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Homework Help: [Linear momentum] When to use which equation.

  1. Dec 10, 2009 #1
    1. The problem statement, all variables and given/known data
    I'm confused about when to use which formula.
    I thought for inelastic collisions,
    for example, when a bullet is shot into a block,
    I should use
    (mbullet)(vinitial) = (mbullet + mblock)(vfinal)

    but it turns out that I must use

    why do i have to use this one?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 10, 2009 #2


    User Avatar

    eqn w/ velocities you use
  4. Dec 10, 2009 #3
    They should really be the same equation. (m1)(vi)+(m2)(vi)=(m1)(vf)+(m2)(vf) will reduce to the top equation you wrote if the block "m2" is initally at rest (vi=0) and upon impact they stick and form a system. Thus they have a combined mass (m1+m2) and the same final velocity (vf). So it reduces to:

    (m1)(vi)bullet = (m1+m2)(vf)system
  5. Dec 10, 2009 #4
    then v1 final and v2 final are the same??
  6. Dec 10, 2009 #5
    Yes, this is true because they stick together to form a system. They must have the same velocity.
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