1. Not finding help here? Sign up for a free 30min 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!

Fluid Mechanics newtons law of viscosity

  1. Jul 25, 2013 #1
    1. The problem statement, all variables and given/known data
    having problems differentiating


    2. Relevant equations
    Tw ( wall shear stress)= -U(viscosity)*du/dr
    been given u as 2V(1-(r/r)^2)


    3. The attempt at a solution
    i substituted u in and got

    d/dr (2V(1-(r/r)^2)
    i tried to multiply out the minus sign and 2V and got a very different answer i got -2V+2V (r/r)^2

    when the answer is supposed to be -4VR/R^2

    please help
     
  2. jcsd
  3. Jul 26, 2013 #2
    What do you mean here:

    been given u as
    2V(1-(r/r)^2) = 2V(1 - 1) = 0

    ?
     
  4. Jul 26, 2013 #3

    pasmith

    User Avatar
    Homework Helper

    Do you mean
    [tex]
    u = 2V\left(1 - \left(\frac{r}{r_0}\right)^2\right)
    [/tex]
    where [itex]r_0[/itex] is the radius of what I assume to be a cylindrical pipe?


    The given answer is for [itex]du/dr[/itex]; to get [itex]T_w[/itex] you have to multiply that by [itex]-\mu[/itex].

    So far all you've done is multiply [itex]u[/itex] by -1. Try differentiating with respect to r first.
     
  5. Jul 26, 2013 #4
    r=R and its (r/R)^2
     
  6. Jul 26, 2013 #5
    okay now i have differentiated it with respect to r i get 2V2r/R^2) or -(4Vr/R^2) thanks again i was then required to use the wall shear stress to determine the frictional force but it is very straight forward from there
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Fluid Mechanics newtons law of viscosity
  1. Fluid mechanics proof (Replies: 2)

Loading...