You are either not logged in or not registered with our community. Click here to register.
 
December 08, 2016, 04:16:02 PM

Welcome, Guest. Please login or register.
Did you miss your activation email?

Login with username, password and session length

Click here if you are having problems.
Default Wide Screen Beige Lilac Rainbow Black & Blue October Send us your theme!

Hark!  The Herald!
Holiday Issue 2016

Wiki Blogs Dicebot

Author Topic: Physics/Theory of Relativity questions  (Read 4026 times)

0 Members and 1 Guest are viewing this topic.

Offline AndyZTopic starter

Physics/Theory of Relativity questions
« on: May 16, 2012, 01:52:19 AM »
Looking for someone who's, if not well knowledgeable on the theory of relativity and experiments therein, at least knows a decent amount about physics and can answer a few questions which have been bugging me.  Really, I'll take what I can get if people can give evidence, reasoning, etc.

I'll pop down one for now, though I'm sure I'll have more:

If a grenade's shock wave normally travels at X miles an hour, and I throw it out of a plane travelling at 600mph, will the shock wave travel at (600+X-Air Resistance) miles an hour, or just X miles an hour?

Online Oniya

  • StoreHouse of Useless Trivia
  • Oracle
  • Carnite
  • *
  • Join Date: Sep 2008
  • Location: Just bouncing through. Hi! City of Roses, Pennsylvania
  • Gender: Female
  • One bad Motokifuka. Also cute and FLUFFY!
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 3
Re: Physics/Theory of Relativity questions
« Reply #1 on: May 16, 2012, 02:23:11 AM »
I'm not much up on the physics of explosions (which involve a chemical reaction instead of merely a moving body), but I may be able to help with some of the other questions.  Did a lot of physics reading not too long ago (including some on relativity), and I still have my old physics textbook for the Newtonian stuff.

Offline gaggedLouise

  • Quim Queen | Collaborative juicy writer
  • Champion
  • Enchanter
  • *
  • Join Date: Jan 2011
  • Location: Scandinavia
  • Gender: Female
  • Bound, gagged and unarmed but still dangerous.
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 0
Re: Physics/Theory of Relativity questions
« Reply #2 on: May 16, 2012, 10:11:25 AM »
I'm no physicist but (like Oniya) I've read some about this over the years; also for what little worth it has, I know some people who have been doing research in astrophysics and have discussed this stuff with them in a relaxed way.

First off, 600 mph is nowhere near the kind of speed where you would begin to have relativistic changes of speed from different angles; 600 miles per second wouldn't be of any consequence either. And as long as the grenade is in the atmosphere, or building up to escape velocity (about 7miles/second from earth - let's suppose the planet in the example has this EV) it doesn't pick up the planet's orbit speed as any part of its own speed from the point of view of someone observing it from the planet. If it were observed from another planet much further outside, yes, then it would be seen to travel with that speed plus its own speed when it was orbiting "forwards" and then v=planetary orbit speed minus object's own moment speed, when it sort of went "around the bend" into the other half of its orbit.)

If it broke free completely from the gravity of the planet it had been sent from, and went into a path taking it away from it - like a probe for Mars or the outer planets - it would keep the speed and orbital direction it had at the point when it completely broke off the "home planet gravity field" which is *not* the same as the point where it enters into orbit around that planet: any satellite has to reach escape velocity to enter into orbit, but if it's lost in space later, near earth, or between the earth and the moon, it will still slip into orbit around the earth for many years. So it's only when they have gone some way beyond the distance to the moon (in earth's case) that these probes or whatever it is escape any substantial bending power from the earth on their paths through space.

