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Conservation of momentum question

  1. Dec 12, 2015 #1
    1. The problem statement, all variables and given/known data
    Physicsquestion.jpg

    2. Relevant equations
    m1v1+m2v2=m1v1+m1v1 after

    3. The attempt at a solution
    my answer = -1m1v1
    because the wall is not moving so its velocity is 0
    therefore
    m1v1 = m1-v1 after
    I put -v because it is the same just bounces in the opposite X direction
    I found somewhere online the answer should be 2 * mv * sin(θ)
    I dont understand why
     
  2. jcsd
  3. Dec 12, 2015 #2

    haruspex

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    Conservation of momentum only applies to a closed system, i.e. no outside forces. The wall exerts a force. If you include the wall, the Earth exerts a force on the wall. If you include the Earth then in fact it will acquire a velocity from the impact, but so small you could not possibly measure it.
    Just consider the change in momentum of the ball (as a vector).
     
  4. Dec 12, 2015 #3
    It is the same as before only its x component changes, first it goes right, then it goes left
     
  5. Dec 13, 2015 #4

    haruspex

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    The magnitude is the same as before, but momentum and velocity are vectors. What is the net change in velocity?
     
  6. Dec 13, 2015 #5
    Oh so I do vector addition
    I did the math out and got the same answer, thank you
     
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