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!

Torque about P (Cross Product?)

  1. Feb 11, 2013 #1
    1. The problem statement, all variables and given/known data

    2mnn6l2.png


    2. Relevant equations

    The book says that

    Torque = the length of the moment arm X the magnitude of the vertical component of F
    = r X F

    3. The attempt at a solution

    My problem is that I don't have any vectors to do a cross product. Instead I tried to just multiply what I know:

    50lbs x 9in x sin60° ≈ 389.71ft-lb

    which is far from the correct answer, 32.5ft-lb. The problem seemed simple enough so am I making some small mistake or going about this completely wrong?
     
  2. jcsd
  3. Feb 11, 2013 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Just because you aren't given any explicit vectors doesn't mean you can't formulate your own.

    Looking at the problem, suppose that we set up an x-axis along the wrench.
    In this case, vector r is (9,0) inches, with the emphasis on inches

    The force vector F is going to be (50 cos (-60), 50 sin (-60)) lbs.

    You can then evaluate r x F in the usual manner

    Remember: in x lbs is not equal to ft-lbs
     
  4. Feb 11, 2013 #3
    Ah, you're right! Thank you, that makes perfect sense.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Torque about P (Cross Product?)
  1. Cross product torque (Replies: 3)

Loading...