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: Help please! cylinder, torque, rolling w/o slipping problem!

  1. Dec 17, 2012 #1
    A massless string is wrapped around a 10 kg cylinder. the picture will look like the letter b. you pull upward on the string with a force of P = 49N. The bottom of the cylinder s on horizontal table and will roll w/o slipping. using dynamics involving center of mass, find the magnitude and the direction of frictional force (f). (no slippage between string and cylinder)


    Torque about cm: RP - Rf = Iα ---> R(P-f) = Iα

    I = 1/2MR^(2)
    α = a/R

    R(P-f) = 1/2MR^(2) (a/R) ---> P-f = 1/2Ma

    Forces Horz: f = ma

    Why do we need to consider the forces horizontal?? Isn't it already included in the torque calculation?

    P-ma = 1/2Ma
    P = 3/2Ma
    Ma = 2/3P
    a = 2/3 (49N) / 10kg ---> 3.2667 N

    My teacher got 32.667N? but how?
  2. jcsd
  3. Dec 17, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    You have one equation with two unknowns. You need another equation in the same two unknowns.

    f = Ma gives you that.
    You solved for a and got 3.2667 m/s2 . (You had the wrong units.)

    What does that give you for f ?
  4. Dec 18, 2012 #3
    ooh ok i got it. thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook