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Homework Help: What type of collision?

  1. Nov 18, 2011 #1
    1. The problem statement, all variables and given/known data
    Block A is moving to the right at 2 m/s when it collides with an identical block B that is initially at rest. After the collision, block A is moving back to the left at 1 m/s and block B is moving to the right at 3 m/s. Which answer best describes the type of collision that occurred?
    a) elastic
    b) inelastic
    c) perfectly inelastic
    d) The answer to this question depends on the precise masses of the two blocks.
    e) This question cannot be answered with the given information.

    2. Relevant equations
    (m1)(v1i) = (m1)(v1f) + (m2)(v2f)
    KE = (1/2)mv^2

    3. The attempt at a solution
    my teacher said the answer was a, but i thought that the answer would be e, because the initial KE of the system is 2m, while the final KE is 5m. this means an increase in KE but i dont know if increase in KE can still be considered elastic.
    Last edited: Nov 18, 2011
  2. jcsd
  3. Nov 18, 2011 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    I agree with you. Energy has been added. But we don't know how much energy was added. We don't know if the collision was inelastic (it lost 1m of energy, say) and that energy was replaced plus additional energy (4m added) or if it was elastic and 3m of energy added.

    Note: This could be a collision between two blocks with already compressed springs. Upon the colliding, the springs release and push the two blocks apart with spring force.

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