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Homework Help: Elastic Collision of two marbles

  1. Jun 17, 2012 #1
    1. The problem statement, all variables and given/known data
    A 45g marble collides with an identical stationary marble, and continues at 55 degrees to it's original direction. The collision is perfectly elastic. What is the angle between the direction taken by the target ball and the original direction of the incident ball?

    2. Relevant equations

    3. The attempt at a solution
    Since elastic collisions in 2 dimensions result in 2 perpendicular velocities after the collision, I'm getting the feeling that the answer is simply 90-55=35 degrees. Is this right?
  2. jcsd
  3. Jun 17, 2012 #2


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    Yes. The final velocities will be perpendicular if all of the following conditions are met

    (1) the collision is elastic
    (2) the two masses are equal
    (3) one of the masses is initially at rest
    (4) the collision is not a direct, head-on collision
    (5) the objects do not pick up spin in the collision

    Are any other conditions required? (Don't know if I'm overlooking something. At first I intended to list only the first three; the other two came to me while typing.)
  4. Jun 17, 2012 #3
    Elastic collision and identical mass with one at rest.
    Conservation of energy

    This is a right angle triangle.


    Head on collision.
    Conservation of momentum.
    1.Ball#1 stop and ball#2 moves.
    2.Ball#2 stays and Ball#1 continue moving. Ball#1 like ghost, passing through. Unphysical.
    Maybe in billions years as I've seen on Discovery Channel, Quantum World.
    Last edited: Jun 18, 2012
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