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Homework Help: Playing hockey

  1. Jun 30, 2005 #1
    A hockey puck B rests on smooth ice surface and is struck by a second puck A, which was originally traveling at 40.0 m/s and which is deflected 30.0° from its original direction. Puck B acquires a velocity at a 45.0° angle to the original direction of A. The pucks have the same mass. Compute the speed of each puck after the collision.

    Va2 =speed puck A after collision
    Vb2 = speed puck B after collision

    So far my work is:

    Since the momentum is conserved for both components I did this.

    Va2 = (40-Vb2*(cos(45) )/cos(30)

    Vb2 = (-Va2*sin(30) )/sin(45)

    Okey. I have 2 unknowns I need to find and 2 equations.
    When I compute these 2 I don't get the right answer.

    The right answer is Va2=29.3 and Vb2=20.7

    Could someone please give me a hint to what I'm doing wrong.
  2. jcsd
  3. Jun 30, 2005 #2


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    It's probably that minus sign in your second equation. Your velocity variables are both positive.
  4. Jun 30, 2005 #3


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    I got
    Va2 = (Va1 - Vb2cos45)/cos30
    Vb2 = Va2sin30/sin45

    So, I have no - (minus) in the latter equation, as the pucks, naturally, deflect in different directions.

    With these equations I get the correct answers.

    EDIT: I'm a slow typer.
  5. Jun 30, 2005 #4
    Thanks guys. :approve:
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