http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html "These are the props. You own a barn, 40m long, with automatic doors at either end, that can be opened and closed simultaneously by a switch. You also have a pole, 80m long, which of course won't fit in the barn. Now someone takes the pole and tries to run (at nearly the speed of light) through the barn with the pole horizontal. Special Relativity (SR) says that a moving object is contracted in the direction of motion: this is called the Lorentz Contraction. So, if the pole is set in motion lengthwise, then it will contract in the reference frame of a stationary observer.....So, as the pole passes through the barn, there is an instant when it is completely within the barn. At that instant, you close both doors simultaneously, with your switch. Of course, you open them again pretty quickly, but at least momentarily you had the contracted pole shut up in your barn. The runner emerges from the far door unscathed.....If the doors are kept shut the rod will obviously smash into the barn door at one end. If the door withstands this the leading end of the rod will come to rest in the frame of reference of the stationary observer. There can be no such thing as a rigid rod in relativity so the trailing end will not stop immediately and the rod will be compressed beyond the amount it was Lorentz contracted. If it does not explode under the strain and it is sufficiently elastic it will come to rest and start to spring back to its natural shape but since it is too big for the barn the other end is now going to crash into the back door and the rod will be trapped in a compressed state inside the barn."
Note that, if the diameter of the rod is equal to the diameter of the barn's hole, Einsteinians will obtain a twofold decrease in the rod's volume as well! Essentially without spending any work! Therefore both the force exerted by the compressed rod on the doors and the work this (enormous?) force can do for Einsteinians are just free lunch. What a breathtaking discovery! And yet Einsteinians seem reluctant to develop the scenario further - what is the magnitude of the force, how much work can be extracted etc. Students should just imagine first the dull Newtonian world where the 80m long rod does not want to hide inside the 40m long barn, then the miraculous Einsteinian world where the rod would hide even inside a 4 cm long barn, and that is enough. Green lights appear in students' eyes and the tunes of "Divine Einstein" and "Yes we all believe in relativity, relativity, relativity" quickly fill the spacetime.
> http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html > "These are the props. You own a barn, 40m long, with automatic doors > at either end, that can be opened and closed simultaneously by a > switch. You also have a pole, 80m long, which of course won't fit in > the barn. Now someone takes the pole and tries to run (at nearly the > speed of light) through the barn with the pole horizontal. Special > Relativity (SR) says that a moving object is contracted in the > direction of motion: this is called the Lorentz Contraction. So, if > the pole is set in motion lengthwise, then it will contract in the > reference frame of a stationary observer.....So, as the pole passes > through the barn, there is an instant when it is completely within the > barn. At that instant, you close both doors simultaneously,
"Simultaneously" ?
What does that mean, exactly?
You should learn a little about SR. There is no concept of absolute simultaniety. Events which appear simultaneous in one frame don't appear simultaneous in a different reference frame (and indeed may not even appear simultaneous in the same reference frame, depending upon how you choose to interpret the word "simultaneous").
If you do learn about SR, you will eventually be able to explain what happens in this thought experiment from both relevant inertial frames - the barn's and the pole's reference frames.
Just to whet your appetite to learn SR, an observer in the inertial frame of the barn could "see" both doors open simultaneously, but from the perspective of the pole they are not simultaneous and the first door opens and closes before the second door.
Its really quite simple when you understand the maths, you might like to think about buying a beginners guide to Relativity. There are many available, including a very good one by Einstein himself.
If there is anything you don't undertsand in whatever book you buy, feel free to ask questions and I am sure people will want to help.
> >http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html > > "These are the props. You own a barn, 40m long, with automatic doors > > at either end, that can be opened and closed simultaneously by a > > switch. You also have a pole, 80m long, which of course won't fit in > > the barn. Now someone takes the pole and tries to run (at nearly the > > speed of light) through the barn with the pole horizontal. Special > > Relativity (SR) says that a moving object is contracted in the > > direction of motion: this is called the Lorentz Contraction. So, if > > the pole is set in motion lengthwise, then it will contract in the > > reference frame of a stationary observer.....So, as the pole passes > > through the barn, there is an instant when it is completely within the > > barn. At that instant, you close both doors simultaneously,
> "Simultaneously" ?
> What does that mean, exactly?
> You should learn a little about SR. There is no concept of absolute > simultaniety. Events which appear simultaneous in one frame don't appear > simultaneous in a different reference frame (and indeed may not even appear > simultaneous in the same reference frame, depending upon how you choose to > interpret the word "simultaneous").
> If you do learn about SR, you will eventually be able to explain what > happens in this thought experiment from both relevant inertial frames - the > barn's and the pole's reference frames.
> Just to whet your appetite to learn SR, an observer in the inertial frame of > the barn could "see" both doors open simultaneously, but from the > perspective of the pole they are not simultaneous and the first door opens > and closes before the second door.
> Its really quite simple when you understand the maths, you might like to > think about buying a beginners guide to Relativity. There are many > available, including a very good one by Einstein himself.
> If there is anything you don't undertsand in whatever book you buy, feel > free to ask questions and I am sure people will want to help.
> HTH, a good luck learning physics!
> Peter Webb
Boy, that's telling him.
I'm sure he'll quit these repetitious, false postings now.
Pentcho Valev wrote: > http://math.ucr.edu/home/baez/physics/Relativity/SR/barn_pole.html > "These are the props. You own a barn, 40m long, with automatic doors > at either end, that can be opened and closed simultaneously by a > switch. You also have a pole, 80m long, which of course won't fit in > the barn. Now someone takes the pole and tries to run (at nearly the > speed of light) through the barn with the pole horizontal. Special > Relativity (SR) says that a moving object is contracted in the > direction of motion: this is called the Lorentz Contraction. So, if > the pole is set in motion lengthwise, then it will contract in the > reference frame of a stationary observer.....So, as the pole passes > through the barn, there is an instant when it is completely within the > barn. At that instant, you close both doors simultaneously, with your > switch. Of course, you open them again pretty quickly, but at least > momentarily you had the contracted pole shut up in your barn. The > runner emerges from the far door unscathed.....If the doors are kept > shut the rod will obviously smash into the barn door at one end. If > the door withstands this the leading end of the rod will come to rest > in the frame of reference of the stationary observer. There can be no > such thing as a rigid rod in relativity so the trailing end will not > stop immediately and the rod will be compressed beyond the amount it > was Lorentz contracted. If it does not explode under the strain and it > is sufficiently elastic it will come to rest and start to spring back > to its natural shape but since it is too big for the barn the other > end is now going to crash into the back door and the rod will be > trapped in a compressed state inside the barn."
> Note that, if the diameter of the rod is equal to the diameter of the > barn's hole, Einsteinians will obtain a twofold decrease in the rod's > volume as well! Essentially without spending any work! Therefore both > the force exerted by the compressed rod on the doors and the work this > (enormous?) force can do for Einsteinians are just free lunch. What a > breathtaking discovery! And yet Einsteinians seem reluctant to develop > the scenario further - what is the magnitude of the force, how much > work can be extracted etc. Students should just imagine first the dull > Newtonian world where the 80m long rod does not want to hide inside > the 40m long barn, then the miraculous Einsteinian world where the rod > would hide even inside a 4 cm long barn, and that is enough. Green > lights appear in students' eyes and the tunes of "Divine Einstein" and > "Yes we all believe in relativity, relativity, relativity" quickly > fill the spacetime.
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