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

Rod in freefall

  1. Nov 22, 2012 #1
    This has been brought up numerous times but I don't really understand it. Consider a rod in freefall.
    If you put your coordinate frame in the center of mass of the rod, there will be no torque around it and the rod as a whole will follow a straightline down. But now put a coordinate frame on one of the end points. Apart from the gravity pulling down on the rod as a whole, there will now be a net torque on the rod (because gravity acts in the center of mass).
    What goes wrong with this picture, because clearly the rod doesn't rotate!
     
  2. jcsd
  3. Nov 22, 2012 #2
    the rod is a rigid body. the other side of the rod also has an equal torque, and due to rigidity, will be in the opposite direction.
     
  4. Nov 22, 2012 #3

    Doc Al

    User Avatar

    Staff: Mentor

    The problem is that you are you using an accelerating point as your 'pivot'. Torque about such an accelerating point does not simply equal the rate of change of angular momentum, unless that point happens to be the center of mass.

    See my post in this thread: https://www.physicsforums.com/showthread.php?p=4097976
     
  5. Nov 22, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    The only external force acting on the rod is gravity.
     
  6. Nov 22, 2012 #5

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    In an accelerated frame that falls with the rod, there is an inertial force upwards:
    http://en.wikipedia.org/wiki/Fictitious_force#Acceleration_in_a_straight_line

    The inertial force cancels gravity at any point of the rod. Regardless if the origin is in the center or the end: There is no net force on any part of the rod in such a frame, and thus no torque.
     
  7. Nov 22, 2012 #6

    Doc Al

    User Avatar

    Staff: Mentor

    That's a good way to look at it (and probably more straightforward).

    The extra terms (beyond the torque due to external forces) you get when you calculate dL/dt about an accelerating point are equivalent to introducing that inertial force.
     
  8. Nov 22, 2012 #7

    mfb

    User Avatar
    Insights Author
    2015 Award

    Staff: Mentor

    In the frame of one of the ends, the rod gains angular momentum - by falling linearly to the floor.
    The torque is present, and required for a linear motion downwards in this frame.
     
  9. Nov 22, 2012 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Viewed from an inertial frame, the rod gains angular momentum. But in the accelerating frame of one of its ends, it does not.
     
  10. Nov 23, 2012 #9

    D H

    User Avatar

    Staff: Mentor

    Yes, there is a torque. It's the same phenomenon that causes spaghettification. Taking advantage of, or otherwise dealing with, gravity gradient torque is an important concept for satellites in low Earth orbit.
     
  11. Nov 23, 2012 #10

    K^2

    User Avatar
    Science Advisor

    Problem assumes uniform gravitational field. There are no tidal forces. Doc Al and A.T. have it covered from both perspectives.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Similar Discussions: Rod in freefall
  1. Freefalling force (Replies: 12)

Loading...