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Homework Help: Collision of 2 balls in air

  1. May 12, 2012 #1
    1. The problem statement, all variables and given/known data
    Two elastic balls of mass m1 = 0.3kg and m2 = 0.2kg are hanging on a thin rope so that their centre of gravity is on the same height. ball 1 goes out of balance raising it up to h0 = 0.05m
    Then letting it go hitting ball 2. Calculate the maximum height of both balls reaching after the collision.

    m1 = 0.3kg
    m2 = 0.2kg
    h0 = 0.05m
    g = 10m/s2
    and since m2 is not moving it's V2 = 0m/s

    h1 = ?
    h2 = ?

    2. Relevant equations
    I'm not 100% sure,
    but I do know I am supposed to use the elastic collision equation:
    m1*V1+m2*V2 = m1*V1'+m2*V2' since V2 = 0m/s it's:
    m1*V1 = m1*V1'+m2*V2'

    And I'm guessing kinetic energy:
    ΔEk = 4 * m1*m2/(m1+m2)2 * Ek1
    Ek1 = 1/2*m1*V12

    3. The attempt at a solution
    First I made a drawing of it to understand it better.
    A 2 ball Newton cradle.
    I just don't see how am I supposed to collide those 2 equations and then the height.
    And what am i supposed to do with g = 10m/s2 ??
    Last edited: May 12, 2012
  2. jcsd
  3. May 12, 2012 #2
    By raising the ball to a ht. of ho, you give it a potential energy change of mg(ho), which will be converted to Kinetic energy when the ball comes down
    mgho = 1/2m(v1^2), thus u get v1.

    Find final velocities. use potential energy again after that.
  4. May 13, 2012 #3
    I've managed to finish the problem after your help. Thank you!

    h1 = 0.0125m
    h2 = 0.072m
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