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Homework Help: Elastic Collision with a Spring Constant

  1. Aug 5, 2012 #1
    1. The problem statement, all variables and given/known data

    A pinball with mass 80g is struck by a lever with mass 80g in a pinball machine in an elastic collision. The lever was pulled back 2 cm by a spring with spring constant k=1.4N/cm. What is the speed of the ball just after collision?

    2. Relevant equations


    M1V1 = M2V2

    3. The attempt at a solution

    1/2 (2.8)(4)=5.6 5.6=1/2 (80) v^2 <-- trying to find velocity of lever just before contact

    5.6/40 = v^2 V=.37

    now this is where i get even more stuck, the masses of the ball and the lever are equal and they cancel out in the equation M1V1 = M2V2, which would leave the velocities as being equal. However, the answer choices are as follows:

    A) 1.1 m/s
    B) 1.4 m/s
    C) 11 m/s
    D) 14 m/s

    If someone can shed some light on how to solve this problem it would be greatly appreciated.
  2. jcsd
  3. Aug 5, 2012 #2
    First thing I would look at is units. g to kg

    centimeters to meters, I would change both and see how it works out.
  4. Aug 7, 2012 #3
    i tried that, and i am still stumped
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