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Homework Help: Static friction with wedges problem

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

    http://www.solutioninn.com/images/E-C-S (808).PNG

    ignore the given FBD below the pic.

    Problem said that: there are 3 blocks like in the picture, with weight A = 600lb, B =150 lb, and C = 500. Find force P such that it will move block C. (everything is static. Coefficient of floor and block is 0.5 and block with block is 0.3 and the angle is 45.

    3. The attempt at a solution
    So there will be 2 cases for this problem:
    case 1: A moving left, B moving up, C moving left.

    case 2: block A stay, C and B moving. I am having problem with this part now.

    When i draw the FBD, I think I dont need to draw one for block A since it is staying. So when I draw the FDB for the wedge B I will have

    - friction b/t A and B
    -Normal N to the floor (I am not sure if this is here)
    -Weight of B
    -friction b/t B and C

    And for the wedge C:
    -W of C
    -friction b/t B and C
    -Normal to the floor, and friction b/t floor and block

    2. Relevant equations
    It seem like I have 5 unknowns and only 4 equations (2 from each diagram) which I am not able to solve by merely equilibrium.
    Is my approach correct? or did I make a mistake in the forces?

    I think the normal force b/t the wedge B and the floor is not necessary, I am not sure about this. If N of B and floor is not there then the problem is solvable at this point.

    PLease help. Thanks.
  2. jcsd
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