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Homework Help: Inelastic collision Two angles and final velocities

  1. Sep 29, 2015 #1
    1. The problem statement, all variables and given/known data
    Consider an inelastic collision between two blocks on a horizontal plane. Block M1 is moving with velocity Vo and collides with block M2 which is at rest. During the collision a fraction Q of the original kinetic energy is lost. It is observed that M1 is deflected by an angle theta (above the x axis), and M2 is deflected at an angle phi (below the x axis). After the collision M1 is moving to the right.

    Find the angle phi, and the final velocities of the blocks.


    2. Relevant equations
    Inital Momentum = Final Momentum (M1iVo+M2iVo =M1fVf+M2fVf)

    KEf = (1-Q)KEi

    *I'm sure there are more but this is what I have from class.

    3. The attempt at a solution

    Finding the final KE was easy enough using the above equation: KE inital =100 KE final =80

    Then I tried to split them up in the x and y components. They both start with zero in the y direction and only M1 has initial momentum in the x direction. In the Y direction after the impact I got some sin and cos directions
    So M1iVo = M1V1fCos(theta)+M2V2fcos(phi)+M1V1f-M2V2fsin(phi) (negative because it is below the x axis)
    If that is right (big if) I have no clue where to go from here. I really don't know how to turn the final kinetic energy into two different objects and I REALLY don't know how to find that phi angle. I'm at a complete loss.

    This is my first post here so hopefully formatted the question correctly. This stuff is really stressing me out. Every time I think that I've stumbled upon something useful online, it just seems way to simple, which is pretty much a guarantee that it can't help me. Our homework consists of just a few problems over the course of a week, and when he shows examples in class my professor fills several chalkboards full of calculations. So anything that is a quick answer must be wrong.

    I don't want the answer, I just need a nudge in the right direction of how to even set up the problem. I have not found a single thing online that can help with these intermediate inelastic collisions with two objects of a different mass.

    This forum is my last hope.

    *Way too many words, not nearly enough physics. Sorry

    Attached Files:

  2. jcsd
  3. Sep 30, 2015 #2


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    you seem to have a basic misapprehension regarding momentum.
    Momentum is a vector, and your 'relevant equation' for its conservation should be interpreted in that light.
    When resolving into separate X and Y directions, that gives you two momentum conservation equations, one for each direction. But you appear to have collapsed them into one scalar equation.
  4. Sep 30, 2015 #3
    Thank you for bringing this up, because it seemed wrong when I was doing it. I guess I just got confused because there is no inital momentum in the y direction.

    Pix=Pfx = M1iVo = M1V1fCos(theta)+M2V2fcos(phi)
    Piy = Pfy = 0 = M1V1fsin(theta)-M2V2fsin(phi)

  5. Sep 30, 2015 #4


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