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Homework Help: Engineering mechanics friction question! HELP PLEASE

  1. Aug 30, 2010 #1
    engineering mechanics friction question! HELP PLEASE!!!!

    I need help for this question, please help me. i cant get the right ans. ):

    6.2) Block B rests on block A and is attached by a horizontal rope BC to the
    wall as shown in Figure-2. What force P is necessary to cause impending motion
    of A? The coefficient of friction between A and B is 0.25 and between A and the
    floor is 0.33. A has a mass of 14 kg and B has a mass of 9 kg. ( P= 102.7 N)
     

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  3. Aug 30, 2010 #2

    PhanthomJay

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    Re: engineering mechanics friction question! HELP PLEASE!!!!

    Welcome to PF! If you show an attempt at your work (which is a requirement in this forum), someone will be better able to assist, to see where you may have gone wrong. Be sure to calculate normal forces correctly.
     
  4. Aug 30, 2010 #3
    Re: engineering mechanics friction question! HELP PLEASE!!!!

    okay, but i dunno my step are correct anot, but i cant get the ans.

    sum of x = 0

    sum of x = -Px - (9 x 9.81 x 0.25) + c + (23 x 9.81 x 0.33)
    = -Px - 22.0725 + c + 74.4579
    = -Px + C + 52.3854
    Px = C + 52.3854

    Sum of y = 0
    Sum of y = Py + Nb + Na - (9.81 x 9) - (14 x 9.81)
    = Py + 88.29 + 225.63 - 88.29 - 137.34
    = 88.29 + Py

    That all i do, and i am stuck, correct me if i am wrong. please.
     
  5. Aug 30, 2010 #4

    PhanthomJay

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    Re: engineering mechanics friction question! HELP PLEASE!!!!

    You are not drawing Free Body Diagrams (FBD's) correctly, and you have signage errors. You might want to start off by first isolating the top block B, using a FBD to note all forces acting on it. By doing this, and looking in the y direction, the normal force on B acts up, and is equal to the 88 N you calculated correctly.

    Now draw a FBD of the bottom block A. Which way does Nb act on A? (think of Newton's third law). What other forces act on A? (the tension force c is not one of them).
    Now use Newton 1 in the y direction and x direction to solve for P, looking at Block A only. Note that friction forces oppose the direction of relative motion between surfaces.
     
  6. Aug 31, 2010 #5
    Re: engineering mechanics friction question! HELP PLEASE!!!!

    i still have problem drawing the FBD of block a. ): how many forces are there?
     
  7. Aug 31, 2010 #6
    Re: engineering mechanics friction question! HELP PLEASE!!!!

    nvm, i got the answer already. but i got a part i dun understand. is Fb pointing to the left? why issit pointing to the left but not the right?
     
  8. Aug 31, 2010 #7

    PhanthomJay

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    Re: engineering mechanics friction question! HELP PLEASE!!!!

    If by Fb you mean the force of friction between A and B, well, if Block A wants to move left, then the friction force of B on A must point right, (the same as the direction of the friction force of the floor on A).
    If you're looking at Block B, however, Newton 3 tells you that the friction force of A on B must point_____?

    You haven't shown how you arrived at the correct answer.
     
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