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Homework Statement
Two plane disks, each of radius 5cm, are mounted coaxially with their adjacent surfaces 1mm apart. They are in a chamber containing gas at STP (viscostiy of gas = 2.1 x 10^-5 Ns/m^2) and are free to rotate about the common acis. One of them rotates with angular velocity of 10 rad/s. Find the couple which must be exerted to keep the other stationary.
Homework Equations
The Attempt at a Solution
So letting the bottom plate be where r=0, top plate is at r=0.001
then i say du/dr = w + rdw/dr
textbook says that only the second term contributes to viscous shearing stress...Why is this?
so assuming that to be true,
F = pi (0.05)^2 n r dw/dr where n is the viscosity..
we can then integrate...but isn't the integral dr from 0 to 0.001? and don't we have a lnr between those limits..and ln 0 is a problem?
Help Please :S!