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Homework Help: Linear momentum carts of mass

  1. Nov 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Two carts with masses m1 = 0.8 kg and m2 = 1.2 kg are moving toward each other with
    speeds v1 = 4.5 m/s and v2 = 3.8 m/s as shown in the diagram below. The two carts
    collide elastically.

    Some reason I'm getting too high of a value, dont know I'm doing wrong...

    2. Relevant equations
    momentum = m1v1 + m2v2 = m1vf1 + m2vf2

    kinetic energy = .5m1v1^2 + .5m2v2^2 = .5m1vf^2 + .5 m1vf2^2

    3. The attempt at a solution

    So I found the momentum which came out to be -.96 = .8vf1 + 1.2vf2
    Rearranged the equation to solve for vf2 = -(2/3)vf1 - .8
    Then tried to find kinetic energy and got 8.1 + 10.83 = .4vf1^2 + .6vf2^2

    Plugged in v2 and got 16.76 = .4vf1^2 + (4/15)vf1^2 + .64vf1 + .384
    Subtracted 16.76 from both sides
    Used quadratic equation

    Came out with 13.5 m/s for vf1.
  2. jcsd
  3. Nov 10, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Check the second term on the left hand side.
  4. Nov 10, 2013 #3
    Fixed it a little, but still getting a pretty high number. Is that really the answer?
  5. Nov 10, 2013 #4

    Doc Al

    User Avatar

    Staff: Mentor

    No. I got the same equation as yours, but got a lower answer.
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