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Homework Help: Block sliding down an incline then collides with another block question.

  1. Nov 4, 2011 #1
    1. The problem statement, all variables and given/known data

    In the diagram below, a block of mass M=17 kg slides down a frictionless inclined plane and collides elastically with another block of mass m=4 kg. If the mass at the top of the wedge is initially at a height of 6 m above the horizontal plane, what is the velocity of m after the collision?


    2. Relevant equations

    conservation of energy formula, conservation of momentum formula. shown below in my attempt at the solution.

    3. The attempt at a solution

    I used the momentum formula to find V2f


    Since V2i of the smaller mass is zero, i'm only left with the first part, which is.


    substituting the masses in I get V2f=34/21 V1i

    To solve for V1i, I used mgh=1/2mv^2. (m=17kg, g=9.8, h=6m) and I got V1i=10.84m/s.

    Then substituting the value i got back into the previous equation, V2f=34/21 * 10.84 = 17.56 m/s. This answer is obviously shown by the computer to be NOT correct. I don't understand where I went wrong, because I even used the conservation of kinetic energy to check that before and after the kinetic energy is the same! Please help!
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Nov 4, 2011 #2

    Nevermind I got it. the right answer is 17.57 m/s, which is extremely close to what i got but the computer program did not approve my first answer for reasons god knows why.
    Last edited by a moderator: Apr 26, 2017
  4. Nov 8, 2011 #3
    CAPA sucks eh...haha. I attempted the problem the same way too and got an incorrect answer, my digits were slightly off.
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