Once it has got that far, and starts travelling freely through interplanetary space, if it has some engines of its own turned on during the entire trip to let's say the next planet, it won't pick up the home planet's orbit speed in any simple way, no matter from where it is being observed. A shock wave erupting from a point hat far out would be, I think, the "shockwave speed" added to the speed of the grenade just before it exploded, and theoretically it would travel back to its home planet, or on to another planet with that speed, if those planets were moving at a slower speed or moving towards the spreading circle of the shock wave, meeting it (actually I think any sizable planet in the inner parts of a planetary system, even Saturn in our own neighbourhood, would be moving much faster through its orbit than most probes or grenades could hope to travel - so they would often outrun the shockwave unless it happened "in front of" said planet). .

The trouble is, most probes we send out don't run on their own engines all the way out. The engines are shut off as soon as the ship or probe has been circling around earth a bit and then pushed itself off into an orbit aimed at the goal it is set to reach in a year or two. Loading Pioneer, Mariner or Cassini with fuel for travelling on their own power all the way through the solar system would be prohibitively expensive, even if it was nuclear fuel. With chemical rocket fuel it would be out of the question (the jury is still out on whether it could be practical to power the first manned trips to Mars with any kind of standard rocket fuel for the full length of the trip, though it owuld make the running time considerably briefer because you could just go the shortest way, pretty much a straight line). Once they have got up to a good level they just coast on the orbits they will get into through the gravity of the sun and the planets nearby, and the speed in that orbit is not a simple product of neither the home planet's orbital speed, nor the vessel's own maximum speed as it took off from the planet.
« Last Edit: May 16, 2012, 12:52:33 PM by gaggedLouise »

Offline ExisD

Re: Physics/Theory of Relativity questions
« Reply #3 on: May 16, 2012, 10:43:34 AM »
I'm someone who's studied engineering so I have a pretty hefty physics background, but it is mostly in physical systems. Though I don't specialize in explosives we did cover them briefly in a course. Within my experience we cared more about the total energy output from the explosion than the speed of the shockwave. Because of this I'll base my analysis off of one of the fragments sent flying and use its velocity as a basis for the velocity of the explosion. Note: I have no idea of what the exact correlation between the two is.



For the grenade question, the big questions here are which frame of reference are you looking from along with the base assumptions for the problem.

The base assumptions I'll use are as such: the explosion is perfectly spherical in nature as are the fragments, the planes is moving at exactly 600mph with no acceleration, no rotational velocity, and no rotational acceleration. All of these are assuming a fragment flies outward parallel to the x axis. If you were looking at one of other axis then X would have to be divided into multiple numbers based on the actual direction.

In all examples X will be speed of the fragment, Y will be the velocity loss from air friction, and Z will be the effects of gravity. In each of these examples the values of Y and Z are going to be based on time. As time increases the value these these will increase with Y having a maximum value of X(in the first two frames and X+600 in the last one) and Z having a maximum value of terminal velocity.

If you are considering the velocity of the explosion from a frame attached to the grenade with the x axis being in the same direction as the velocity of the plane and the z axis being perpendicular to the ground. Then the velocity of the fragment will be X-Y in the x axis and 0 in the z axis.

If you are considering the velocity of the explosion from a frame attached to the plane with the x axis being in the same direction as the velocity of the plane and the z axis being perpendicular to the ground. Then the velocity of the fragment will be X-Y in the x axis and Z in the z axis.

If you are considering the velocity of the explosion from a frame attached to the Earth with the x axis being in the same direction as the velocity of the plane and the z axis being perpendicular to the ground. Then the velocity of the fragment will be 600mph+X-Y in the x axis and Z in the z axis.

This is because when looking at velocity you ignore the velocity and acceleration of the object your frame of reference rests on. As an example when driving it doesn't look like you are moving, rather that the entire world besides you is moving. Yet when you are standing on the side of the road you appear stationary to yourself and all of the cars are moving.

Does that answer your question in a manner that makes sense?

Reason for edit: Posting while brain dead made me forget to name the frame in one of the maximums I listed.

