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: Torques and a Traffic Light

  1. Apr 2, 2012 #1
    1. The problem statement, all variables and given/known data

    A traffic light hangs from a pole as shown in the figure . The uniform aluminum pole AB is 7.50 m long and has a mass of 12.0 kg. The mass of the traffic light is 21.5 kg.

    2. Relevant equations

    clockwise torques = counterclockwise torques

    3. The attempt at a solution

    This may seem silly but my professor said to use the sin of 53° but one of the tutors told me to use the sin of 37 ° (which ended up being incorrect) because of the parallel line theorem. Could someone explain to me why the tutor was mistaken?

    (12.0 kg)(3.75m)(9.8 m/s^2)(sin 53°) + (21.5 kg)(9.8 m/s^2)(7.50 m)(sin 53°) = F * 3.80 m

    Attached Files:

  2. jcsd
  3. Apr 2, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The tutor may have just erred in the geometric or trigonometric relationships. The moment, M, of a force, F, about a point, is the cross product of the position vector, r, times the force , that is, M = r X F = (r)(F)(sin theta), where theta is the angle in between the force and position vectors.
  4. Apr 2, 2012 #3


    User Avatar

    Staff: Mentor

    Is it possible that the tutor suggested the use of cos(37°) rather than sin(37°)?
  5. Apr 2, 2012 #4
    Thanks you two! No, he definitely said (sin 37°).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook