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: Angular Velocity of Earth

  1. Nov 13, 2011 #1
    1. The problem statement, all variables and given/known data
    Find the hours it takes for the earth to rotate in one day if the mass is reduced to 2/3 of its original mass and it is shrunk to 3/4 its original size.

    2. Relevant equations


    3. The attempt at a solution

    To start this problem I assumed that angular momentum is conserved from the original earth to the final earth. This means the angular velocity must change in order for the angular momentum to remain unchanged. So used this equation: Iearthωearth=Ikωk and got that Iearthωearth/Ikk. I plugged in the data and got that the new speed is twice the speed of the original earth. This would mean the earth rotates about 12 hrs a day. This SEEMS wrong but I am not sure.

    Note: k is the final earth.
    Last edited: Nov 13, 2011
  2. jcsd
  3. Nov 13, 2011 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I would be inclined to do this in two steps. First assume that the mass changed as a result of density, for a start - giving an identical 'size' of sphere as it started off with. That gives a new I. (2/3 of the original) But I can't see how this is relevant to the final answer, actually, because you could have two, spheres rotating around a common axis and each sphere could reduce in size, giving the same answer.
    Then conserve angular momentum for the new reduced radius to find the new angular velocity. I is proportional to radius squared so new I is 9/16 of original. this gives a day length of 24X9/16 = 18hours

    Buit the question is SLOPPY because what does "size" mean? Volume or radius?
  4. Nov 13, 2011 #3
    OKay, well I didn't copy the exact questions...Here: Suppose the Earth were to suddenly shrink to 3/4 of its initial radius and 2/3 of its initial mass. What would the duration of one day be?
  5. Nov 13, 2011 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Haha. Sloppy student not sloppy question.
    I guess the 9/16 is what you're after then.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook