Verified answer

As Catarthur says, it would be like a thermonuclear bomb going off, because the kinetic energy would be equal to annihilating 4 times the mass of the original rod. But let's assume, then, that the means of stopping the rod "cold" sucked up this kinetic energy. Then in fact, at least for an instant, the rod is still 6m, not 30m, and so it's actually compressed lengthwise to 1/5 of its length, so we're talking about millions of atmospheres pressure, which alone is sufficient to make a bomb out of the pipe containing it. There is no relation between the potential energy in this compression and its relativistic energy before it was stopped. So, where did this "extra" energy come from? Same place where you got your means of "stopping the rod cold".

I suppose it depends on the definition of "instantly trapped" if one uses a strict enough definition, then I suspect the universe as we know it may cease to exist.

Even if "instantly" is just "pretty quick", while the mass or the two objects is not given, I don't think I'd want to be too close when it happens, v is not an insignificant number.

Edit; Some further explanation, my reasoning for speculating that the universe would end, is that I know of no way an instantaneous transfer of energy could occur, so my reasoning is that the universe would have to change to one with different laws of physics. However upon reflection, (and having read some science fiction in the past) I realize a simpler explanation is that the rod and tube will move to to another universe with different laws, there is no reason for our universe to change. So to an observer in this universe, the rod and tube will simply disappear. Since we don't know what the laws of physics are in the hypothetical universe, that is as far as we can go with the problem.

On the other hand if both the rod and tube are massless, the transfer of energy is not a problem, but are dimensions really relevant for massless objects? What are the physical properties of massless objects? If the pipe is massless, then as soon as the rod is trapped it will be traveling the same speed as the rod, and now being in the same frame of reference the original ratio of lengths will exist, but here the physical properties are important, can the pipe stretch and/or the rod compress? These seem to be unanswerable questions.

At this speed the apparent length of the rod is 6m. By suddenly stopping, its length will jump to 30m.

First of all there will be a thermonuclear bomb equivalent release of energy, enough to vaporize both the rod and the pipe.

But I see your point, assuming no energy release (let's say the weight of rod is close to 0), what happens to the pipe, is it stretched to 30 meter by the rod?

Hmmm something to ponder tonite while trying to sleep.

it may force the pipe inside to shoot out. idk in 9th grd physics in hawaii i studied similar.

i would suppose it would do his becuase the legnth is greater. thus it would increase speed velocity. if it was short expect a rate of air decreasement.

Yeah , this pipe is gona shoot out, if the pipe at rest = 6 M then lol its gona shoot.

but it wont travel far...

Good luck, physics seems fun... :)

I am not sure what's going to happen; but I am sure there will not be many happy campers!