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: Friction Help

  1. Feb 13, 2010 #1
    Problem: Ive been assigned a problem that involves a block on a table that is attached by a string to a block hanging over the edge. They are of different masses, and I am given coefficients for both static and kinetic friction. The former is .50, and the latter is .30. I have to find the acceleration of the system if it is released from rest.

    My problem: ok, so do I need to use the coefficient of kinetic friction at all? I do not think so...that's pretty much my question.
  2. jcsd
  3. Feb 13, 2010 #2
    I got a negative answer..
  4. Feb 13, 2010 #3
    Can you show what equations you used? Kinetic friction should be used for surfaces moving relative to each other.
  5. Feb 13, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hi ƒ(x)! :wink:

    Do good ol' Newton's second law on each block separately, plus the fact that their accelerations must be the same (because the string length is constant).

    What do you get? :smile:
  6. Feb 13, 2010 #5
    m1 (on table) = 10 kg
    m2 = 4 kg

    .5*Fn = Fs
    .5*10*9.8 = 49 N = Fs

    Fnet = Fx - Fs = 4*9.8 - 49 = -9.8 N

    ma = -9.8
    a = -9.8/m = -9.8/(4+10) = -.7 m/s/s
  7. Feb 13, 2010 #6


    User Avatar
    Science Advisor
    Homework Helper

    ohh! you didn't give the masses before :rolleyes:

    no wonder you got a negative answer
    That doesn't make sense … how can the mass be accelerating upward?

    What does it mean if the weight is less than the µsN ?
  8. Feb 14, 2010 #7
    I'm guessing it means that the system isn't moving.
  9. Feb 14, 2010 #8


    User Avatar
    Science Advisor
    Homework Helper

    (why guessing? :rolleyes:)

    That's right! …

    if the system is released from rest, it will never move (even though if it was given a little nudge, the low µk would enable it to keep accelerating). :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook