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

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Kinetic friction acts in such a direction to reduce the relative motion between two bodies.
Static friction tries to prevent relative motion between two bodies.
In the case of static friction it can range from zero up to the coefficient of static friction times the normal reaction.

In the steady state with both disc rotating at the same constant angular speed there is no need for a frictional force.

If one of the discs were to slow down then frictional forces would act between the discs to prevent (or reduce) relative motion between them.

  • $\begingroup$ So do I have to conclude kinetic friction between two disks is proportional to their relative velocity. Although if the disks are rotating along different directions then there should be a relative velocity. And static friction is dependant on the normal force. So do i have to conclude the normal force is zero at steady state (or walter lewin stops pressing as the disks reach steady state or even pressing them against each other at steady state doesn't create frictional forces ? Please elaborate. $\endgroup$ – Nobody recognizeable Dec 27 '18 at 0:28
  • $\begingroup$ Force causes acceleration; a constant velocity (even an angular velocity) implies zero net force, i.e. zero friction force. Normal force is not relevant (it doesn't act in the angular-velocity direction). $\endgroup$ – Whit3rd Dec 27 '18 at 2:57
  • 1
    $\begingroup$ @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. $\endgroup$ – Farcher Dec 29 '18 at 10:44

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