Suppose we throw a metallic plate through the space between two poles of a horseshoe magnet. There is no gravity or air drag. Eddy currents will be generated in the metal, which will try to oppose change of magnetic flux through it, and the plate will slow down.
However, since the magnetic field can't do work, what is slowing the loop down? Here there is no battery or external agent which might do work.
I know that the kinetic energy of the metal will be converted to heat. But here I am asking about the force (it must be some electric force) which is doing negative work on the metal.
Something similar to this is done in Introduction to Electrodynamics by David J. Griffiths (in the chapter magnetostatics, where it turns out that the magnetic field it forcing the current source to do work), but there is no such current source.