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Homework Help: Momentum conservation - particle collision

  1. Jun 7, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution

    Working out how momentum is consered in this situation,...

    I know that the mass of an alpha particle is 4*mass of proton....however, I am still slighly concerned because the alpha particles move in opposite vertical directions hence their momenta cancel...how is momentum conserved here (I know that horizontal momentum is conserved but how is vertical momentum conserved)??
  2. jcsd
  3. Jun 7, 2012 #2


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    From the picture, the alpha particles don't seem to quite move in opposite directions. In any case, the horizontal momentum is absorbed by the foil, and ultimately by what's holding the foil in place.
  4. Jun 7, 2012 #3
    but you cannot deny both alpha particles have horizontal momenta (but they must cancel to conserve linear momentum)....I guess that if the examiners were to ask a question about this in my exam they would make the diagram clearer...or they would simply ask me what I knew about the momenta of the particles.

    Would the following answer be correct:

    "As no external forces act the total momentum is conserved. Therefore the two alpha particles must move in opposite horizontal directions to conserve horizontal momentum. The total vertical momentum of both alpha particles must be equal in magnitude AND direction to the momentum of the proton"
  5. Jun 7, 2012 #4
    I have drawn a blue line to show that the vertical momentum are in differnt directions...

    Attached Files:

  6. Jun 7, 2012 #5
    on reading about
    "A proton is directed at a stationairy lithium target. The law of conservation of momentum states that the two alpha particles must have equal and opposite momentum"

    How does this conserve momentum? If they are equal and opposite, it implies total momentum after is zero BUT total momentum before is not zero...what is going on?
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