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Homework Help: 2 blocks collide, 5% energy loss. 2 answers, which correct?

  1. Sep 12, 2015 #1
    1. The problem statement, all variables and given/known data

    The question I have is not how to arrive at the correct values of final velocity, but once I have the values of final velocity, how do I know which velocities (which are computed from a quadratic equation) are correct?

    Two blocks, block A and Block B, are travelling to the right.

    Before collision:

    Block A is 4kg at 10m/s
    Block B is 2kg at 5m/s

    After collision:

    Block A is 4kg at vf = ?
    Block B is 2kg at Vf = ?

    Energy loss is .05

    2. Relevant equations

    mvi + MVi = mvf + MVf

    1/2mvf^2 + 1/2MVf^2 = .95[1/2mvi^2 + 1/2MVi^2]

    3. The attempt at a solution

    mvi + MVi = mvf + MVf ==> Vf = 25 - 2vf

    1/2mvf^2 + 1/2MVf^2 = .95[1/2mvi^2 + 1/2MVi^2] ==> 4vf^2 + 2Vf^2 = 427.5

    4vf^2 + 2(25 - 2vf) = 427.5 ==> 12vf^2 - 200vf + 822.5 = 0

    Quadratic Formula ==> vf = 9.28m/s and 7.38m/s

    Vf = 25 - 2(9.28) = 6.44m/s and Vf = 25 - 2(7.38) = 10.24m/s

    So the possible answers are:

    [vf = 9.28m/s Vf = 6.44m/s] or [vf = 7.38m/s Vf = 10.24]

    Given the energy loss of .05, how could I know which set of final velocities are correct? I don't think I can just rationalize it because in either case, the faster, heavier block A slows down upon colliding with the lighter, slower block B. And Block B speeds up upon being struck by the faster, heavier block A. What I mean is, in both final answers, Block A is slowing down and Block B is speeding up.

    Is there a computation that will determine which set of velocities are correct?

  2. jcsd
  3. Sep 13, 2015 #2


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    In the first case the larger block is moving faster than the smaller block after the collision. Do you think this is possible?
  4. Sep 13, 2015 #3
    Makes perfect sense PeroK. The larger block must be moving slower that the smaller block after collision because the larger block is behind the smaller block. It can't physically pass the smaller block. I was simply looking at the numbers in a scalar sense and not considering the bigger picture.

    Thank you.
  5. Sep 13, 2015 #4


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    If you solve the problem with no energy loss, you'll find one solution is where the blocks miss each other and continue with their original velocities. The first case is, therefore, a variation of this with the 5% energy loss.
  6. Sep 14, 2015 #5
    Thanks PeroK
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