As for why the reference frames are important to the theory of relativity. The frame you are looking at an objects velocity relative to its own is an important part of the problem to be solved.
« Last Edit: May 16, 2012, 04:31:42 PM by ExisD »

Offline Vekseid

Re: Physics/Theory of Relativity questions
« Reply #4 on: May 16, 2012, 01:04:47 PM »
If a grenade's shock wave normally travels at X miles an hour, and I throw it out of a plane travelling at 600mph, will the shock wave travel at (600+X-Air Resistance) miles an hour, or just X miles an hour?

This isn't a relativity question.

When an explosion occurs, a small amount of solid matter turns into a large amount of gaseous matter at very high speed, displacing the surrounding air at the same speed. Here you determine 'X' to be the volume taken up in an amount of time given by the explosive's own velocity, which is dependent on the explosive, and of course, it's a volume, rather than something 'simple' like calculating the muzzle velocity of a rifle round. But this wave uses the grenade's frame of reference, and is 600+X.

Equations for this sort of thing with nuclear weapons are publicly available, but above-ground nuclear explosions have a lot to do with lighting the surrounding air on fire, making something a bit more slower than the mach ~4 missile (or whatever) the main reactant in the explosion.

Offline AndyZTopic starter

Re: Physics/Theory of Relativity questions
« Reply #5 on: May 18, 2012, 09:54:31 PM »
Thanks for responding, folks ^_^

First off, I wanted to clarify that I realize there's a difference between general and special theories of relativity.  This part was more general.

If you're on a bike going 120mph, and you throw a baseball 90mph, then from the perspective of someone watching along the side of the road, the baseball will appear to travel 210mph.  Mostly I was wondering if that works the same for non-kinetic forces.  Then again, maybe an explosion is kinetic?

Sadly, I'm not educated enough to really understand all the formulas.  When I read through Einstein's book, I kinda glazed over the letters and numbers; the principles made sense, the calculations not so much.

Offline gaggedLouise

  • Quim Queen | Collaborative juicy writer
  • Champion
  • Enchanter
  • *
  • Join Date: Jan 2011
  • Location: Scandinavia
  • Gender: Female
  • Bound, gagged and unarmed but still dangerous.
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 0
Re: Physics/Theory of Relativity questions
« Reply #6 on: May 18, 2012, 10:50:01 PM »
Are you thinking of explosions that would happen in outer space (between planets) or in the planet's atmosphere? The pattern and speed of spreading would be quite different, both because there is no continuous medium in outer space (such as air, water or earth) to forward the explosion, and because the changes in speed of the object before it explodes would be very affected by its being up close to a planet. Comets and meteorites that approach earth increase their speed and temperature as they get closer to us, because they are pulled in by the gravity force of earth. They gain more momentum - and then it might end up in their being repelled and tossed out into space again, so they don't crash down on us.
« Last Edit: May 18, 2012, 10:55:26 PM by gaggedLouise »

Offline Darius

Re: Physics/Theory of Relativity questions
« Reply #7 on: May 18, 2012, 10:56:30 PM »
Ok, assuming something the size of a round concussion grenade to alleviate all kinds of drag problems.

If you throw the grenade out perpendicular to the plane the air pressure against the mass of the grenade will slow it sufficiently in the 4-8 seconds (depending on fusing) prior to the explosion to make the velocity of the grenade insignificant compared to the speed of the shock wave it generates.

If you throw it out the back of your plane at that speed you likely get the same results.

If you throw it forward from the plane you asking to be hit in the face with a hand grenade, or to have it sucked through the air intake of your jet engine with sufficiently poor results that you will likely not care about the speed of shock waves.

Offline Samnell

Re: Physics/Theory of Relativity questions
« Reply #8 on: May 18, 2012, 11:08:45 PM »
If you're on a bike going 120mph, and you throw a baseball 90mph, then from the perspective of someone watching along the side of the road, the baseball will appear to travel 210mph.  Mostly I was wondering if that works the same for non-kinetic forces.  Then again, maybe an explosion is kinetic?

Explosions involve kinetic energy. If there's motion, it's kinetic.

