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Homework Help: Find the Acceleration of the two joined Blocks

  1. Feb 15, 2013 #1
    1. The problem statement, all variables and given/known data
    The sliders A and B are connected by a light rigid bar of length l = 0.50 m and move with negligible friction in the slots, both of which lie in a horizontal plane. For the position where xA = 0.4 m, the velocity of A is vA = 0.80 m/s to the right. Determine the acceleration of each slider and the force in the bar at this instant. The acceleration of A is positive if to the right. The acceleration of B is positive if up (if that is the right word in this horizontal plane). The force in the rod is positive if in tension. Remember that the motion takes place in a horizontal plane, so the force of gravity is not a factor.

    2. Relevant equations

    3. The attempt at a solution

    I drew FBD of blocks A and B.

    Block A has force P acting on it to the right, and there is a force F pointing along the direction of the attached rod.

    ƩRx = maA = P - Fcos(phi)

    Then I drew a FBD of block B which has the force F acting on it as well in the direction of the rod. Aside from this I can't see any other forces acting on block B.

    ƩFz = maA = -Fcos(θ)

    After this I'm stuck. I think I need another relationship, one relating the acceleration of block A and B. I'm thinking the Pythagoras theorem would work, but I'm not sure, how I should adapt it for this situation.

    Any input would be appreciated.

    Attached Files:

  2. jcsd
  3. Feb 15, 2013 #2


    User Avatar

    Staff: Mentor

    For the triangle, x² + y² = 0.5²

    If you differentiate this twice w.r.t. time, you'll have the acceleration of x ❲viz., d²x/dt²❳ related to the acceleration of y ❲viz., d²y/dt²

    Does this sound like the method you expect? I don't want to mislead you, I don't know whether calculus is essential to solving this.
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