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Homework Help: Linear Momentum and Collisions

  1. Jan 15, 2007 #1
    H guys,

    I want to first start off by saying that I am not here for handout to my homework. I am here because maybe you guys can help me understand physics. I am currently enrolled in a calculus-base physics that is kicking my ass and I need some help understand the different subjects.

    Currently I am reviewing, Linear Momentum and Collisions. I wanted to know if there is method that I can use to determine what formula to use.

    I know: mV=mv ( V = velocity sub one and v= velocity sub two)

    Sometime, depending on the question I can use: mV= mv+mV or mV=(m+m)V etc.

    I understand the these formulas can only be used when momentum is conserved but what I do not understand is how to determine what formula to use. Can someone explain that to me?

  2. jcsd
  3. Jan 15, 2007 #2


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    Staff Emeritus
    Science Advisor

    Hi, and welcome to PF.

    The equations you are quoting are all special cases of the law of conservation of total momentum This means that total momentum before the collision = total momentum after the collision. The number of terms on each side of the equation just depends on the number of objects there are in the problem!

    If you are having trouble with any particular problem, then feel free to post it with your thoughts, and we'll try and help you understand further.
    Last edited: Jan 15, 2007
  4. Jan 15, 2007 #3
    (m+M)V is used when objects share the same velocity after a collision. This is a perfectly inelastic collision because they objects stick together.

    mv = mv' + MV' would be used when one object is initially at rest. The complete equation would be mv +MV = mv' + MV' , but MV is zero because this mass is initially at rest.

    Every problem with momentum is a version of this general case:
    m1v1 + M2V2 = m1v1' + M2V2' (the ' indicates after the collision)
    Things get simpler when certain things are zero or certain things are common to both objects
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