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.
Pentcho Valev
pvalev@yahoo.com