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Conservation of momentum in one direction (add or subtract?)

  1. Jun 25, 2015 #1
    1. The problem statement, all variables and given/known data
    Hey everyone! I'm new to these forums so I apologize in advance if I do something wrong! I just had a quick question about the momentum in one direction equation m_1v_1o +/- m_2v_2o = m_1v_1f +/- m_2v_2f; I'm just not sure when I'm supposed to add these values and when I'm supposed to subtract them. I thought it had to do with the objects vector, but there is conflicting questions in my lesson (I think at least). I'm doing an online course so I don't really have a teacher to ask lol
    For example:
    Football player 1 with a mass of 110kg is moving at 8.0m/s when he crashes into football player 2 with a mass of 105kg, moving at 12 m/s [N]. During the tackle football player 1 holds onto football player 2, giving them the same velocity after 0.30s.
    a) Find the final velocity of each player after collision

    In a Physics experiment, a 1.2kg cart1 is moving at 3.2m/s [R], when it collides with a 1.8kg cart 2 which is moving at 5.8m/s[L]. After the collision cart1 is moving at 7.6m/s[L].Find the velocity for cart2 after the collision.
    2. Relevant equations
    m_1v_1o + m_2v_2o= (m_1+m_2)v_f
    m_1v_1o - m_2v_2o = m_1v_1f + m_2v_2f
    3. The attempt at a solution
    So for the football one my book shows;
    Let [N] be positive
    P_to=P_tf
    m_1v_1o + m_2v_2o= (m_1+m_2)v_f
    (110kg)(-8.0m/s)+(105kg)(12m/s)= (110kg +105kg)v_f
    v_f=1.77m/s[N]

    For the Physics experiment one:
    P_1o-P_2o=P_1f+P_2f
    Let [R] be positive;
    m_1v_1o - m_2v_2o = m_1v_1f + m_2v_2f
    (1.2kg)(3.2m/s)-(1.8kg)(5.8m/s)=-(1.2kg)(7.6m/s)+(1.8kg)v_2f
    -6.6kgm/s= -9.12kgm/s + (1.8kg)v_2f
    v_2f= 1.4m/s [R]
    So Is it because the two forces are combining in the first that you add? and in the second one they bounce away so you subtract?
    Sorry if this is a really simple question, I haven't done physics in almost 8 years!
    Thank you in advance!
     
  2. jcsd
  3. Jun 25, 2015 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Is there a reason most of your text is struck out? Did you still have a question?
     
  4. Jun 26, 2015 #3
    Put (+) everywhere and use vector direction to see the correct sign.
     
  5. Jul 13, 2015 #4
    Hey sorry everyone I didn't respond I went on vacation and dint have internet but i figured it out thank you both!
     
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