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Machine part ABC is supported by a frictionless hinge

  1. Nov 22, 2009 #1
    The machine part ABC is supported by a frictionless hinge at B and a 10degree wedge at C. There is also a 120 lb force pushing to the right of point A. Knowing that the coefficient of static friction is 0.20 at both surfaces of the wedge, determine (a) the force P required to move the wedge to the left, (b) the components of the corresponding reaction at B.

    I found the moment about C to be Mc=(-Byx10)+(-120x8)=> By=-96N
    But I'm not sure where to go from there : /
     
  2. jcsd
  3. Nov 22, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi link13! Welcome to PF! :wink:
    You haven't used µ = 0.2 yet …

    show us what equations you get, and where you're stuck, and then we'll know how to help! :smile:
     
  4. Nov 22, 2009 #3
    Re: Friction

    thats where I got stuck actually : / I know that to find the friction you would have to have a normal force to multiply M=0.2 by, I'm just not sure how to find the normal force : /
     
  5. Nov 22, 2009 #4

    tiny-tim

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    Hi link13! :smile:

    Acceleration normal to the surface is usually zero, so you usually find the normal force by taking components normal to the surface, and equating to zero. :wink:
     
  6. Nov 22, 2009 #5
    Re: Friction

    So would the Moment about B or Mb = (-120x8) + (Nx10) work to find the normal force?
     
  7. Nov 23, 2009 #6

    tiny-tim

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    (just got up :zzz: …)

    Sorry, I'm confused because I don't really understand what the thing looks like,

    (what shape is ABC? where is the wedge? is it fixed? etc :confused:)

    but I don't see 10º or µ anywhere in that, so I'll assume it's wrong.
     
  8. Nov 23, 2009 #7
  9. Nov 23, 2009 #8

    tiny-tim

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    link13, that's hilarious … however did you think we were going to get that from your description?! :rofl:

    ok, a horizontal force P is trying to push a wedge to the left (at C), the top of the wedge slopes down to the left at 10º, and is held in position by pressure on its upper surface from an L-shaped object hinged at B (10 in. to the left of C) and pushed to the right by a horizontal force at A (8 in. above B).

    Since the wedge is (obviously) not accelerating vertically, the net vertical force on it is zero, so the vertical reactions on both the upper and lower surfaces must be the same.

    Find that reaction by considering the forces on ABC. Then find the friction forces. Then find P and the reaction at B. :smile:
     
  10. Nov 23, 2009 #9
    Re: Friction

    alright i think i got got it now! Thanks! : )
     
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