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 Posted: Tue Sep 05, 2006 3:58 pm
Alright, I just started my 3 year (at least) course through highschool and early college physics, and my teacher tends to go one quite a few topics, which I always enjoy researching long after class ends.
Anyway, we were discussing acceleration due to gravity, which someone brought up terminal velocity. While that exist due to air resistance, I was wondering if it existed within a vacuum, which he said there wouldnt be any... unless you go by the speed of light being the fastest speed in the universe.
That brings me here. What is terminal velocity in a vacuum? Is the speed of light the limit? In constant acceleration, for however long in a space shuttle, those inside would eventually reach a restive state (while still accelerating), and they too would reach near that speed as well?
And if it isn't? Let's say we someone made a golfball reach that point. What would it do?
Thanks for any ideas 3nodding
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Posted: Tue Sep 05, 2006 9:04 pm
Mr. Mage Anyway, we were discussing acceleration due to gravity, which someone brought up terminal velocity. While that exist due to air resistance, I was wondering if it existed within a vacuum, which he said there wouldnt be any... unless you go by the speed of light being the fastest speed in the universe. That brings me here. What is terminal velocity in a vacuum? Your question is incomplete because you do not provide the specifics of your projectile--the most relevant parameters are its geometry and the driving force. That said, the speed of light is the only barrier in a perfect vacuum. However, such a thing doesn't exist. Interstellar dust will produce drag, and the blueshifted background radiation will produce pressure on the front of the projectile as well. Mr. Mage In constant acceleration, for however long in a space shuttle, those inside would eventually reach a restive state (while still accelerating), and they too would reach near that speed as well? During the acceleration, the occupants will of course feel a force towards the back of the shuttle and therefore be affected by it.
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Posted: Tue Sep 05, 2006 9:32 pm
This is sort of like the max velocity of a Mag lev train in an unending Vacuum, I asked my friend who is finishing his PhD in Physics, and he said the limiting facort would be energy.....
But as far as terminal velocity in a true vacuum, it should be as you approach the speed of light untill you reach the speed of light. One of the assumptions you need to figure out though is how strong the gravity pull is going to be, that sorta thing, because you would never come close to terminal velocity if you are using earth as a gravity source, because just a distance thing.
Anyways, I am not sure if there really is an answer because we arent sure if the speed of light can be reached, and if its not possible to go past it or not.
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Posted: Tue Sep 05, 2006 9:40 pm
KaitlynintheGoal But as far as terminal velocity in a true vacuum, it should be as you approach the speed of light untill you reach the speed of light. One of the assumptions you need to figure out though is how strong the gravity pull is going to be, that sorta thing, because you would never come close to terminal velocity if you are using earth as a gravity source, because just a distance thing. Anyways, I am not sure if there really is an answer because we arent sure if the speed of light can be reached, and if its not possible to go past it or not. On the contrary, the speed of light limit is more certain than practically everything else in physics. As an interesting note, gravity is repulsive at strongly relativistic speeds.
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Posted: Sat Sep 09, 2006 2:29 pm
VorpalNeko blueshifted background radiation will produce pressure on Not to get off topic, but what the heck is that? =D!
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Posted: Sat Sep 09, 2006 2:38 pm
AirisMagik VorpalNeko blueshifted background radiation will produce pressure on Not to get off topic, but what the heck is that? The background radiation I was referring to is the extremely uniform remnant of the big bang mostly in the microwave band. Even neglecting interstellar particles, space is filled with this radiation, giving it a temperature of about 3K. Look up the COBE and WMAP missions by NASA for more detailed results. If a ship travels at a strongly relativistic speed relative to the frame of maximal uniformity of this background radiation, the radiation will be blueshifted in the front and redshifted in the back, giving a pressure differential that will put drag on the ship. (Realistically, however, interstellar dust is the more important issue.)
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Posted: Sun Sep 10, 2006 6:32 pm
How is gravity repellant at relativistic speeds? I assume that's a formation from GTR.
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Posted: Mon Sep 11, 2006 6:47 pm
Swordmaster Dragon How is gravity repellant at relativistic speeds? I assume that's a formation from GTR. I should make that statement more precise for it to be valid. Let's say there is a stationary observer some distance from a spherically symmetric body, who then sends an inward probe equipped with a mirror along a radial path. By bouncing light signals off the probe, the observer can calculate the probe's position and velocity at any given time. Now, if the body is sufficiently small (down to black hole size), what the observer should measure is that the probe eventually slows down and hovers just outside the event horizon. If the probe's initial velocity is sufficiently large, the probe will be observed to always slow down, and it would not matter if the body is not a black hole, for every spherically symmetric body has the same external field by Birkhoff's theorem. By symmetry, an outwardly directly probe with sufficient velocity will be seen to speed up; it's not too difficult to calculate that this happens when the proble's velocity at infinity is no less than sqrt(3)/3 of the speed of light. It should be emphasized that what the observer sees and what the probe experiences are not the same thing.
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Posted: Wed Sep 13, 2006 8:15 pm
What do you mean by "outwardly directly probe"? And does this imply that at higher and higher speeds, the external fields of different bodies become more and more similar (i.e. at low velocities, the size of the object becomes more and more relevant)?
I have another question about Birkhoff's theorem, but I really don't know much about it.
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