Offline AndyZTopic starter

Re: Physics/Theory of Relativity questions
« Reply #9 on: May 18, 2012, 11:36:46 PM »
Are you thinking of explosions that would happen in outer space (between planets) or in the planet's atmosphere? The pattern and speed of spreading would be quite different, both because there is no continuous medium in outer space (such as air, water or earth) to forward the explosion, and because the changes in speed of the object before it explodes would be very affected by its being up close to a planet. Comets and meteorites that approach earth increase their speed and temperature as they get closer to us, because they are pulled in by the gravity force of earth. They gain more momentum - and then it might end up in their being repelled and tossed out into space again, so they don't crash down on us.

I understand about comets speeding up as gravity suddenly kicks in, especially before they have any wind resistance which would cause terminal velocity, but I don't really understand how explosions in space are different from explosions on Earth.  If I must pick one, though, I'd go with explosions on Earth, though I'd like to know how each are different.

Ok, assuming something the size of a round concussion grenade to alleviate all kinds of drag problems.

If you throw the grenade out perpendicular to the plane the air pressure against the mass of the grenade will slow it sufficiently in the 4-8 seconds (depending on fusing) prior to the explosion to make the velocity of the grenade insignificant compared to the speed of the shock wave it generates.

If you throw it out the back of your plane at that speed you likely get the same results.

If you throw it forward from the plane you asking to be hit in the face with a hand grenade, or to have it sucked through the air intake of your jet engine with sufficiently poor results that you will likely not care about the speed of shock waves.

See, I'm cursed with the curiosity inherent on problems which may not be practical.  For example, I want to know what happens after I die, even though I'm not willing to kill myself purely to find out.  Thanks, though.

Explosions involve kinetic energy. If there's motion, it's kinetic.

Does that apply to anything that moves, including light, particles and things like that?

Offline Darius

Re: Physics/Theory of Relativity questions
« Reply #10 on: May 18, 2012, 11:53:16 PM »
Does that apply to anything that moves, including light, particles and things like that?

Absolutely

Offline Samnell

Re: Physics/Theory of Relativity questions
« Reply #11 on: May 19, 2012, 02:38:22 PM »
See, I'm cursed with the curiosity inherent on problems which may not be practical.  For example, I want to know what happens after I die, even though I'm not willing to kill myself purely to find out.  Thanks, though.

That's a biology question. :) Granted biology exists inside physics, just like chemistry, but it's running at different levels of complexity.

Does that apply to anything that moves, including light, particles and things like that?

Like Darius said, yes. If it helps, kinetic energy is a type of energy that includes lots of other things that we call energy: heat, radiation, electricity, etc. All those guys are in motion. With regard to kinetic energy, its opposite number is potential energy. When objects are in motion we're seeing their potential energy turn into kinetic energy and kinetic into potential, minus some energy that will leave the system from things like friction.

Offline AndyZTopic starter

Re: Physics/Theory of Relativity questions
« Reply #12 on: May 19, 2012, 11:20:49 PM »
I guess the issue I've had is stuff like that I've heard that the speed of sound is constant through air.  Is this incorrect as well?  Does a moving person's voice move faster than a still person's voice?

Offline Vekseid

Re: Physics/Theory of Relativity questions
« Reply #13 on: May 20, 2012, 12:19:35 AM »
Thanks for responding, folks ^_^

First off, I wanted to clarify that I realize there's a difference between general and special theories of relativity.  This part was more general.

None of what you are asking involves general relativity in any sense. The only part of relativity you need to know for any of this is the concept of frames of reference. You measure things in miles per hour and there are less than seven digits involved, relativity doesn't really cause any issues except for things like GPS.

I guess the issue I've had is stuff like that I've heard that the speed of sound is constant through air.  Is this incorrect as well?  Does a moving person's voice move faster than a still person's voice?

Air can move. So does spacetime, of course, but these are exceptional situations (spacetime expands at some 70 km/sec/megaparsec, spacetime gets 'swallowed' by a black hole).

