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Homework Help: Ball moving up a wedge

  1. Aug 22, 2007 #1
    1. The problem statement, all variables and given/known data
    a ball with mass m and velocity v is moving up a wedge of mass M. What height does the ball reach if:
    a) the ramp is fixed on the table
    b) the ramp is able to move on a frictionless surface

    2. Relevant equations

    3. The attempt at a solution
    for a) i just solved for h using both equations given above, since there is a loss in kinetic energy which is converted to a gain in gravitational potential energy as the ball moves up the wedge. so this gives me that the h = v(squared)/2g.
    im stuck on b) however.
  2. jcsd
  3. Aug 22, 2007 #2


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    Staff Emeritus
    Science Advisor

    If the ramp is supported by a 'frictionless' surface, then as the ball of mass m goes up the ramp, it imparts a force on the ramp, and that force is related the weight.

    So now the ball of mass m is doing work on the ramp of mass M, and both accelerate.

    Think about using conservation of momentum in addition to conservation of energy.
  4. Aug 22, 2007 #3
    well that was sort of my train of thought as well. the ball will exert a force onto the wedge thus making both move.
    we know that v'2 (that is the new velocity of the wedge with mass M) = (2M/m+M) * v1 (that is the original velocity of the ball)

    but im confused because isnt the angle a factor aswell, i mean the force exerted on the wedge by the ball is different depending on how steap it is.
    i mean taken the extreme case we could say the wedge is in fact block, in that case i can see the conservation of momentum as given above applying to this situation, but in a case where the angle is flat the force exterted on the wedge would be less, or at least thats what im thinking.
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