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Friction with two stacked blocks and friction

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data

    Here is the problem:

    Block A in the figure weighs 1.05 , and block B weighs 3.90 . The coefficient of kinetic friction between all surfaces is 0.32. Find the magnitude of the horizontal force necessary to drag block B to the left at constant speed if and are connected by a light, flexible cord passing around a fixed, frictionless pulley.See the attachment for a picture.

    https://www.physicsforums.com/attachment.php?attachmentid=44607&stc=1&d=1330645523


    2. Relevant equations

    ƩFx=Max, Friction=uk*n

    3. The attempt at a solution

    My FBD for the upper block has N going up MG going down Fk going left and T going right (positive)

    My FBD for lower has N going up MG going down (I included both weights) Tension and then both frictions going to the right, and the force required going to the left (which i made positive for this one)

    I am not sure what i am missing in solving this. Help me see if I have the right set up please. For the bottem block I have:

    Sum of forces in x = M*Ax

    Pull - Tension - .32(4.95) -.32(1.05)= M*Ax

    For the top block I have:

    T-.32(1.05)=M*Ax

    Am I correct for these equations, especially in the bottem block where I have subtracted both tensions, I am assuming this is correct because the table exerts friction but so would the top block as it is sliding across the top block.
     

    Attached Files:

  2. jcsd
  3. Mar 1, 2012 #2

    ehild

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    It is correct if you specify what M*Ax means in both equations.

    ehild
     
  4. Mar 1, 2012 #3
    M*Ax is mass * accel in the x direction from newton 2

    Sum of forces in x = mass time accel in x
     
  5. Mar 1, 2012 #4
    Can you possibly point me in the direction of what i am missing... i have 3 equations with 2 unknowns right now.
     
  6. Mar 1, 2012 #5

    ehild

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    Is it the same for both blocks???????

    ehild
     
  7. Mar 1, 2012 #6

    ehild

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    You meant two equation with three unknowns, didn't you?

    What do you know about the tension along a single rope ?

    Do you know the acceleration if the blocks if they move along a straight line with constant speed?

    ehild
     
  8. Mar 2, 2012 #7
    Ehild thanks for your help yeah I did mean 2 eq and 3 unknown. After looking at this question that was exactly what I was was missing I must have read it too quickly constant speed tells me exactly what the acceleration is.
     
  9. Mar 3, 2012 #8

    ehild

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    You are welcome. :smile:

    ehild
     
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