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Homework Help: Problems with frictionless wedges

  1. Oct 4, 2011 #1
    1. The problem statement, all variables and given/known data
    A 2.3kg block rests on a frictionless wedge that has an inclination of 30° and an acceleration to the left such that the block remains stationary relative to the wedge; i.e., the block does not slide up or down the wedge. The acceleration of gravity is 9.81 m/s^2.

    Find the magnitude of the acceleration of the block-wedge system.

    2. Relevant equations
    ƩFx=ma=mgsinθ (?)

    3. The attempt at a solution
    [PLAIN]http://img36.imageshack.us/img36/456/physicsdiagram.png [Broken]
    (near-exact recreation of the diagram given, with force vectors added.)
    ƩFx = 2.3kg * 9.81m/s^2 * sin30°
    .... = 11.2815 kg*m/s^2

    If anyone is familiar with the UT Quest system, that's what we're using for this homework, and it's said that this answer is wrong. Enlightenment is encouraged, assistance is welcomed, etc. Your help would be greatly appreciated. Thank you! :)
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 4, 2011 #2

    Try to draw your FBD a little bit precise. You've got three forces acting on your block: ma, G=mg and N.
  4. Oct 4, 2011 #3

    Doc Al

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    Staff: Mentor

    There are only two forces acting on the block: The normal force and gravity. Those are shown correctly in your diagram. But if the block is not sliding down the wedge, what must be the direction of its acceleration?

    Apply Newton's 2nd law to horizontal and vertical forces to solve for the acceleration.
  5. Oct 4, 2011 #4


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    Staff Emeritus
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    The "block remains stationary relative to the wedge". The acceleration of the wedge is to the left. (It's on a horizontal surface.)
    Last edited: Oct 4, 2011
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