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 and weight

  1. Apr 10, 2007 #1
    1. The problem statement, all variables and given/known data

    A lightweight rope is wrapped around a 100lb drum, passes over a frictionless pulley, and is attached to weight, W. The coefficient of friction is 0.50. Determine the maximum amount of weight that can be supported by this arrangement.

    I've drawn a picture, showing what I think are the forces on everything.

    I need help solving this, as I'm not sure how to actually do it. The answer, I have(given to me) is W=75lbs.

    2. Relevant equations

    3. The attempt at a solution

    Forces in x direction = 0 = Fa - Nb + Tw
    Forces in y direction = 0 = Na + Fb - 100

    Not sure if there is a moment(and if so, where do I take it about).....also, if not.....I'm not sure how to solve this next.

    Thank you,
    View attachment friction.bmp
  2. jcsd
  3. Apr 10, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Why not take moments about the center of the drum? Hint: If the drum is just about to turn, what can you say about the friction forces?
  4. Apr 10, 2007 #3


    User Avatar
    Science Advisor
    Homework Helper

    You have 5 unknowns Fa Na Fb Nb and Tw so you need 5 equations.

    You have 2 equations already.

    When the drum starts to rotate, Coulombs law of friction says
    Fa = mu.Na and Fb = mu.Nb That makes 4 equations.

    You can get a 5th equation by taking moments. If doesn't matter where you take moments about. Choose a point which several forces pass through, so they have zero moment and you get a simpler equation.
  5. Apr 10, 2007 #4
    Ok......so now, I took the moment about the center.


    M = 0 = Fb(d/2) + Fa(d/2) - Tw(d)

    And the 2 equations above. I'm totally lost as to how I mathematically solve these though, or where I start. Also, I don't know the diameter(d)....so not sure how the moment equation will help.....
  6. Apr 10, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor


    Why is one force times d while others are times d/2?

    The d drops out, so you don't need the actual value.

    If you answer my question/hint about friction, you'll be able to rewrite Fb and Fa in terms of Nb and Na. (Similar to what AlephZero told you to do.) Then you'll have 3 equations and 3 unknowns--solve!
  7. Apr 10, 2007 #6
    Ok......I was saying it was the diameter/2.....or the radius. I don't know what I was thinking....I found that mistake, and will try to solve it now....see if I can do it....not sure.
  8. Apr 10, 2007 #7
    yes, I did solve it(it feels good....haha). Thank you for all of your help, I sure appreciate it!!!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook