Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The pen revolves

  1. May 28, 2007 #1
    Suppose that I am holding a pen in my hand and then i move my hand in such a way that the pen starts revolving around my index finger. It rotates in such a way that it seems that my finger is the center of the circular path(the path, through which the pen is revovling).
    I hope I have described the problem properly. Now can you tell me why the pen revolves around my finger without any support and what are the componets of force acting on it
  2. jcsd
  3. May 31, 2007 #2
    well, your pen could be rotating. If you put a pen on a table, you can apply torque to the pen so that it will rotate around its centre of gravity. The same will happen with your pen around your finger, because you place your finger close to the pen's center of gravity, and then apply torque to begin the rotation. You have gravitational force which is negligible since I assume you preform this pen trick rather rapidly, then plain kinetic energy and angular momentum. Which are not forces, only gravity is. I hope I understood your question correctly.
  4. May 31, 2007 #3


    User Avatar
    Homework Helper

    I can do simple pencil / spin spinning around my thumb, but my thumb is big so I start with the center of mass a bit a way from my thumb, and the center of mass approaches my thumb while the pen/pencil rotates around my thumb.

    Here's one link to a web site that includes a link to a video.


    Good baton twirlers can do similar movements on a larger scale:

    Last edited by a moderator: Sep 25, 2014
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook