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Impulse/final velocity question confusion

  1. Jul 8, 2015 #1
    I uploaded my question below; basically I am confused in part b) where they show that:
    50 kgm/s = (5kg)v2 - (5kg)(-4m/s) then they say that
    30 kgm/s = 5kg v2, but shouldn't it be 70 kgm/s = 5kg v2?

    As noted at the beginning of the question North is positive and thus since the cart moves south is negative (-4m/s).
    Could this be a typo ?
     

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  3. Jul 8, 2015 #2
    If south is negative and that was the initial direction of momentum then the second term is written correctly. Note carefully the signs (subtracting a negative on the RHS) so the second line is correct because the equation reads (without units) 50 = 5v + 20.
     
  4. Jul 8, 2015 #3
    yeah I totally missed the subtracting a negative on the RHS that makes more sense now thank you
     
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