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

Conservation of angular momentum and energy

  1. Nov 29, 2004 #1


    User Avatar
    Homework Helper

    if you have a spinning body, and then its shape changes so that the moment of inertia changes, what happens? If angular velocity is conserved, since theres no net torque, then it spins a different speed, but the same direction. But then has the rotational kinetic energy changed? And if so, does this mean the excess kinetic energy must take the form of translational motion? Does it matter if the change in shape was symmetrical?
  2. jcsd
  3. Nov 29, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Consider a very simple example - two weights connected by a cord spinning around their common center.

    If you pull the weights in, it will require work to do so. (The easiest way to see this is to go to a non-inertial frame that's corotating with the weights, and consider the centrifugal force). Similarly, if you let the weights go out, work will be generated and must be dissipated (perhaps by friction).

    Energy and angular momentum will both be conserved.
  4. Nov 29, 2004 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    The (instantaneous) angular velocity may well change its direction, since (for one) we cannot assume that the shapechange does not affect the inertial tensor (with respect to the C.M).
    The angular momentum, remains, however, constant.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook