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Collision problem

  1. May 29, 2014 #1
    1. The problem statement, all variables and given/known data
    A 90-kg halfback running north with a speed of 9 m/s is tackled by a 120-kg opponent running south with a speed of 3 m/s. If the collision is perfectly inelastic and head-on, calculate. (a) the velocity of the players just after the tackle and (b) the total energy lost as a result of the collision.

    2. Relevant equations
    m1v1i + m2v2i = (m1 + m2)vf

    3. The attempt at a solution
    I plugged the masses and velocities into the equation for perfectly inelastic collisions and solved for vf. I got 5.57 m/s and the answer is 2.14 m/s.
  2. jcsd
  3. May 29, 2014 #2
    Which direction have you picked as positive, north or south?
  4. May 29, 2014 #3
    OK I see what I did. I didn't take in to account negative velocity. Thanks!
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