1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Rotational Kinematics - Angular velocity

  1. Jan 18, 2010 #1
    1. The problem statement, all variables and given/known data
    http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys211/oldexams/exam3/fa07/fig19.gif [Broken]
    A dumbbell consists of a slender rod of negligible mass and length L = 1 m and small steel balls attached to each end with mass 1 kg and 2 kg, respectively. It is pivoted at its center about a horizontal frictionless axle and initially held in place horizontally. The dumbbell is then released. What is the angular velocity of the system when the rod is vertical?

    2. Relevant equations

    3. The attempt at a solution

    I have figured out the angular acceleration upon release is 6.5rad/s^2 and the angular acceleration is zero at vertical (because of torque = 0 at vertical)
    But from there, i got stuck and i don't know how to link this acceleration into the velocity..

    Please could someone help me out here?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 18, 2010 #2


    User Avatar
    Homework Helper

    It is easier to apply conservation of energy. When the rod is vertical, the centre of gravity is lower than in the horizontal position, the difference is equal to the kinetic energy.

  4. Jan 20, 2010 #3
    I have tried to use the hint you have given..

    Iw^2/2 = mgh

    h = (2/3 - 1/2) ; length of center of gravity from pivot when it's at vertical

    And i took moment of inertia I as 3*(2/3-1/2)^2

    But when i calculated this, it was wrong...

    I think RHS of the eqn is correct...

    Is something wrong in LHS of the equation?

    Please help me out....:(

    I would really appreciate it .
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Rotational Kinematics Angular Date
Rotational Motion of a thin rod about a pivot Nov 10, 2016
Rotational: angular momentum Oct 17, 2016
Rotational Kinematics Apr 9, 2016
Coin on a turntable Mar 10, 2015
Angular kinematics: determining satellite orbital radius Dec 22, 2014