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Homework Help: Finding angular velocity after asteroid collision

  1. Dec 1, 2012 #1
    1. The problem statement, all variables and given/known data
    In the figure a spherical non spinning asteroid of mass M = 4E16 kg and radius R = 1.5E4 m moving with speed v1 = 2.4E4 m/s to the right collides with a similar non spinning asteroid moving with speed v2 = 5.9E4 m/s to the left, and they stick together. The impact parameter is d = 1.4E4 m. Note that I_sphere = 2/5*M*R^2.
    After the collision, what is the velocity of the center of mass and the angular velocity about the center of mass? (Note that each asteroid rotates about its own center with this same angular velocity. Assume that the asteroids move in the x-y plane, and that the asteroid of speed v1 moves in the positive x direction.)

    2. Relevant equations
    L_A,f = L_A,i
    L_Rot = Iω, L_Rot = r1cm x p1 + r2cm x p2
    L_tran = r_a,cm x p_tot

    3. The attempt at a solution
    I have found v_cm. Now I am looking for the angular velocity. I have considered both asteroids to be included in the system and the surroundings to be nothing. Because there are no surroundings dL_A/dt is 0. Therefore, L_Af = L_Ai. If this is the case, the angular velocity should be the same in the initial and final conditions. Is this true?

    I then said that L_rot = Iω, and L_rot = r_cm1 X p_1 + r_cm2 X p_2. So I had the following equation:

    Iω = r_cm1 X p_1 + r_cm2 X p_2.

    Then I solved for ω,

    ω = (r_cm1 X p_1 + r_cm2 X p_2)/I.

    Is this correct reasoning? Are the initial and final angular velocities different? Should I consider a system with just one asteroid instead?

    Thank you
  2. jcsd
  3. Dec 1, 2012 #2
    Use the conservation of angular momentum equation.
  4. Dec 1, 2012 #3
    Okay. If I use the conservation form I will need to change my system, correct? So now my system is just one asteroid and my surroundings are the other asteroid.

    Can I still use the equations I included or will I need to use another definition L_rot and L_trans?

    And to add to my many questions I have, I watched a lecture video on youtube with a similar problem.

    The professor explained that the center of mass changed throughout the collision. Will I need to take this into account?
    Last edited by a moderator: Sep 25, 2014
  5. Dec 2, 2012 #4


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    No, it should be simplest and safest to take the mass centre of the system as reference.
    You didn't define the variables in your equations, so I cannot say whether they're right.
    What is the moment of each about the common mass centre before collision?
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