So your question about the grenade's shock wave - eventually, yes, it's leading edge won't be any faster than the speed of sound. But how fast this is in which direction depends on its altitude (the current pressure), which way the wind is blowing and how fast, etc.

Thinking of the speed of sound as being a microcosm of the speed of light in spacetime is useful for describing certain concepts - the Doppler effect, for example. But there's no observed and verifiable physical transformation of the same sort that occurs as things approach the speed of light.

Offline Samnell

Re: Physics/Theory of Relativity questions
« Reply #14 on: May 20, 2012, 06:59:39 PM »
I guess the issue I've had is stuff like that I've heard that the speed of sound is constant through air.  Is this incorrect as well?  Does a moving person's voice move faster than a still person's voice?

I don't have much to add to what's been said already, but it might be helpful to remember that sound is just vibration going through a medium. There's no sound particle generated by noise. It's just like tossing a stone in still water and watching the waves emanate from the impact site.

Light is different. There is actual light stuff which behaves in some ways that are hard to understand without knowing the math. Which I do not. :)

Online Oniya

  • StoreHouse of Useless Trivia
  • Oracle
  • Carnite
  • *
  • Join Date: Sep 2008
  • Location: Just bouncing through. Hi! City of Roses, Pennsylvania
  • Gender: Female
  • One bad Motokifuka. Also cute and FLUFFY!
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 3
Re: Physics/Theory of Relativity questions
« Reply #15 on: May 21, 2012, 10:04:25 AM »
Light has instances where it behaves like a wave (interference patterns, red-shifts) and instances where it behaves like a particle (photons can be detected individually) - this is one of the reasons that talking about things involving light get messy.

Offline gaggedLouise

  • Quim Queen | Collaborative juicy writer
  • Champion
  • Enchanter
  • *
  • Join Date: Jan 2011
  • Location: Scandinavia
  • Gender: Female
  • Bound, gagged and unarmed but still dangerous.
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 0
Re: Physics/Theory of Relativity questions
« Reply #16 on: May 24, 2012, 03:50:19 AM »
I understand about comets speeding up as gravity suddenly kicks in, especially before they have any wind resistance which would cause terminal velocity, but I don't really understand how explosions in space are different from explosions on Earth.  If I must pick one, though, I'd go with explosions on Earth, though I'd like to know how each are different.


I guess I was thinking of how the thrust of the explosion moves forward. The "wave" part of a powerful explosion (or an eruption, for that matter) will reach much further than the direct throw-out of splinters of the object that exploded, provided that there is a medium the wave can move in. On earth we don't think of that because there's always a continuing medium of some kind: water, air, earth, so the wave will have something to move in - you have to offset the resistance of that medium of course.

In space there is no physical medium, so a wave of matter, or even of sound, can't really travel very far unless it has a carrier that keeps on pushing it when the opening impact has gone down (it does keep travelling until it hits into something that stops it or bounces it, but it's no longer traveling "on its own speed", only on a gravity controlled orbit). An explosion in outer space wouldn't make any sound - I reckon you knew that already - since there is no atmosphere or liquid for the sound waves to travel in. And with solid matter - parts of a bomb or a blown-up spaceship - the concerted force of an explosion (if it were like any man-made explosion we can set off today) would soon be gone: every individual piece of scrap would just slip off into a trajectory that would really be controlled by the gravity of distant objects, such as planets and the star of the system. I don't think you would get any "added speed" to the explosion due to the orbit and speed the object was on at the moment before it exploded, as would have happened on earth.

