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Block Moving Down A Moving Wedge

  1. Oct 19, 2011 #1
    1. The problem statement, all variables and given/known data

    A mass of 5kg and wedge of 10kg are held at rest. All surfaces are frictionless. No outside forces are acting on the system. θ of the wedge is 30°. Once the block and wedge are let go, what happens to both objects? What is the acceleration of the block left/right and what is the acceleration of the block down the wedge?
    2. Relevant equations

    I seriously do not get this problem at all.

    3. The attempt at a solution

    When I made a fbd, and split both masses into x and y components, it showed that the wedge will accelerate in the opposite direction of the block's acceleration.

    Anyone understand this?
     
  2. jcsd
  3. Oct 19, 2011 #2

    Doc Al

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    Why not? The block slides down the wedge, pushing the wedge backwards. If you viewed their motion with respect to the ground, how would it look?
     
  4. Oct 19, 2011 #3
    Thank you for the reply.

    I understand that the block will continue to move down along the block... But we had a problem with this concerning conservation of energy. But the work that the block does on the wedge is positive and the work that the wedge does on the block is positive, why is energy conserved? It doesn't make sense.
     
  5. Oct 19, 2011 #4

    Doc Al

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    Why doesn't it make sense? Don't forget about gravity.
     
  6. Oct 19, 2011 #5
    Because a positive work is acted on in the system. Therefore, energy must've been added to it.
     
  7. Oct 19, 2011 #6

    SammyS

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    Please, read the Forum rules regarding bumping.
     
  8. Oct 19, 2011 #7

    SammyS

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    Have you looked at the links at the bottom of this web page. This problem has been addressed many times, sometimes quite well.
     
  9. Oct 20, 2011 #8

    Doc Al

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    Once again I will remind you not to forget about gravity.

    Also you may want to compare what happens when the wedge cannot move versus when it is free to move (as in this case).
     
    Last edited: Oct 20, 2011
  10. Oct 20, 2011 #9
    I am taking gravity into account. The wedge moves left and the block moves right and down. There is a positive force to the left acting on the wedge and a positive force to the right acting on the block, along with gravity acting down. Therefore, all work done in this problem is positive. Since we area adding positive work into the system, Wnc= delta E
     
  11. Oct 20, 2011 #10

    Doc Al

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    There is no non-conservative work being done here. The increase in KE of the block and wedge is due to the work done by gravity.
     
  12. Oct 20, 2011 #11
    So, the work done by gravity is shraed between the two objects, correct?

    But looking at it from a Newton's Laws perspective, how would you go about solving for the accelerations of each objects? Like, how does the acceleration of the wesdge relate to the acceleration of the block?
     
  13. Oct 20, 2011 #12

    Doc Al

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    Right.

    Start by identifying the forces acting on each object and draw yourself a FBD of each. (You might have already done that.)

    One constraint to apply is that the block maintains contact with the wedge. Call the acceleration of the wedge aw and the acceleration of the block with respect to the wedge a. (aw will be horizontal and a will be parallel to the wedge.)
     
  14. Oct 20, 2011 #13

    SammyS

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    Check these out !!!
     
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