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: Pivoting Stick

  1. Nov 5, 2007 #1
    [SOLVED]Pivoting Stick

    1. The problem statement, all variables and given/known data
    A uniform stick of mass M = 1.3 kg and length L = 2.3 m is pivoted at one end. It is held horizontally and released. Assume the pivot is frictionless. Find the angular acceleration (in rad/s) of the stick immediately after it is released.

    Continuation: Find the magnitude in newtons of the force Fo exerted on the stick by the pivot immediately after it is released.

    http://img134.imageshack.us/img134/1377/prob06azg9.gif [Broken]

    2. Relevant equations

    3. The attempt at a solution

    I set T=Ia


    It tells me that is the wrong answer

    For the second part, shouldn't the force be 0?(0is not the right answer)
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Nov 5, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Not OK. What's the rotational inertia of a thin rod about one end? (When analyzing rotational motion, you can't treat an extended body as if its mass were concentrated at its center of mass.)
  4. Nov 5, 2007 #3
    Ah, thanks, using I =1/3mL^2 I get the correct answer.

    So for the second part is the force=9.81(1.3)-6.39(1.15)(1.3)

    or in other words the force on the rod it it was not connecting to a pivot point, minus the acceleration of the rod while it is connected to the pivot point?

    EDIT: i just put in my answer as caclulated above and it was correct.

    thanks Doc Al for the Help
  5. Nov 5, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Cool. The way to think of the second part is just to apply Newton's 2nd law to the vertical direction:
    F - mg = ma
    (where a is the acceleration of the center of mass, which you can figure out from the first answer)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook