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Homework Help: Momentum of Bird on Swing after takeoff

  1. Jan 7, 2009 #1
    A 94 kg fullback moving east with a speed of 9.0 m/s is tackled by a 99 kg opponent running west at 3.0 m/s, and the collision is perfectly inelastic.

    (a) Calculate the velocity of the players just after the tackle.
    ___________________ m/s to the east
    (b) Calculate the decrease in kinetic energy during the collision.
    ___________________ J

    2. eqns
    conservation of energy/momentum
    Ke=1/2 * m * v^2

    3. The attempt at a solution
    a. 99*3=297

    I AM CLUELESS? I GOT 2695.6696
  2. jcsd
  3. Jan 7, 2009 #2


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    How did you get that number for part b?

    If the collision is perfectly inelastic, then the two objects stick together. Since you have the velocity after impact, you can get the kinetic energy after the impact.
  4. Jan 7, 2009 #3
    Last edited by a moderator: May 3, 2017
  5. Jan 7, 2009 #4
    what exactly should i do?
  6. Jan 8, 2009 #5


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    You have not correctly calculated the initial kinetic energy of the the 2 players. Before the collision, each has its own mass, speed, and KE. KE is a scalar quantity. Just add them up. And your title is for the birds. :wink:
  7. Jan 8, 2009 #6


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    I don't follow your thinking for the collision. We can't talk, so we have to write very clearly. Does your teacher begin with a general statement for a collision problem such as:

    momentum before collision = momentum after collision

    This is most helpful and worth writing! You put the formulas on the next line (one mv for each moving object or stuck-together object), then the numbers on the next and finally the answer on the next line.
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