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Homework Help: Elastic collision of a ball

  1. Oct 17, 2006 #1
    Ball #1 moving at a speed of +4.4 m/s along x-axis collides with an identical ball (#2). The initial velocity of ball #2 is equal to zero. Assume that this is a perfectly elastic collision.

    I know that Pbefore = Pafter - but since I'm not given the mass of either ball how am I to know what their velocities are after they hit?
  2. jcsd
  3. Oct 17, 2006 #2
    I have another problem that asks a similar question, no mass given:

    Two balls of equal mass approach the coordinate origin where they collide. Assume that this is a perfectly elastic collision. Before collision, one ball moves along the y-axis at +4.5 m/s and the other ball moves along the x-axis at +4.4 m/s. After they collide, one of the balls moves along the x-axis at +1.2 m/s.

    Find the x-component of velocity of the other ball after the collision =

    How do I get started on these problems? Thanks
  4. Oct 17, 2006 #3
    Identical = equal masses
  5. Oct 17, 2006 #4


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    Hint 1: use the facts that momentum and kinetic energy are conserved, if the collision if perfectly elastic.

    Hint 2: when there are more directions, use vectors, and then deal with their components, in order to keep things more clear.
  6. Oct 17, 2006 #5
    ok, for the first question; since they're equal mass and hit directly, total energy is transferred to ball 2. Vball1 = 0, Vball2 = 4.4

    Now, for the second problem I'm supposed to be using vectors to find the x and y components. I will attempt this one later - thanks.
  7. Oct 17, 2006 #6


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    Your notation is inconsistent - what exactly do you need to find in 1) ? The velocity of the second ball after the collision?

    Edit. Actually, if the solution is correct, It doesn't matter.
    Last edited: Oct 17, 2006
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