# Why should the friction between two disks be zero when there velocity is same? [duplicate]

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.

## marked as duplicate by sammy gerbil, Kyle Kanos, Jon Custer, M. Enns, Qmechanic♦Dec 30 '18 at 16:46

• @Nobodyrecognizeable You do not have to make any assumption about the kinetic friction other than it tries to reduce the relative motion between the two discs. The range of possible values of static friction is $0\le F_{\rm static} \le \mu_{\rm static} N$ and the value adjusts itself to try and prevent relative motion between the discs. When both discs have the same angular velocity the static frictional force is zero. – Farcher Dec 29 '18 at 10:44