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 and Pulley Problem

  1. Dec 19, 2005 #1
    "Two blocks are attached to a string that passes over a frictionless pulley. One of the blocks is 4kg and rests on a rough horizontal surface. The other block is a 2kg block that hangs over the edge of the surface. When the system is released, the 2 kg mass falls from rest and attains the speed of 2m/s after traveling a distance of 3 meters"

    The first question is "what is the loss in mechanical energy" and I get 56J, which is right according to my teacher. However, the second question is "what is the coefficient of kinetic friction between the 4 kg block and the rough surface?"

    What I do is:

    Loss in mechanical energy = energy of friction


    56 = Fd =
    56 = umgd =
    56 = u * 4 * 9.8 * 3m

    and I get .47. However, the answer is .40. What am I doing wrong?
  2. jcsd
  3. Dec 19, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    How did you obtain 56 J?

    Also think, what is the tensile force in the string?

    There is friction, and there is the force required to accelerate the 4 kg mass laterally. What is the lateral acceleration of the 4 kg block?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook