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!

Friction Force on a System at Rest

  1. Oct 23, 2007 #1
    1. The problem statement, all variables and given/known data
    http://img337.imageshack.us/img337/2514/diagramfn4.png [Broken]

    3. The attempt at a solution
    [tex]\sum F_{x} = w_{x} - T = m_{x}a[/tex]
    [tex]T = (6.9)g - (6.9)a[/tex]

    [tex]\sum F_{a} = T - w_{a} - f_{s} = m_{a}a[/tex]
    [tex]T - m_{a}g - m_{a}a = f_{s}[/tex]
    [tex](6.9g - 6.9a) - m_{a}g - m_{a}a = f_{s}[/tex]
    I don't see a 5kg block with friction, but I do see a 3kg block with friction:
    [tex]6.9g - 6.9a - 3g - 3a = f_{s}[/tex]
    [tex]3.9g - 9.9a = f_{s}[/tex]

    Since the system is at rest, [tex]a = 0[/tex]
    [tex]38N = f_{s}[/tex]
    So the answer is B

    But I have two problems with my answer:
    1) [tex]f_{s-max} = (0.40)(50) = 20N < 38N[/tex] in my answer
    2) Using the "5kg" block, [tex]f_{s} = 19N[/tex]

    Am I missing something?
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 23, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks to me like the 3 kg was a typo and they meant to write 5 kg. (Otherwise the problem doesn't work, as you point out.)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook