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Homework Help: Hockey Puck Momentum Problems

  1. Sep 10, 2007 #1
    1. The problem statement, all variables and given/known data

    1. Two magnetic hockey pucks sliding on a flat plane of frictionless ice attract each other, changing each other's direction of motion as they pass. We can (or cannot) realistically apply conservation of momentum to this situation because

    a. The system of interest floats in space
    b. The pucks are a functionally isolated system
    c. We can treat the interaction as a collision
    d. Conservation of momentum does not apply here

    2. Two hockey pucks are sliding on a flat, horizontal, frictionless sheet of ice. One puck has twice the mass of the other. Initially, the light puck is moving east at 3 m/s while the heavy puck is moving west at 2 m/s. The pucks collide and stick together. The final velocity of the joined pucks is

    a. Eastward
    b. Westward
    c. Zero
    d. Impossible to determine
    e. Some other direction

    2. Relevant equations

    3. The attempt at a solution

    1. b? I don't really understand conservation of momentum.
    2. d. because we don't know mass?
    Last edited: Sep 10, 2007
  2. jcsd
  3. Sep 10, 2007 #2
    1) Perhaps you should state your ideas/guesses why the respective statements do/can/do not apply for each letter.
    2) The masses are X and 2*X with a realistic chance that X>0 kg. See if that helps you (hint: Conservation of momentum :wink:).
  4. Sep 10, 2007 #3
    Alright, let me give a try.

    a. because of the ice, the pucks aren't floating in space
    b. because the surface of the ice is frictionless, they are probably isolated, which is why momentum is conserved
    c. collision doesn't seem to make sense without friction
    d. false, because we can treat the two pucks as an isolated system; the external interactions are zero

    therefore, B?

    p1 = 2 * -3m/s = -6 m/s
    p2 = 1 * 2 m/s = 2 m/s

    -6 + 2 = 4

    therefore, a eastward?
    is momentum conserved?
  5. Sep 11, 2007 #4
    a) It's a possible statement. I am not familiar with multiple-choice questions in physics and don't know their style. For me, the statement would possibly by incorrect because nowhere it was said they did - so it could be true or not.
    b) Yes. I would add the intermediate step "frictionless => no external force (e.g. from the ice) on the pucks) => isolated system without external force => conservation of momentum".
    c) Collision can absolutely happen without friction. Collision means collisions between the pucks, not between puck and ice. You can collide a spacecraft with an asteroid (where in reality you'd rather put some effort into not making that happen). Yet, the question is "does it make sense to treat the interaction as a collision?". Going back to space to find an example you could consider the system of earth and sun as an analogy. Both travel through space quite frictionless and both attract each other. Would you treat the system of earth and sun as a collision? Why or why not?
    d) agreed.

    The calculation attempt (i.e. determining p = p1 + p2 and then obtaining the direction from p) is correct as such. Momentum seems to be conserved and atm I couldn't even think of a collision where that wouldn't be the case. Two notes:
    - You mixed up the masses (or equivalently the velocities).
    - -6+2 = -4, not +4 :rolleyes:
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