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Homework Help: Help! Conserv. of Energy Momentum is Driving Me Crazy

  1. Oct 27, 2008 #1
    1. The problem statement, all variables and given/known data

    A 100 g granite cube slides down a 35 degree frictionless ramp. At the bottom, just as it exits onto a horizontal table, it collides with a 215 g steel cube at rest.
    How high above the table should the granite cube be released to give the steel cube a speed of 200 cm/s?

    2. Relevant equations

    Conservation of momentum and energy equations. I don't know if there are more?

    3. The attempt at a solution

    For momentum: .1v1=.1v1_f+.215v2_f

    For energy: .1(9.81)h=.5(.1)v1_f^2+.5(.215)v2_f^2

    I know I'm supposed to solve a system of equations, but I have way too many variables right now. What am I doing wrong? What should I do next?
  2. jcsd
  3. Oct 28, 2008 #2
    Have you done some sketches? Can you see a way of breaking this problem down into smaller parts?
  4. Jul 12, 2010 #3

    Ill tell you the path to solve this. it seems that there is a lack of given data. to overcome that you have to do the following assumption.

    after the collision the granite cube and the steel cube behaves as a single unit.

    from the equations of conservation of momentum find the velocity of the granite cube before the collision.

    then apply v^2=u^2+2as along the ramp. here v is the velocity of the granite cube before the collision, u=0. s is the distance traveled by the granite cube along the ramp.
    to find a apply F=ma along the ramp.

    finally s*sin35=the height you want

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