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Determine static coefficient of friction

  1. Oct 25, 2015 #1
    1. The problem statement, all variables and given/known data
    why the µ for block is 0.3(between B and A) ? why cant be µ= 0.4( between B and C) ??

    2. Relevant equations


    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Oct 25, 2015 #2

    PhanthomJay

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    The problem statement is worded incorrectly. It apparently means to consider friction between blocks and the inclined surface, not between blocks. Move on to the next problem, and study the formulas for static and kinetic friction, and when each applies.
     
  4. Oct 25, 2015 #3
    Can you explain further ??
     
  5. Oct 25, 2015 #4

    haruspex

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    The first difficulty is that there is no actual question. It describes a set-up, but doesn't say what is to be determined.

    Following the calculation, we see that the first step shows that block C is not about to slide down, nor does it require any support from block B. So the conclusion is that there is no force between those two blocks. However, the calculation makes use of a coefficient of static friction between block C and the ramp, whereas the description gave this as the coefficient between blocks B and C.

    Next, we perform the same calculation for block B. This time we find that it will slide, if not held in place by block A. Again, it uses the stated coefficients between A and B as though they are between B and the ramp.

    The calculation for block A completely ignores the force that would be exerted by block B, so disagrees with the diagram. Again, it uses the stated coefficients between A and B as though they are between A and the ramp.

    In short, the question and solution are nonsense from start to finish.
     
  6. Oct 26, 2015 #5
    well , i still dont understand why 0.3 is used for B , why not 0.4 is used for block B ?
     
  7. Oct 26, 2015 #6

    haruspex

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    if you take the question statement as correct, there is no reason for using either. There simply is no information on the coefficients of friction between the blocks and the ramp.
    If you accept that the question statement is garbled, the only way we can deduce what it should have said is by looking at the solution; what it actually said becomes irrelevant.
     
  8. Oct 27, 2015 #7
    Ok, I know my mistakes already. Thank you for your help.
     
  9. Nov 28, 2015 #8
    why the µ for block is 0.3(between B and A) ? why cant be µ= 0.4( between B and C) ??
    can you explain on that ?
     
  10. Nov 29, 2015 #9

    PhanthomJay

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    Stop trying to make sense out of a problem that makes no sense, and move on.
     
  11. Nov 29, 2015 #10
    i still didnt get you . can you explain further ?
     
  12. Nov 29, 2015 #11

    haruspex

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    I see no way to make it any clearer than in my post #6.
     
  13. Nov 29, 2015 #12
    tat means there is something wrong with the question ?
     
  14. Nov 29, 2015 #13

    haruspex

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    Yes!!!!
     
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