1. The problem statement, all variables and given/known data PLEASE help! I really should be able to do this but am so stuck: Two plane disks, each of radius 5 cm, are mounted coaxially with their adjacent surfaces 1 mm apart. They are in a chamber containing Ar gas at S.T.P. (viscosity 2.1×10−5 Nsm−2) and are free to rotate about their common axis. One of them rotates with an angular velocity of 10 rad s−1. Find the couple which must be applied to the other to keep it stationary. 2. Relevant equations I know there's an equation of F/A = viscosity*d<ux>/dz Where <ux> is the average x-velocity at a given z (z is along the axis). 3. The attempt at a solution I tried taking ux to be the component of a particle's velocity along the theta direction (perpendicular to the radius of the disc), and replacing it with rw. I then tried to integrate that equation up there wrt z, so that the RHS gives viscosity*r*(w - w0) where w0 is the value given for angular velocity and w is the value at the other disc. However I am confused as to what F is. I think it's the force exerted on the spinning disc, but if so how do I know what it is? I thought about using Idw0/dt = torque, but w0 is constant. I also thought about centripetal force acting on parts of the disc, but I don't understand how that could be right. I'm very confused! Thanks!