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!

Homework Help: Rotational inertia cockroach

  1. Oct 4, 2007 #1
    A cockroach of mass m runs counterclockwise around the rim of a lazy Susan (a circular disc mounted on a vertical axle) of radius R and rotational inertia I with frictionless bearings. The cockroach's speed relative to the earth is v, where as the lazy Susan turns clockwise with angular speed wo. The cockroach finds a bread crumb on the rim and, or course stops. a) what is the angular speed of the lazy susan after cockroach stops. b.) Is mechanical energy conserved?

    I'm not sure how to relate the cockroach's linear speed with the angular speed of the disc......

    I'd love any help..
  2. jcsd
  3. Oct 5, 2007 #2


    User Avatar
    Homework Helper

    Use conservation of angular momentum... take clockwise positive... counterclockwise negative... what is the angular momentum of the cockroach before it stops... what is the angular momentum of the lazy Susan before the cockroach stops?

    angular momentum afterwards the roach stops equals angular momentum before...
  4. Oct 5, 2007 #3


    User Avatar
    Homework Helper

    Try and use conservation of angular momentum - assume that the LS has none when the cc gets on it.
  5. Oct 5, 2007 #4
    How do I manage to use a linear speed of (cockroach) when its spinning though? isnt rotational motion acceleration?

    It doesnt make sense to me...
  6. Oct 5, 2007 #5

    Doc Al

    User Avatar

    Staff: Mentor

    The cockroach has both linear momentum and angular momentum. Given the first and the distance from the axis, you can calculate the second. (Treat the cockroach as a point mass.)
  7. Feb 14, 2008 #6
    w= ( mRv - Iwo ) / ( I + mR^2 )
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook