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: Torque Equalibrium

  1. May 7, 2007 #1
    Problem: A 40kg, 5.0m long beam is supported by, but not attached to, the two posts in the figure. A 20kg boy starts walking along the beam. How close can he get to the right end of the beam without it falling over?
    http://student.ucr.edu/~meliz003/phy.JPG [Broken]

    My answer:
    \tau_net = (F_1*x_F_1)-(F_Beam*X_Beam)-(F_Boy*X_Boy)
    Since, \tau_net = 0

    0 = (F_1*x_F_1)-(F_Beam*X_Beam)-(F_Boy*X_Boy)
    0 = (40*3)-(40*2)-(20*X_boy)

    Comes out to X_Boy = 2?
    So the boy has to step at the very edge of the beam for it to start falling over?
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. May 7, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Looks like you are treating the beam as have two parts, which is OK. But:
    (1) 40 kg is the mass of the entire beam, not each piece;
    (2) The weight of an object acts through its center of mass.
    You need to redo those torque calculations.

    Even better is to treat the beam as one piece--that way you only have two torques to worry about. Where is its center of mass with respect to the pivot?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook