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: Calculate Inertial tensor of 5000 points

  1. Jan 21, 2013 #1
    1. The problem statement, all variables and given/known data

    I was given a sample of 5000 points from an ellipsoidal blob (in 3D) that has some orientation. Assuming that each point has equal mass, I was asked to calculate the inertia tensor of this blob, then find the principal axes to determine the orientation of this blob.

    2. Relevant equations

    R = [itex]\frac{1}{M} [/itex][itex]\sum m_{i}r_{i}[/itex]
    where M is the sum of the masses of every point
    R is the center of mass

    I = - [itex]\sum m_{i} [r_{i} - R][r_{i} - R][/itex]

    3. The attempt at a solution

    Since all the mass for every particle is the same, the formula for center of mass becomes,

    R = [itex]\frac{1}{5000} [/itex][itex]\sum r_{i}[/itex]

    however I'm not sure how to find the inertial tensor since I don't know the mass of each particle.

    Any help would be much appreciated, thanks!
  2. jcsd
  3. Jan 21, 2013 #2
    Perhaps you do not know the mass of each particle (in your line with the CM, you seem to assume that every mass is of mass "m", therefore, M = 5000*m). Why not scale out this unknown m, adn compute I/m? I bet you know all the r_i from your initial data set, and it seems like you can compute R. Is there a more complicated thing stopping you, even in light of this simple suggestion?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook