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Homework Help: Not enough information in this momentum question?

  1. Jun 1, 2010 #1
    1. The problem statement, all variables and given/known data
    Two air track gliders of a mass 300 g and 200 g are moving towards each other in opposite directions with speeds of 50 cm/s and 100cm/s respectively. Take the direction of the more massive glider as positive.

    A.) determine the velocity of each glider after the collision if the collision is elastic.

    b.) The most inelastic collision would occur if the two gliders stuck together on impact. If this was the case, find the velocity of the pair after the collision, and the kinetic energy in standard units of "J".

    2. Relevant equations
    m1v1i +m2v2i = m1v1f + m2v2f

    3. The attempt at a solution

    a.) in order to find the unknown velocites you need to know at least one of the rebound velocities do you not?

    0.3(0.50) + 0.2(-1.00) = 0.3v1f + 0.2v2f

    b.)0.3(0.50) + 0.2(-1.00) = 0.5vf

    vf = (0.15-0.2)/0.5
    = -1.0 m/s

    KE = 1/2mv^2

    = 1/2(0.5)(-1.0)^2
    = 0.25 J
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 1, 2010 #2
    Is this elastic collision only one-dimensional?
  4. Jun 1, 2010 #3


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    Homework Helper
    Gold Member

    You have enough information to solve it (given that this is a 1 dimensional problem -- things can get more complicated if the objects can freely move in more than 1 dimension).

    You have the conservation of momentum equation that you listed above. But since you are assuming it is a perfectly elastic collision, you can also use conservation of kinetic energy (conservation of kinetic energy only applies if the collision is perfectly elastic). So you have 2 equations and 2 unknowns, which is solvable.

    Try that last calculation again. I think you missed a decimal point somewhere.
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