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Homework Help: Help! Conservation of Momentum

  1. Feb 27, 2012 #1
    1. The problem statement, all variables and given/known data

    Object 1 has a mass of 2.8 kg and is moving eastward with a velocity of 7.7 m/s. It collides with a stationary object. After the collision object 1 recoils with a velocity of -2.2 m/s What is the mass of object 2?

    2. Relevant equations

    As far as I know:

    Conservation of Momentum




    3. The attempt at a solution

    I keep trying to solve by using m1v1=m2v2

    m2 = m1v1/v2
  2. jcsd
  3. Feb 27, 2012 #2


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    You have two unknowns: the mass and velocity of object 2 after the collision.
    You will need two equations to solve this. If you know it is an elastic collision, you can use conservation of energy as your second equation. If it is not elastic, then you cannot find the solution. The result of the collision depends on how much energy is lost in the collision.
  4. Feb 27, 2012 #3
    but don't you need the mass of the second object to calculate using the elastic collision formula?
  5. Feb 27, 2012 #4


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    No. With two equations, momentum and energy, you can find two unknowns.
    It will not be straightforward - you would have to write the two equations, then solve one of them for either m or v, substitute into the other to eliminate one unknown, then solve for the other one.
  6. Feb 27, 2012 #5
    Any chance you could show me? I'm at a major loss and I have a huge exam coming up:/
  7. Feb 27, 2012 #6


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    Glad to help. But you can't just add "elastic collision" to the question.
    As it stands, there is no solution.
    Take another look at it - if it says "elastic collision" in the question, then write
    energy before = energy after
    ½mv² = ½mv² + ½mv²
    and fill in the known quantities. Once you have your two equations with two unknowns, I can offer further advice.
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