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Homework Help: Total momentum after collision physics

  1. Jan 17, 2010 #1
    1. A proton, moving to the left at 7.0 x 106m/s collides with a neutron (mass approx. same as that of the proton) moving at 4.0x106m/s to the right to form a deuteron. Find the velocity of the deuteron.



    2. Total momentum before collision = Total momentum after collision



    3. 7.0 x 106m/s - 4.0x106m/s = v
    v= 3.0x106m/s. This is an inelastic collision


    Could someone check that please?
     
  2. jcsd
  3. Jan 17, 2010 #2
    Re: collisions

    Sure, and your answer is...wrong. The mass of the deuteron is twice that of the proton/neutron. You might want to write out the full COLM expression to avoid careless mistakes like that.
     
  4. Jan 17, 2010 #3
    Re: collisions

    (Mn x 4.0 x 106m/s) - (Mp x 7.0 x 106m/s) = (Mn+Mp) + v
    4.0 x 106m/s - 7.0 x 106m/s = 2M +v
    -3.0 x 106m/s = 2Mv
    v = -3.0 x 106/2 x M

    That seems wrong:confused:
     
  5. Jan 17, 2010 #4
    Re: collisions

    (Mn+Mp)v, not (Mn+Mp) + v !
     
  6. Jan 17, 2010 #5
    Re: collisions

    (Mn x 4.0 x 106m/s) - (Mp x 7.0 x 106m/s) = (Mn+Mp)v
    4M-7M = (Mn+Mp)v
    -3M = 2Mv
    v = -3/2 = -1.5
     
  7. Jan 17, 2010 #6
    Re: collisions

    Yup, that's it, 1.5 X 10^6 m/s to the left.
    (always try to give an interpretation to a negative sign)
     
  8. Jan 17, 2010 #7
    Re: collisions

    great!
    Thank you Fightfish
     
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