With cases like the remains of a supernova, where there's sheets of gas that keep moving at high speed in a clearly visible way for many thousands of years after the star died, the force that "carries" the motion is heated gas and the ability to keep up chemical reactions and ionization long after the explosion, and it creates a wave motion in the interstellar medium. So it's not just a mechanical thrust from the point where the star stood before it exploded, it's mostly a wave of heat and radiation - unlike matter and sound, these do not need a medium to keep actively moving forward.
« Last Edit: May 24, 2012, 04:14:39 AM by gaggedLouise »

Online Oniya

  • StoreHouse of Useless Trivia
  • Oracle
  • Carnite
  • *
  • Join Date: Sep 2008
  • Location: Just bouncing through. Hi! City of Roses, Pennsylvania
  • Gender: Female
  • One bad Motokifuka. Also cute and FLUFFY!
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 3
Re: Physics/Theory of Relativity questions
« Reply #17 on: May 24, 2012, 07:29:25 AM »
Well - radiation doesn't need a medium, since it provides its own (as alpha and beta particles).  Heat, which is created by kinetic energy, is stopped by a vacuum.  This is actually why a Thermos bottle works - a thin layer of vacuum between the inner and outer walls prevents the kinetic energy from your hot coffee from getting out as quickly (some is still transferred through the points of contact between the inner shell and the outer shell, but that's limited by design.)

Offline gaggedLouise

  • Quim Queen | Collaborative juicy writer
  • Champion
  • Enchanter
  • *
  • Join Date: Jan 2011
  • Location: Scandinavia
  • Gender: Female
  • Bound, gagged and unarmed but still dangerous.
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 0
Re: Physics/Theory of Relativity questions
« Reply #18 on: May 24, 2012, 07:45:08 AM »
Okay, thanks for enlightening  me on that one, Oniya, but with "absence of a medium" (having a void, that is) I wasn't thinking of a perfect vacuum, which is something that really has to be constructed by some engineers or physicists - it takes considerable skill to create a reliable lab vacuum that CERN would recognize - but rather discontinuity and sparseness of medium, a region of space where the particles do not really add up to a continuing medium that the waves in question could travel in. If you only have a hundred atoms per litre/cubic decimeter of space, excepting the sudden burst of foreign particles from the exploding object, then it stops any sound wave and probably doesn't do much in order to carry a grenade tremor forward
« Last Edit: May 24, 2012, 07:46:13 AM by gaggedLouise »

Online Oniya

  • StoreHouse of Useless Trivia
  • Oracle
  • Carnite
  • *
  • Join Date: Sep 2008
  • Location: Just bouncing through. Hi! City of Roses, Pennsylvania
  • Gender: Female
  • One bad Motokifuka. Also cute and FLUFFY!
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 3
Re: Physics/Theory of Relativity questions
« Reply #19 on: May 24, 2012, 07:55:09 AM »
True enough (and the void in the Thermos bottle probably isn't CERN-level vacuum either), but in order for heat to be transferred, the particles need to collide with each other often enough to transfer kinetic energy.  Heat from a non-nuclear explosion outside an atmosphere is going to drop off incredibly fast.  Now, radiation (being particulate in itself, and generating new particles as atoms are broken up by the collisions) can transfer/generate heat, but you're still dealing with a rapid increase in volume that those particles are bouncing around in.

Offline gaggedLouise

  • Quim Queen | Collaborative juicy writer
  • Champion
  • Enchanter
  • *
  • Join Date: Jan 2011
  • Location: Scandinavia
  • Gender: Female
  • Bound, gagged and unarmed but still dangerous.
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 0
Re: Physics/Theory of Relativity questions
« Reply #20 on: May 24, 2012, 09:01:18 AM »
Spirits in Space

Speaking of sparse interstellar medium that yet adds up to something, I remember reading somewhere that there are enough ethanol molecules in the interstellar space of the Milky Way alone that they would make up an estimated billion of litres of erm, liquid (raw) liquor if they were pulled together.  :-) The trouble being, of course, to find them.
« Last Edit: May 24, 2012, 09:02:49 AM by gaggedLouise »

Offline AndyZTopic starter

Re: Physics/Theory of Relativity questions
« Reply #21 on: May 24, 2012, 01:33:35 PM »
Just wanted to say thanks to people who are posting.  Haven't had much to say but I'm reading it all and trying my best to understand.

