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Homework Help: Velocity and Direction of a System after Inelastic Collision

  1. Mar 26, 2016 #1
    1. The problem statement, all variables and given/known data
    A hockey player with a mass of 30.0-kg is initially moving 2.00-m/s to the right. He catches on the stick a puck initially moving at 35.0-m/s at an angle of 60 degrees. The puck's mass is .18 kg and the player and puck form a single object for a few seconds. (A) Determine the direction angle and speed of the puck AND skater after the collision. (B) Was this collision elastic or inelastic? Prove it with numbers.
    SORRY FOR THE TERRIBLE DRAWING... kind of in a hurry.

    2. Relevant equations

    Pbefore = Pafter >>> MV + mv = (M+m)V
    Conservation of kinetic energy and 1/2mv2

    3. The attempt at a solution
    The answers are given:
    (A): 1.89-m/s , 5.48 degrees
    (B): inelastic because K < K0

    I understand B, mostly. You just find the kinetic energies of the systems before and after and compare them. If they don't match up, it has lost some KE and you know it's inelastic. But, part A really messes me up. I tried just plugging in the numbers for my first relevant equation, then I realised I probably needed to find the components of each and then find the resultant ones. Any help would be amazing.
  2. jcsd
  3. Mar 26, 2016 #2


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    First, let me say this is a terrible problem. A hockey player is not an inert object that gets knocked around by a puck. He could stop a puck without moving if he wanted to. It's not that different from stopping a soccer ball without getting knocked backwards.

    That said, if you imagine the player has no grip on the ice, you need to use momentum conservation in two directions for part A.
  4. Mar 26, 2016 #3


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    You cannot use energy because you do not know how much was lost, so you need another equation. You are told that the two form a single object for a while. What equation does that give you?
    With regards to @PeroK's critique, I offer a slightly different view. A reasonable supposition is that a skater has negligible friction in the direction of travel, but will never slide orthogonally to that. On that basis, you could use conservation of momentum in the skater's original direction, but assume the skater's direction does not change. Whether that is what the question intends I cannot tell.
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