Short story could be from 1960-1980s. In English.
Scientists are frantically working in a lab trying to develop a impenetrable force field. They have created one that does not use much power, but it is weak. (it might stop a fist from going through, but a bullet will go through.)
When you add more power to make it stronger, (i.e. able to stop a bullet) in a few minutes, it becomes unstable, and explosively fails. destroying anything near (maybe the room too) And the Math and equations say it is a fixed limit, so it will never work.
There is some sort of back story, aliens on a very high G world (like Jupiter), living under tremendous pressure, have been discovered, however they are very hostile. Fortunately, neither they nor humanity have any materials or alloys strong enough, that would allow them to leave their planet, due to the need to maintain internal pressure at the surface pressure as they rise in the atmosphere. i.e. their space ships would pop like balloons.
The scientists and leaders and diplomats are happy that the aliens would not be able to use force shields to escape their planet.
Meanwhile, someone else has been working on a force field (Engineer type?) and we are witnessing a discussion between him and someone else as they fly around in a spaceship who's exterior wall IS a force field.
How did he do it? He cranked the power up way high, so the field is so strong that nothing can penetrate it, Although light will pass through. But at that power level, the field will only last about .00001 second ( 1/100,000th of a second) So he has a relay operating at a millionth of a second, turning the power off and on, so the field does not have time to become unstable.
There was some punch line or pun as the last line. (Very Asimovian, So it might have been an Asimov story) but I dont remember what it was.