wrote: That's kinda neat. So it sounds like if you're protected from heat and debris, unless you're really close to the source, nothing "dangerous" happens.

Correct, since there's no air or other molecules for the explosion to transfer energy to by compressing those molecules together, all or most of the energy from the explosion is transferred to the debris.

As for heat, it can't travel in a vaccum, except by radiation (electromagnetic waves), so you wouldn't have to worry about the heat either. Since you wouldn't be in contact with any of the debris, no heat would be transferred to you by means of conduction. As for convection, since there's no air, it wouldn't happen either.

Post by **Nightshade** » Wed Nov 16, 2005 1:02 pm

It took me a minute to realize that you're right in saying that heat can't travel in a vacuum. I hate it when I do that.

Post by **menkent** » Wed Nov 16, 2005 8:27 pm

and no oxygen so no firey death.

and yes, a shockwave is just one strong sound wave (or at least a tightly bunched packet of compression waves). a good explosion will cause a strong enough compression that the trough (vacuum) behind the shockwave can suck the paint off of paintings.

anyway waves are not force but rather energy. a generally trivial lexical discrepency in this context, but one might as well have the language match the physics.

Post by **Nightshade** » Wed Nov 16, 2005 9:48 pm

And as such, it's more proper to call a sound wave a pressure wave. :icon25:

Post by **menkent** » Thu Nov 17, 2005 4:51 am

it's longitudinal!

Post by **Nightshade** » Thu Nov 17, 2005 5:32 am

Transverse. :icon26:

Post by **Canidae** » Thu Nov 17, 2005 5:41 am

I've emailed George Lucas on this one.
I should have his answer by tomorrow

Post by **Canis** » Thu Nov 17, 2005 6:41 am

There would be no shockwave, but just an expansion of gasses determined by the velocity generated by the explosion itself. The gasses would expand forever, and as they got further and further away they'd dilute so they'd have less and less impact (given by density of particles that all have a given velocity and direction) on an object of a given size depending on the distance between that object and the explosion. The materials of the explosion wouldnt have an atmosphere to slow them down and contain them in an area around the explosion. Rather, they'd just continue outwards from the center of the explosion. The shockwave is just the compression of atmospheric gasses, that then traveles as a wave. However, since there are no gasses in space (or they're so dilute it essentially creates a vacuum) there will be no shockwave.