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!

Time period of oscillation of a physical pendulum and spinning disk

  1. Jul 15, 2013 #1
    1. The problem statement, all variables and given/known data

    Find the period of a pendulum consisting of a disk of mass M and radius R fixed to the end of the rod of length l and mass m. How does the period change if the disk is mounted to the rod by a friction less bearing so that it is perfectly free to spin? The centre of the disk is attached to the rod.

    3. The attempt at a solution

    I can find the position of the centre of mass of the system, the torque due to gravity and the moment of inertia of the system about the pivot. From this quantities i shall form the equation of motion and thus i can find the time period.

    Now i see no reason why the period will change if the disk is free to spin. Firstly i think the disk will not spin since there is no torque acting on it. Both gravity and the force exerted by the rod pass through the bearing. If it also spins the position of centre of mass doesn't change. The moment of inertia of the system too remains unchanged.

    Am I correct since I am feeling that I missed something?
  2. jcsd
  3. Jul 15, 2013 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    You are correct that the disk will not rotate if the disk is free to spin.

    But, how would you express the disk's angular momentum about the pivot of the pendulum
    (a) if the disk rotates with the pendulum?
    (b) if the disk does not rotate?
    Last edited: Jul 15, 2013
  4. Jul 16, 2013 #3
    If the spin velocity is constant then the time period will be the same. But if the spin has some acceleration then the oscillation may be damped or lose it's oscillatory character. Am i correct?
  5. Jul 16, 2013 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    You have two situations. The first is where the disk and rod form one rigid body so that the disk rotates with the pendulum as the pendulum swings. The second is where the disk does not rotate about its center as the pendulum swings. See picture. The red dot is just a reference mark painted on the disk.

    Attached Files:

    Last edited: Jul 16, 2013
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - period oscillation physical Date
Period (T) of a pendulum Jan 29, 2018
Period of Oscillation for mass on a spring. Jan 23, 2018
Time period in harmonic oscillation. Mar 31, 2016
Period of Vibration of Torsion Pendulum Dec 12, 2015
How to find wavelength of wave? Oct 8, 2015