This question already has an answer here:
In this video here, Walter Lewin mentions two discs of different radius $r_1$ and $r_2$, of the same density, where the first disk is initially rotating, while the second is at rest. In the videos scenario, both disks are eventually rotating with constant speed pressed against each other. Here the video states that the friction between them becomes zero.
My question is: Why should that happen? If I run on a surface and it recedes at the same rate I am running, should not it pose a reaction force on me, which will cause some friction? This friction should be diminished if the reaction force is zero, but that's does not the case, so perhaps somebody could explain how it is going to zero, when the two disks come to a kind of steady state.