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Homework Help: Need help on Collision problem

  1. Nov 9, 2004 #1
    After a completely inelastic collision, two objects of the same mass and initial speed are found to move away together at 1/2 their initial speed. Find the angle between the initial velocities of the objects.

    Solution is here http://www.physics.sfsu.edu/~senglish/APphysicswebsite/collisions/solutions%20on%20collisions/17.pdf [Broken]

    My first atempt to solve this problem was drawing my diagram so that both objects converge towards the positive y-axis instead of the positive x-axis as seen in solution.

    v=initial velocity vf= final velocity
    so the x-comp mv cos A - mv cos B= 0
    y-comp mv sin A + mv sin A = 2m vf

    Well it didn't work, so i gave up and redrew my diagram like the one in the solution and solved it that way. I'm starting to conclude that my original idea solved for a different angle for the problem.

    What did i do wrong in solving the problem with my original diagram with the objects converging towards the positive y-axis
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Nov 9, 2004 #2
    The x component momentum is

    mv cosA - mv cosB = 0


    mv cosA - mv sinA
  4. Nov 9, 2004 #3
    typo :mad:

    anyway your right

    my calculations so far
    x-comp mv cos A - mv cos B= 0 A=b but then its 0=0 which doesn;t really help

    so looking at the y-comp
    y-comp mv sin A + mv sin A = 2m vf

    2mv sinA = 2mv 1/2v

    sin A = 1/2
    A= 30 degrees x2 60 degrees

    The answer is suppose to be 120 degrees, but then my answer plus the actual is 180 degrees. Therefore i think the angle i solved for was the outside angle, but why :confused:
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