Two plane discs, each of radius 5 cm, are mounted coaxially with their adjacent surfaces 1 mm apart. They are in a chamber containing Ar gas at STP (viscosity 2.1×10−5 N s m−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 torque that must be applied to the other to keep it stationary.