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Homework Help: Elastic collision problem?

  1. Sep 3, 2014 #1
    Two blocks are released from rest on either side of a frictionless
    half-pipe. Block B is more massive than
    block A. The height HB from which block B is released is less
    than HA, the height from which block A is released. The blocks
    collide elastically on the flat section. After the collision, which
    is correct?
    A. Block A rises to a height greater than HA and block B
    rises to a height less than HB.
    B. Block A rises to a height less than HA and block B
    rises to a height greater than HB.
    C. Block A rises to height HA and block B rises to
    height HB.
    D. Block A rises to height HB and block B rises to
    height HA.
    E. The heights to which the blocks rise depends on where
    along the flat section they collide.
    I honestly didn't understand where to start with this problem, and got confused on the solution walkthrough. It basically wrote out two equations based on the conservation of momentum and conservation of energy, which said could be used to calculate the final speeds and then the final heights. Is there a more intuitive, less complicated way of figuring out this problem or does it require multiple equations? If someone could kindly give me an explanation for this problem it would be much appreciated!
  2. jcsd
  3. Sep 3, 2014 #2


    User Avatar
    Gold Member

    Well we can't really give you an explanation, that would be doing your homework for you. Conservation of energy and momentum are good places to start, do you know why?
  4. Sep 4, 2014 #3
    Well since it's an elastic collision I know Kf=Ki and due to the conservation of momentum Pf=Pi but i don't know how to put these two equations together to determine which height each of the blocks rises too. I can't really understand how the book uses the equations since the explanation is very vague. I guess i'm more confused since they don't give number values in this problem.
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