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New velocity between two spacecraft moving in opposing directions

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data

    A 60-tonne (1 t = 1000 kg) spacecraft moving in the + x-direction at 0.70 m/s docks with a 65-tonne craft moving in the -x-direction at 0.64 m/s

    Find the velocity of the joined spacecraft

    2. Relevant equations

    Conservationg of kinetic energy?

    KE_i=KE_f
    3. The attempt at a solution

    I don't know if this is silly, but I tried solving for the velocity using kinetic energies of the seperate space ships equalling the joined space ships:

    1/2mv_1^2+1/2mv_2^2=1/2mv_3^2
    1/2(60000kg)(0.70m/s)^2+1/2(65000kg)(0.64m/s)^2=(125000)v_3^2
    14700+13312=62500v_3^2
    v_3^2=0.4482
    v_3=0.6695m/s
     
  2. jcsd
  3. Oct 10, 2011 #2

    lightgrav

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    Which direction do you think that velocity would be?

    no, they "lock them together" before they can bounce apart.
    what's conserved here is total momentum (vector).
     
  4. Oct 10, 2011 #3
    Okay, so I can use P=mv for each respective part?
     
  5. Oct 10, 2011 #4
    The velocity would be in the direction the heavier ship is going
     
  6. Oct 10, 2011 #5

    lightgrav

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    the heavier ship isn't going as fast ... just add them up, being careful with +/- signs.
     
  7. Oct 10, 2011 #6
    Okay so the lighter ship is going faster but the larger ship is going slower, I don't understand what equation im supposed to use to find the velocity of the combined*
     
  8. Oct 10, 2011 #7

    lightgrav

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    you compute each one's momentum, then you add them.

    Total momentum is the same after connecting as it was before connecting.
     
  9. Oct 10, 2011 #8
    So I'll be subtracting the second ships momentum* since it's moving in the opposite direction, correct?
     
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