1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Kinetic Theory of Gas question involving viscosity

  1. Oct 25, 2008 #1
    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!

  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted