I remember one of our physics tests at school many years ago. Our teacher had provided one solenoid with a magnetic bar in the center; He put one metallic ring on magnetic bar and connected terminal of the coil to 50HZ, 220V power supply. Suddenly, we observed a wonderful phenomenon. The metallic ring jumped up and stood over the magnetic coil. We had to explain the reason of that phenomenon. I think if our teacher had replaced solenoid magnetic rod with an other inferior core, we would have observed a higher jumping. Indeed, the applied force to the metallic ring is similar to the transformer winding stresses which meet short circuit fault currents.