Online Oniya

  • StoreHouse of Useless Trivia
  • Oracle
  • Carnite
  • *
  • Join Date: Sep 2008
  • Location: Just bouncing through. Hi! City of Roses, Pennsylvania
  • Gender: Female
  • One bad Motokifuka. Also cute and FLUFFY!
  • My Role Play Preferences
  • View My Rolls
  • Referrals: 3
Re: Physics/Theory of Relativity questions
« Reply #22 on: May 24, 2012, 02:35:41 PM »
If you want some offline references, I recommend the 'Blackboard Book' E=Mc2, and The Dancing Wu Li Masters.  The first one is a book designed as a refresher of basic concepts - not much math, a few tables as I recall, and a good dose of humor.  The second one is specifically geared as physics without the math - almost as philosophy.

Offline Exelion

Re: Physics/Theory of Relativity questions
« Reply #23 on: May 26, 2012, 01:47:50 PM »
Well, if you start to REALLY think about it...that grenade and helicopter are on the earth, which is currently travelling at something like 66,000 mph..in a galaxy traveling tens of thousands of mph...

I try not to think about these things. My head might explode.

Offline dutchprof

Re: Physics/Theory of Relativity questions
« Reply #24 on: May 26, 2012, 11:05:20 PM »
In space there is no physical medium, so a wave of matter, or even of sound, can't really travel very far unless it has a carrier that keeps on pushing it when the opening impact has gone down (it does keep travelling until it hits into something that stops it or bounces it, but it's no longer traveling "on its own speed", only on a gravity controlled orbit). An explosion in outer space wouldn't make any sound - I reckon you knew that already - since there is no atmosphere or liquid for the sound waves to travel in. And with solid matter - parts of a bomb or a blown-up spaceship - the concerted force of an explosion (if it were like any man-made explosion we can set off today) would soon be gone: every individual piece of scrap would just slip off into a trajectory that would really be controlled by the gravity of distant objects, such as planets and the star of the system. I don't think you would get any "added speed" to the explosion due to the orbit and speed the object was on at the moment before it exploded, as would have happened on earth.
In otherwise empty space, matter particles resulting from an explosion might travel pretty far. In principle, they would travel indefinitely at a constant speed in a straight line; that is different than on earth, where interaction with surrounding air would slow debris down rather quickly.

In the gravitational field of a planet or star, the gravitational force would affect the trajectory of the debris. However, if you look at an explosion from a perspective that is in free fall, this falling motion would not be visible. (Compare to the motion of astronauts in the Space Station: they are in free fall and, contrary to popular belief, still experience about 98% of the earth's usual gravitational pull, but since the camera falls along with them they appear to float.) The only difference between an explosion in truly empty space and near a planet is due to "tidal forces", which seem to pull the exploding matter apart if it is spread out over larger distances. These tidal forces are due to the fact that the gravitational field is not constant but has varying strength and direction. These forces are usually rather weak.

If a satellite or spaceship would explode while in orbit around a planet, the debris particles would on average have the same speed and direction as the original object; this follows from the law of conservation of momentum. If you would travel in a second ship along with the exploding object, it would appear (from your perspective) that the explosion is symmetric, but an outsider would see the explosion move. In particular, when the debris impacts a nearby planet, they have more speed in the original direction of the exploding object; although air resistance is likely to reduce this forward speed before impact.

If a very large amount of matter explodes in otherwise empty space, the debris will gradually slow down because the particles attract each other. However, gravitational forces are very weak, so that this only strongly affects planetary and stellar quantities of matter. If debris moves outward at a speed faster than the escape speed at the surface of the object, it will be able to escape this gravitational pull and keep moving outward indefinitely; it will be slowed down a bit by gravity but will not fall back. For a 100-m radius, 1-million ton spaceship, this escape speed is about 0.1 mph.