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Homework Help: Finding velocity of elastic collision with angles

  1. Nov 17, 2015 #1
    1. The problem statement, all variables and given/known data
    A red ball and blue ball are sliding on a frictionless surface, each ball has a mass of 40g. They collide in an elastic collision. Initially the red one is at rest and the blue one is traveling in the x direction with a speed of 4 m/s. After the collision the blue one is traveling in the direction +35 degrees, with the red one traveling in the direction ‐55 degrees. Using conservation of energy and momentum find the speed of each puck after the collision

    2. Relevant equations
    Conservation of momentum: M1Vi1+ M2Vi2 = M1Vf1 + M2Vf2
    Conservation of kinetic energy: 1/2MVi12 + 1/2MVi22 = 1/2MVf12 + 1/2MVf22

    3. The attempt at a solution
    Started using conservation of kinetic energy and got 4 = Vf1 + Vf2
    Wasn't sure how to isolate V plugged it into Conservation of momentum factored out m setting Vf1 + Vf2 to 4
    0.04kg(4m/s)+ 0 = 0.04kg (4)
    It didn't seem to do anything and i'm not exactly sure where i'm supposed to use the angles
  2. jcsd
  3. Nov 17, 2015 #2

    Mister T

    User Avatar
    Science Advisor
    Gold Member

    Good start.

    To use the conservation of momentum equation you have take into account that it's a vector equation. Which means it's really two equations. One in the x-direction, one in the y-direction. That's where those angles will enter.
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