1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mass frame.

  1. Oct 24, 2007 #1
    1. The problem statement, all variables and given/known data

    Nucleus A,which has rest mass [tex]m_A[/tex],collides with nucleus B,which has rest mass [tex]m_B[/tex].In the laboratory frame,nucleus A has energy [tex]E>m_Ac^2[/tex]
    abd nucleus B is stationery.Find the total energy of the system in center of mass frame in terms of [tex]m_A,m_B and E[/tex].

    3. The attempt at a solution

    What's the center of mass frame?Is it a frame of nucleus A?
  2. jcsd
  3. Oct 24, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Consider the frame in which the total spatial momentum is zero.
  4. Oct 24, 2007 #3
    Is it a center of two nuclei?
  5. Oct 24, 2007 #4
    Then check my solution.
    Velocity of A in labframe is
    velocity of center of mass

    In frame which moves with velocity v_c,nucleus A has velocity

    [tex]v'_A=\frac{v-v_c}{1-\frac{vv_c}{c^2}}[/tex].And nucleus B has velocity [tex]v'_B=v_c[/tex].So the total energy in this frame is

  6. Oct 24, 2007 #5
    I think from the frame moving with velocity v_c,velocities for A and B arer true.But as seems to me [tex]m_Av_A\neq m_Bv_B[/tex] in this frame.
  7. Oct 30, 2007 #6
    Here I tried another solution:

  8. Oct 30, 2007 #7


    User Avatar
    Homework Helper

    Looks right to me.
  9. Oct 30, 2007 #8
    Thanks very much.

    And ,if after colission reaction occurs [tex]A+B\rightarrowC+D+Q[/tex].[tex]m_A+m_B=m_C+m_D+Q[/tex].Is it

    [tex]E'^2=(m_Cc^2+m_Dc^2+Q)^2[/tex] ? For E minimum
  10. Oct 30, 2007 #9


    User Avatar
    Homework Helper

    I don't understand. What is the question asking for?
  11. Oct 30, 2007 #10
    Oh,sorry.After collision reaction occurs [tex]A+B\rightarrow C+D[/tex].Suppose that

    [tex]m_A+m_B=m_C+m_D+Q[/tex].Find the minumum E for reaction to occur?And I think that in center of mass frame after reaction total momentum is zero.
  12. Oct 30, 2007 #11


    User Avatar
    Homework Helper

    sorry. still a little confused. what does Q represent here?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook