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: Coefficient of friction

  1. Nov 12, 2005 #1
    If the coefficient of static friction between a table and a rope is [tex] \mu_{s} [/tex], what fraction of the rope can hang over the edge of a table without the rope sliding?

    Ok, so I declared two variables, P and 1-P . From here, all I know is that mass and weight are not of any concern in this problem. Could someone please offer some help in solving this problem? I know the answer is [tex] \frac{\mu_{s}}{1+\mu_{s}} [/tex]

  2. jcsd
  3. Nov 12, 2005 #2
    Are you sure the answer you have is right? i get something slightly different.

    In any case, I think you should start by equating the two forces acting on your rope, [itex] F_g[/itex] and [itex]F_f[/itex]. You know that
    [tex] F_g=mg[/tex], where m is the mass that's hanging, and that
    [tex] F_f=\mu_s(M-m)[/tex], where M is the total mass

    With these equations in hand, you can now find the critical percentage, M/m.

    Hope it's useful, but once again, this leads to a different answer from that which you've got.
  4. Nov 12, 2005 #3


    User Avatar
    Gold Member

    Lets say that p is hanging of the table and 1-p is on the table. Think how much force (mg) p is pulling down with and how much friction is resisting due to the 1-p on the table. Then equate the two. Oops! once again I post a second after someone else!
  5. Nov 13, 2005 #4
    Did you really mean [tex]F_f=\mu_s(M-m)[/tex]? The part in brackets has to be a force for the equation to be homogenous, so I think you are missing a 'g' in this equation.

    I agree with the answer that you are looking for

  6. Nov 13, 2005 #5
    Oops, yes, there's a g missing. So yeah, the answer is perfectly right.
    Sorry, my bad.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook