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Homework Help: Tension problem help

  1. Jul 15, 2007 #1
    In Figure 5-50, three ballot boxes are connected by cords, one of which wraps over a pulley having negligible friction on its axle and negligible mass. The masses are mA = 28.0 kg, mB = 40.0 kg, mC = 22.0 kg.

    Box A is on the table and B/C are hanging from the table from the pulley system. So B/C would be on the vertical

    (a) When the assembly is released from rest, what is the tension in the cord that connects boxes B and C?




    F=ma



    I'm not really sure where to start for finding the tension between the 2 boxes. Any hints to get me started would be nice, thanks.
     
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  3. Jul 15, 2007 #2

    PhanthomJay

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    Draw free body diagrams of each block. Apply Newton 2 for each block. What can you say about the acceleration of each block?
     
  4. Jul 15, 2007 #3
    I'm pretty sure I drew my free body diagrams right, but that leaves me with mg, T and a. of which I only know mg, so I don't really know how to figure out my tension or acceleration. I know that Acceleration is the same for all the blocks.
     
  5. Jul 15, 2007 #4

    PhanthomJay

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    You have 3 fbd's, and 3 unknowns, the tension in the cable between block A and B, the tension in the cable between block B and C, and the acceleration. Solve for these values using 3 equations with 3 unknowns.
     
  6. Jul 15, 2007 #5
    I don't know. I'm absolutely lost right now with these tension problems. I guess I'm going to go back and re-read the chapter to see if I can get a better understanding of it. the problem is that there is about 1/3 of a page on tension in the book. thanks for the help though.
     
  7. Jul 15, 2007 #6

    PhanthomJay

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    What did you come up with in your FBD's? Here's the bottom block C FBD: [tex]M_c(g) -T_c = M_c(a)[/tex]
     
  8. Jul 15, 2007 #7
    - I had that for the block C, but I have 2 unknowns in that equation (a and T). The only other thing I could think of would be to figure out the equation for block B and then relate the 2 equations together. I'm just not sure how to set the equation up for block B since it's mass is larger than Block C.
     
  9. Jul 16, 2007 #8

    PhanthomJay

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    Draw the FBD for block B and see what you get. You have 2 tension forces plus the block B weight, all acting on Block B. Tension forces always pull away from the object. Then draw the FBD of Block A. You will now have 3 equations with 3 unknowns, and the problem is solvable.
     
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