Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Very difficult Hockey stick puck problem. Please help

  1. Dec 10, 2008 #1
    1. The problem statement, all variables and given/known data

    Please help. Have no clue?????
    A hockey stick of mass m_s and length L is at rest on the ice (which is assumed to be frictionless). A puck with mass m_p hits the stick a distance D from the middle of the stick. Before the collision, the puck was moving with speed v_0 (note: a v with subscript zero) in a direction perpendicular to the stick, as indicated in the figure. The collision is completely inelastic, and the puck remains attached to the stick after the collision.

    2. Relevant equations
    i suppose v_f=v_0+m_p/m_s+m_p
    but it gives a wrong answer.
    As for the others, i have no clue. please help..

    3. The attempt at a solution
  2. jcsd
  3. Dec 10, 2008 #2
    All you've done is told us a story about a hockey puck. What is the question?

    (Unless I'm blind)

  4. Dec 10, 2008 #3
    thats why i called myself madman, innit.
    anyway, here are the question, if any one can have a go at them, id be greatful..
    1.Find the speed v_f of the centre of mass of the stick+puck combination after the collision.
    Express v_f in terms of some of the following quantities: v_0, m_p, m_s, and L.
    2.After the collision, the stick and puck will rotate about their combined centre of mass. How far is this centre of mass from the point at which the puck struck? In the figure, this distance is (D-b).
    3. What is the angular momentum L_cm of the system before the collision, with respect to the centre of mass of the final system?
    Express L_cm in terms of the given variables (do not use b).
    4. What is the angular velocity omega of the stick+puck combination after the collision? Assume that the stick is uniform and has a moment of inertia I_0 about its centre. Your answer for omega should not contain the variable b
    5. Which of the following statements are TRUE?

    1) Kinetic energy is conserved.
    2) Linear momentum is conserved.
    3) Angular momentum of the stick+puck is conserved about the centre of mass of the combined system.
    4) Angular momentum of the stick+puck is conserved about the (stationary) point where the collision occurs.
    i told you it was tough.
  5. Dec 10, 2008 #4
    It looks like since you arn't give any actual values in the question, you can only answer the question in terms of the variables given.

    Try using the formula for completely inelastic collisions.

    M1v1+m2v2 = (m1 + m2) V'
    OR (mass 1)(initial velocity of mass 1) + (mass 2)(initial velocity of mass 2) = (Mass 1 + mass 2)(Final velocity of both masses as one)

    Just try plugging in what you're given.
  6. Dec 10, 2008 #5
    We had this question on the Mastering Physics online assignment thing last week ;-)
  7. Dec 10, 2008 #6
    Thats why I wan't to see an attempt at a solution first.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook