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!

Friction in a 3 block and pulley system

  1. Jun 21, 2015 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 is on top of a block of mass m2. Block 2 is connected by an ideal rope passing through a pulley to a block of unknown mass m3 as shown. The pulley is massless and frictionless. There is friction between block 1 and 2 and between the horizontal surface and block 2. Assume that the coefficient of kinetic friction between block 2 and the surface, μ, is equal to the coefficient of static friction between blocks 1 and 2.
    What is the minimum value of m3 for which block 1 will start to move relative to block 2?

    2. Relevant equations
    F=ma
    f= mu*N

    3. The attempt at a solution

    I've taken a couple of approaches. The one I think is most valid is this: The point just before block 2 slips relative to block 1 is where the static friction between m1 and m2 reaches its maximum value. At this point the magnitude of acceleration between all the blocks is the same. I'm then solving for:

    m1*a = mu*m1*g
    m2*a = T-mu*m1*g-mu*(m1+m2)*g
    m3*a=T-m3*g

    Which gives me:
    m3=2*mu*(m1+m2)/(1+mu)

    Which is wrong. Am I making a math error or taking the wrong approach entirely?

    Thanks in advance for any help!
     
  2. jcsd
  3. Jun 21, 2015 #2
    Can you post the diagram or facsimile thereof?
     
  4. Jun 21, 2015 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

  5. Jun 21, 2015 #4

    Nathanael

    User Avatar
    Homework Helper

    You have it almost right, except your sign-convention is inconsistent:
    "a" is supposed to be the same for all three equations, right?
    In the first two equations, "a" takes a positive value, but in the third equation "a" takes a negative value (m3g ≥ T)
     
  6. Jun 21, 2015 #5

    Got it. Thanks.
     
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: Friction in a 3 block and pulley system
  1. 3 block pulley system (Replies: 2)

  2. 3 Blocks and a Pulley (Replies: 5)

  3. 3 blocks and pulley (Replies: 1)

Loading...